linux/drivers/gpib/lpvo_usb_gpib/lpvo_usb_gpib.c

// SPDX-License-Identifier: GPL-2.0

/***************************************************************************
 *  This code has been developed at the Department of Physics (University  *
 *  of Florence, Italy) to support in linux-gpib the open usb-gpib adapter *
 *  implemented at the University of Ljubljana (lpvo.fe.uni-lj.si/gpib)	   *
 *									   *
 *  copyright		 : (C) 2011 Marcello Carla'			   *
 ***************************************************************************/

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#define dev_fmt pr_fmt
#define NAME KBUILD_MODNAME

/* base module includes */

#include <linux/module.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/tty.h>
#include <linux/types.h>
#include <linux/slab.h>
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include <linux/spinlock.h>
#include <linux/file.h>
#include <linux/timer.h>
#include <linux/delay.h>
#include <linux/sched/signal.h>
#include <linux/usb.h>

#include "gpibP.h"

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("GPIB driver for LPVO usb devices");

/*
 * Table of devices that work with this driver.
 *
 * Currently, only one device is known to be used in the lpvo_usb_gpib
 * adapter (FTDI 0403:6001) but as this device id is already handled by the
 * ftdi_sio USB serial driver the LPVO driver must not bind to it by default.
 *
 * If your adapter uses a different chip, insert a line
 * in the following table with proper <Vendor-id>, <Product-id>.
 *
 * To have your chip automatically handled by the driver,
 * update files "/usr/local/etc/modprobe.d/lpvo_usb_gpib.conf"
 * and /usr/local/etc/udev/rules.d/99-lpvo_usb_gpib.rules.
 *
 */

static const struct usb_device_id skel_table[] = {
	{ }					   /* Terminating entry */
};
MODULE_DEVICE_TABLE(usb, skel_table);

/*
 *   ***  Diagnostics and Debug  ***
 * To enable the diagnostic and debug messages either compile with DEBUG set
 * or control via the dynamic debug mechanisms.
 * The module parameter "debug" controls the sending of debug messages to
 * syslog. By default it is set to 0
 * debug = 0: only attach/detach messages are sent
 *         1: every action is logged
 *         2: extended logging; each single exchanged byte is documented
 *	(about twice the log volume of [1])
 * To switch debug level:
 *         At module loading:  modprobe lpvo_usb_gpib debug={0,1,2}
 *         On the fly: echo {0,1,2} > /sys/modules/lpvo_usb_gpib/parameters/debug
 */

static int debug;
module_param(debug, int, 0644);

#define DIA_LOG(level, format, ...)			   \
	do { if (debug >= (level))					\
			dev_dbg(board->gpib_dev, format, ## __VA_ARGS__); } \
	while (0)

#define WQT wait_queue_entry_t
#define WQH head
#define WQE entry

/* standard and extended command sets of the usb-gpib adapter */

#define USB_GPIB_ON	 "\nIB\n"
#define USB_GPIB_OFF	 "\nIBO\n"
#define USB_GPIB_IBm0	 "\nIBm0\n"   /* do not assert REN with IFC */
#define USB_GPIB_IBm1	 "\nIBm1\n"   /* assert REN with IFC */
#define USB_GPIB_IBCL	 "\nIBZ\n"
#define USB_GPIB_STATUS	 "\nIBS\n"
#define USB_GPIB_READ	 "\nIB?\n"
#define USB_GPIB_READ_1	 "\nIBB\n"
#define USB_GPIB_EOI	 "\nIBe0\n"
#define USB_GPIB_FTMO	 "\nIBf0\n"    /* disable first byte timeout */
#define USB_GPIB_TTMOZ	 "\nIBt0\n"    /* disable byte timeout */

/* incomplete commands */

#define USB_GPIB_BTMO	 "\nIBt"      /* set byte timeout */
#define USB_GPIB_TTMO	 "\nIBT"      /* set total timeout */

#define USB_GPIB_DEBUG_ON    "\nIBDE\xAA\n"
#define USB_GPIB_SET_LISTEN  "\nIBDT0\n"
#define USB_GPIB_SET_TALK    "\nIBDT1\n"
#define USB_GPIB_SET_LINES   "\nIBDC.\n"
#define USB_GPIB_SET_DATA    "\nIBDM.\n"
#define USB_GPIB_READ_LINES  "\nIBD?C\n"
#define USB_GPIB_READ_DATA   "\nIBD?M\n"
#define USB_GPIB_READ_BUS    "\nIBD??\n"

/* command sequences */

#define USB_GPIB_UNTALK "\nIBC_\n"
#define USB_GPIB_UNLISTEN "\nIBC?\n"

/* special characters used by the adapter */

#define DLE ('\020')
#define STX ('\02')
#define ETX ('\03')
#define ACK ('\06')
#define NODATA ('\03')
#define NODAV ('\011')

#define IB_BUS_REN  0x01
#define IB_BUS_IFC  0x02
#define IB_BUS_NDAC 0x04
#define IB_BUS_NRFD 0x08
#define IB_BUS_DAV  0x10
#define IB_BUS_EOI  0x20
#define IB_BUS_ATN  0x40
#define IB_BUS_SRQ  0x80

#define INBUF_SIZE 128

struct char_buf {		/* used by one_char() routine */
	char *inbuf;
	int last;
	int nchar;
};

struct usb_gpib_priv {		/* private data to the device */
	u8 eos;			/* eos character */
	short eos_flags;	/* eos mode */
	int timeout;		/* current value for timeout */
	void *dev;		/* the usb device private data structure */
};

#define GPIB_DEV (((struct usb_gpib_priv *)board->private_data)->dev)

static void show_status(struct gpib_board *board)
{
	DIA_LOG(2, "# - buffer_length %d\n", board->buffer_length);
	DIA_LOG(2, "# - status %lx\n", board->status);
	DIA_LOG(2, "# - use_count %d\n", board->use_count);
	DIA_LOG(2, "# - pad %x\n", board->pad);
	DIA_LOG(2, "# - sad %x\n", board->sad);
	DIA_LOG(2, "# - timeout %d\n", board->usec_timeout);
	DIA_LOG(2, "# - ppc %d\n", board->parallel_poll_configuration);
	DIA_LOG(2, "# - t1delay %d\n", board->t1_nano_sec);
	DIA_LOG(2, "# - online %d\n", board->online);
	DIA_LOG(2, "# - autopoll %d\n", board->autospollers);
	DIA_LOG(2, "# - autopoll task %p\n", board->autospoll_task);
	DIA_LOG(2, "# - minor %d\n", board->minor);
	DIA_LOG(2, "# - master %d\n", board->master);
	DIA_LOG(2, "# - list %d\n", board->ist);
}

/*
 * GLOBAL VARIABLES: required for
 * pairing among gpib minor and usb minor.
 * MAX_DEV is the max number of usb-gpib adapters; free
 * to change as you like, but no more than 32
 */

#define MAX_DEV 8
static struct usb_interface *lpvo_usb_interfaces[MAX_DEV];   /* registered interfaces */
static int usb_minors[MAX_DEV];			   /* usb minors */
static int assigned_usb_minors;		   /* mask of filled slots */
static struct mutex minors_lock;     /* operations on usb_minors are to be protected */

/*
 * usb-skeleton prototypes
 */

struct usb_skel;
static ssize_t skel_do_write(struct usb_skel *, const char *, size_t);
static ssize_t skel_do_read(struct usb_skel *, char *, size_t);
static int skel_do_open(struct gpib_board *, int);
static int skel_do_release(struct gpib_board *);

/*
 *  usec_diff : take difference in MICROsec between two 'timespec'
 *		 (unix time in sec and NANOsec)
 */

static inline int usec_diff(struct timespec64 *a, struct timespec64 *b)
{
	return ((a->tv_sec - b->tv_sec) * 1000000 +
		(a->tv_nsec - b->tv_nsec) / 1000);
}

/*
 *  ***  these routines are specific to the usb-gpib adapter  ***
 */

/**
 * write_loop() - Send a byte sequence to the adapter
 *
 * @dev:      the private device structure
 * @msg:      the byte sequence.
 * @leng:     the byte sequence length.
 *
 */

static int write_loop(void *dev, char *msg, int leng)
{
	return skel_do_write(dev, msg, leng);
}

/**
 * send_command() - Send a byte sequence and return a single byte reply.
 *
 * @board:    the gpib_board_struct data area for this gpib interface
 * @msg:      the byte sequence.
 * @leng:     the byte sequence length; can be given as zero and is
 *	      computed automatically, but if 'msg' contains a zero byte,
 *	      it has to be given explicitly.
 */

static int send_command(struct gpib_board *board, char *msg, int leng)
{
	char buffer[64];
	int nchar;
	int retval;
	struct timespec64 before, after;

	ktime_get_real_ts64 (&before);

	if (!leng)
		leng = strlen(msg);
	retval = write_loop(GPIB_DEV, msg, leng);
	if (retval < 0)
		return retval;

	nchar = skel_do_read(GPIB_DEV, buffer, 64);

	if (nchar < 0) {
		dev_err(board->gpib_dev, " return from read: %d\n", nchar);
		return nchar;
	} else if (nchar != 1) {
		dev_err(board->gpib_dev, " Irregular reply to command: %s\n", msg);
		return -EIO;
	}
	ktime_get_real_ts64 (&after);

	DIA_LOG(1, "Sent %d - done %d us.\n", leng, usec_diff(&after, &before));

	return buffer[0] & 0xff;
}

/*
 * set_control_line() - Set the value of a single gpib control line
 *
 * @board:    the gpib_board_struct data area for this gpib interface
 * @line:     line mask
 * @value:    line new value (0/1)
 */

static int set_control_line(struct gpib_board *board, int line, int value)
{
	char msg[] = USB_GPIB_SET_LINES;
	int retval;
	int leng = strlen(msg);

	DIA_LOG(1, "setting line %x to %x\n", line, value);

	retval = send_command(board, USB_GPIB_READ_LINES, 0);

	DIA_LOG(1, "old line values: %x\n", retval);

	if (retval == -EIO)
		return retval;

	msg[leng - 2] = value ? (retval & ~line) : retval | line;

	retval = send_command(board, msg, 0);

	DIA_LOG(1, "operation result: %x\n", retval);

	return retval;
}

/*
 * one_char() - read one single byte from input buffer
 *
 * @board:	the gpib_board_struct data area for this gpib interface
 * @char_buf:	the routine private data structure
 */

static int one_char(struct gpib_board *board, struct char_buf *b)
{
	struct timespec64 before, after;

	if (b->nchar) {
		DIA_LOG(2, "-> %x\n", b->inbuf[b->last - b->nchar]);
		return b->inbuf[b->last - b->nchar--];
	}
	ktime_get_real_ts64 (&before);
	b->nchar = skel_do_read(GPIB_DEV, b->inbuf, INBUF_SIZE);
	b->last = b->nchar;
	ktime_get_real_ts64 (&after);

	DIA_LOG(2, "read %d bytes in %d usec\n",
		b->nchar, usec_diff(&after, &before));

	if (b->nchar > 0) {
		DIA_LOG(2, "--> %x\n", b->inbuf[b->last - b->nchar]);
		return b->inbuf[b->last - b->nchar--];
	}
	return -EIO;
}

/**
 * set_timeout() - set single byte / total timeouts on the adapter
 *
 * @board:    the gpib_board_struct data area for this gpib interface
 *
 *	   For sake of speed, the operation is performed only if it
 *	   modifies the current (saved) value. Minimum allowed timeout
 *	   is 30 ms (T30ms -> 8); timeout disable (TNONE -> 0) currently
 *	   not supported.
 */

static void set_timeout(struct gpib_board *board)
{
	int n, val;
	char command[sizeof(USB_GPIB_TTMO) + 6];
	struct usb_gpib_priv *data = board->private_data;

	if (data->timeout == board->usec_timeout)
		return;

	n = (board->usec_timeout + 32767) / 32768;
	if (n < 2)
		n = 2;

	DIA_LOG(1, "Set timeout to %d us -> %d\n", board->usec_timeout, n);

	sprintf(command, "%s%d\n", USB_GPIB_BTMO, n > 255 ? 255 : n);
	val = send_command(board, command, 0);

	if (val == ACK) {
		if (n > 65535)
			n = 65535;
		sprintf(command, "%s%d\n", USB_GPIB_TTMO, n);
		val = send_command(board, command, 0);
	}

	if (val != ACK)
		dev_err(board->gpib_dev, "error in timeout set: <%s>\n", command);
	else
		data->timeout = board->usec_timeout;
}

/*
 * now the standard interface functions - attach and detach
 */

/**
 * usb_gpib_attach() - activate the usb-gpib converter board
 *
 * @board:    the gpib_board_struct data area for this gpib interface
 * @config:   firmware data, if any (from gpib_config -I <file>)
 *
 * The channel name is ttyUSBn, with n=0 by default. Other values for n
 * passed with gpib_config -b <n>.
 *
 * In this routine I trust that when an error code is returned
 * detach() will be called. Always.
 */

static int usb_gpib_attach(struct gpib_board *board, const struct gpib_board_config *config)
{
	int retval, j;
	u32 base = config->ibbase;
	char *device_path;
	int match;
	struct usb_device *udev;

	DIA_LOG(0, "Board %p -t %s -m %d -a %p -u %d -l %d -b %d\n",
		board, board->interface->name, board->minor, config->device_path,
		config->pci_bus, config->pci_slot, base);

	board->private_data = NULL;  /* to be sure - we can detach before setting */

	/* identify device to be attached */

	mutex_lock(&minors_lock);

	if (config->device_path) {
		/* if config->device_path given, try that first */
		for (j = 0 ; j < MAX_DEV ; j++) {
			if ((assigned_usb_minors & 1 << j) == 0)
				continue;
			udev =	usb_get_dev(interface_to_usbdev(lpvo_usb_interfaces[j]));
			device_path = kobject_get_path(&udev->dev.kobj, GFP_KERNEL);
			match = gpib_match_device_path(&lpvo_usb_interfaces[j]->dev,
						       config->device_path);
			DIA_LOG(1, "dev. %d: minor %d  path: %s --> %d\n", j,
				lpvo_usb_interfaces[j]->minor, device_path, match);
			kfree(device_path);
			if (match)
				break;
		}
	} else if (config->pci_bus != -1 && config->pci_slot != -1) {
		/* second: look for bus and slot */
		for (j = 0 ; j < MAX_DEV ; j++) {
			if ((assigned_usb_minors & 1 << j) == 0)
				continue;
			udev =	usb_get_dev(interface_to_usbdev(lpvo_usb_interfaces[j]));
			DIA_LOG(1, "dev. %d: bus %d -> %d  dev: %d -> %d\n", j,
				udev->bus->busnum, config->pci_bus, udev->devnum, config->pci_slot);
			if (config->pci_bus == udev->bus->busnum &&
			    config->pci_slot == udev->devnum)
				break;
		}
	} else {		/* last chance: usb_minor, given as ibbase */
		for (j = 0 ; j < MAX_DEV ; j++) {
			if (usb_minors[j] == base && assigned_usb_minors & 1 << j)
				break;
		}
	}
	mutex_unlock(&minors_lock);

	if (j == MAX_DEV) {
		dev_err(board->gpib_dev, "Requested device is not registered.\n");
		return -EIO;
	}

	board->private_data = kzalloc_obj(struct usb_gpib_priv);
	if (!board->private_data)
		return -ENOMEM;

	retval = skel_do_open(board, usb_minors[j]);

	DIA_LOG(1, "Skel open: %d\n", retval);

	if (retval) {
		dev_err(board->gpib_dev, "skel open failed.\n");
		kfree(board->private_data);
		board->private_data = NULL;
		return -ENODEV;
	}

	show_status(board);

	retval = send_command(board, USB_GPIB_ON, 0);
	DIA_LOG(1, "USB_GPIB_ON returns %x\n", retval);
	if (retval != ACK)
		return -EIO;

	/*
	 * We must setup debug mode because we need the extended instruction
	 * set to cope with the Core (gpib_common) point of view
	 */

	retval = send_command(board, USB_GPIB_DEBUG_ON, 0);
	DIA_LOG(1, "USB_GPIB_DEBUG_ON returns %x\n", retval);
	if (retval != ACK)
		return -EIO;

	/*
	 * We must keep REN off after an IFC because so it is
	 * assumed by the Core
	 */

	retval = send_command(board, USB_GPIB_IBm0, 0);
	DIA_LOG(1, "USB_GPIB_IBm0 returns %x\n", retval);
	if (retval != ACK)
		return -EIO;

	retval = set_control_line(board, IB_BUS_REN, 0);
	if (retval != ACK)
		return -EIO;

	retval = send_command(board, USB_GPIB_FTMO, 0);
	DIA_LOG(1, "USB_GPIB_FTMO returns %x\n", retval);
	if (retval != ACK)
		return -EIO;

	show_status(board);
	DIA_LOG(0, "attached\n");
	return 0;
}

/**
 * usb_gpib_detach() - deactivate the usb-gpib converter board
 *
 * @board:    the gpib_board data area for this gpib interface
 *
 */

static void usb_gpib_detach(struct gpib_board *board)
{
	int retval;

	show_status(board);

	DIA_LOG(0, "detaching\n");

	if (board->private_data) {
		if (GPIB_DEV) {
			write_loop(GPIB_DEV, USB_GPIB_OFF, strlen(USB_GPIB_OFF));
			msleep(100);
			DIA_LOG(1, "%s", "GPIB off\n");
			retval = skel_do_release(board);
			DIA_LOG(1, "skel release -> %d\n", retval);
		}
		kfree(board->private_data);
		board->private_data = NULL;
	}

	DIA_LOG(0, "detached\n");
}

/*
 *   Other functions follow in alphabetical order
 */
/* command */
static int usb_gpib_command(struct gpib_board *board,
			    u8 *buffer,
			    size_t length,
			    size_t *bytes_written)
{
	int i, retval;
	char command[6] = "IBc.\n";

	DIA_LOG(1, "enter %p\n", board);

	set_timeout(board);

	*bytes_written = 0;
	for (i = 0 ; i < length ; i++) {
		command[3] = buffer[i];
		retval = send_command(board, command, 5);
		DIA_LOG(2, "%d ==> %x %x\n", i, buffer[i], retval);
		if (retval != 0x06)
			return retval;
		++(*bytes_written);
	}
	return 0;
}

/**
 * usb_gpib_disable_eos() - Disable END on eos byte (END on EOI only)
 *
 * @board:    the gpib_board data area for this gpib interface
 *
 *   With the lpvo adapter eos can only be handled via software.
 *   Cannot do nothing here, but remember for future use.
 */

static void usb_gpib_disable_eos(struct gpib_board *board)
{
	((struct usb_gpib_priv *)board->private_data)->eos_flags &= ~REOS;
	DIA_LOG(1, "done: %x\n",
		((struct usb_gpib_priv *)board->private_data)->eos_flags);
}

/**
 * usb_gpib_enable_eos() - Enable END for reads when eos byte is received.
 *
 * @board:    the gpib_board data area for this gpib interface
 * @eos_byte: the 'eos' byte
 * @compare_8_bits: if zero ignore eigthth bit when comparing
 *
 */

static int usb_gpib_enable_eos(struct gpib_board *board,
			       u8 eos_byte,
			       int compare_8_bits)
{
	struct usb_gpib_priv *pd = (struct usb_gpib_priv *)board->private_data;

	DIA_LOG(1, "enter with %x\n", eos_byte);
	pd->eos = eos_byte;
	pd->eos_flags = REOS;
	if (compare_8_bits)
		pd->eos_flags |= BIN;
	return 0;
}

/**
 * usb_gpib_go_to_standby() - De-assert ATN
 *
 * @board:    the gpib_board data area for this gpib interface
 */

static int usb_gpib_go_to_standby(struct gpib_board *board)
{
	int retval = set_control_line(board, IB_BUS_ATN, 0);

	DIA_LOG(1, "done with %x\n", retval);

	if (retval == ACK)
		return 0;
	return -EIO;
}

/**
 * usb_gpib_interface_clear() - Assert or de-assert IFC
 *
 * @board:    the gpib_board data area for this gpib interface
 * @assert:   1: assert IFC;  0: de-assert IFC
 *
 *    Currently on the assert request we issue the lpvo IBZ
 *    command that cycles IFC low for 100 usec, then we ignore
 *    the de-assert request.
 */

static void usb_gpib_interface_clear(struct gpib_board *board, int assert)
{
	int retval = 0;

	DIA_LOG(1, "enter with %d\n", assert);

	if (assert) {
		retval = send_command(board, USB_GPIB_IBCL, 0);

		set_bit(CIC_NUM, &board->status);
	}

	DIA_LOG(1, "done with %d %d\n", assert, retval);
}

/**
 * usb_gpib_line_status() - Read the status of the bus lines.
 *
 *  @board:    the gpib_board data area for this gpib interface
 *
 *    We can read all lines.
 */
static int usb_gpib_line_status(const struct gpib_board *board)
{
	int buffer;
	int line_status = VALID_ALL;   /* all lines will be read */
	struct list_head *p, *q;
	WQT *item;
	unsigned long flags;
	int sleep = 0;

	DIA_LOG(1, "%s\n", "request");

	/*
	 * if we are on the wait queue (board->wait), do not hurry
	 * reading status line; instead, pause a little
	 */

	spin_lock_irqsave((spinlock_t *)&board->wait.lock, flags);
	q = (struct list_head *)&board->wait.WQH;
	list_for_each(p, q) {
		item = container_of(p, WQT, WQE);
		if (item->private == current) {
			sleep = 20;
			break;
		}
		/* pid is: ((struct task_struct *) item->private)->pid); */
	}
	spin_unlock_irqrestore((spinlock_t *)&board->wait.lock, flags);
	if (sleep) {
		DIA_LOG(1, "we are on the wait queue - sleep %d ms\n", sleep);
		msleep(sleep);
	}

	buffer = send_command((struct gpib_board *)board, USB_GPIB_STATUS, 0);

	if (buffer < 0) {
		dev_err(board->gpib_dev, "line status read failed with %d\n", buffer);
		return -1;
	}

	if ((buffer & 0x01) == 0)
		line_status |= BUS_REN;
	if ((buffer & 0x02) == 0)
		line_status |= BUS_IFC;
	if ((buffer & 0x04) == 0)
		line_status |= BUS_NDAC;
	if ((buffer & 0x08) == 0)
		line_status |= BUS_NRFD;
	if ((buffer & 0x10) == 0)
		line_status |= BUS_DAV;
	if ((buffer & 0x20) == 0)
		line_status |= BUS_EOI;
	if ((buffer & 0x40) == 0)
		line_status |= BUS_ATN;
	if ((buffer & 0x80) == 0)
		line_status |= BUS_SRQ;

	DIA_LOG(1, "done with %x %x\n", buffer, line_status);

	return line_status;
}

/* parallel_poll */

static int usb_gpib_parallel_poll(struct gpib_board *board, u8 *result)
{
	/*
	 * request parallel poll asserting ATN | EOI;
	 * we suppose ATN already asserted
	 */

	int retval;

	DIA_LOG(1, "enter %p\n", board);

	retval = set_control_line(board, IB_BUS_EOI, 1);
	if (retval != ACK)
		return -EIO;

	*result = send_command(board, USB_GPIB_READ_DATA, 0);

	DIA_LOG(1, "done with %x\n", *result);

	retval = set_control_line(board, IB_BUS_EOI, 0);
	if (retval != 0x06)
		return -EIO;

	return 0;
}

/* read */

static int usb_gpib_read(struct gpib_board *board,
			 u8 *buffer,
			 size_t length,
			 int *end,
			 size_t *bytes_read)
{
#define MAX_READ_EXCESS 16384

	struct char_buf b = {NULL, 0};

	int retval;
	char c, nc;
	int ic;
	struct timespec64 before, after;
	int read_count = MAX_READ_EXCESS;
	struct usb_gpib_priv *pd = (struct usb_gpib_priv *)board->private_data;

	DIA_LOG(1, "enter %p -> %zu\n", board, length);

	*bytes_read = 0;      /* by default, things go wrong */
	*end = 0;

	set_timeout(board);

	/* single byte read has a special handling */

	if (length == 1) {
		char inbuf[2] = {0, 0};

		/* read a single character */

		ktime_get_real_ts64 (&before);

		retval = write_loop(GPIB_DEV, USB_GPIB_READ_1, strlen(USB_GPIB_READ_1));
		if (retval < 0)
			return retval;

		retval = skel_do_read(GPIB_DEV, inbuf, 1);
		retval += skel_do_read(GPIB_DEV, inbuf + 1, 1);

		ktime_get_real_ts64 (&after);

		DIA_LOG(1, "single read: %x %x %x in %d\n", retval,
			inbuf[0], inbuf[1],
			usec_diff(&after, &before));

		/* good char / last char? */

		if (retval == 2 && inbuf[1] == ACK) {
			buffer[0] = inbuf[0];
			*bytes_read = 1;
			return 0;
		}
		if (retval < 2)
			return -EIO;
		else
			return -ETIME;
	}

	/* allocate buffer for multibyte read */

	b.inbuf = kmalloc(INBUF_SIZE, GFP_KERNEL);
	if (!b.inbuf)
		return -ENOMEM;

	/* send read command and check <DLE><STX> sequence */

	retval = write_loop(GPIB_DEV, USB_GPIB_READ, strlen(USB_GPIB_READ));
	if (retval < 0)
		goto read_return;

	if (one_char(board, &b) != DLE || one_char(board, &b) != STX) {
		dev_err(board->gpib_dev, "wrong <DLE><STX> sequence\n");
		retval = -EIO;
		goto read_return;
	}

	/* get data flow */

	while (1) {
		ic = one_char(board, &b);
		if (ic == -EIO) {
			retval = -EIO;
			goto read_return;
		}
		c = ic;

		if (c == DLE)
			nc = one_char(board, &b);
		if (c != DLE || nc == DLE) {
			/* data byte - store into buffer */

			if (*bytes_read == length)
				break; /* data overflow */
			if (c == DLE)
				c = nc;
			buffer[(*bytes_read)++] = c;
			if (c == pd->eos) {
				*end = 1;
				break;
			}

		} else {
			/* we are in the closing <DLE><ETX> sequence */
			c = nc;
			if (c == ETX) {
				c = one_char(board, &b);
				if (c == ACK) {
					*end = 1;
					retval = 0;
					goto read_return;
				} else {
					dev_err(board->gpib_dev, "wrong end of message %x", c);
					retval = -ETIME;
					goto read_return;
				}
			} else {
				dev_err(board->gpib_dev, "lone <DLE> in stream");
				retval = -EIO;
				goto read_return;
			}
		}
	}

	/* we had a data overflow - flush excess data */

	while (read_count--) {
		if (one_char(board, &b) != DLE)
			continue;
		c = one_char(board, &b);
		if (c == DLE)
			continue;
		if (c == ETX) {
			c = one_char(board, &b);
			if (c == ACK) {
				if (MAX_READ_EXCESS - read_count > 1)
					dev_dbg(board->gpib_dev, "small buffer - maybe some data lost");
				retval = 0;
				goto read_return;
			}
			break;
		}
	}

	dev_err(board->gpib_dev, "no input end - board in odd state\n");
	retval = -EIO;

read_return:
	kfree(b.inbuf);

	DIA_LOG(1, "done with byte/status: %d %x %d\n",	(int)*bytes_read, retval, *end);

	if (retval == 0 || retval == -ETIME) {
		if (send_command(board, USB_GPIB_UNTALK, sizeof(USB_GPIB_UNTALK)) == 0x06)
			return retval;
		return	-EIO;
	}

	return retval;
}

/* remote_enable */

static void usb_gpib_remote_enable(struct gpib_board *board, int enable)
{
	int retval;

	retval = set_control_line(board, IB_BUS_REN, enable ? 1 : 0);
	if (retval != ACK)
		dev_err(board->gpib_dev, "could not set REN line: %x\n", retval);

	DIA_LOG(1, "done with %x\n", retval);
}

/* request_system_control */

static int usb_gpib_request_system_control(struct gpib_board *board, int request_control)
{
	if (!request_control)
		return -EINVAL;

	DIA_LOG(1, "done with %d -> %lx\n", request_control, board->status);
	return 0;
}

/* take_control */
/* beware: the sync flag is ignored; what is its real meaning? */

static int usb_gpib_take_control(struct gpib_board *board, int sync)
{
	int retval;

	retval = set_control_line(board, IB_BUS_ATN, 1);

	DIA_LOG(1, "done with %d %x\n", sync, retval);

	if (retval == ACK)
		return 0;
	return -EIO;
}

/* update_status */

static unsigned int usb_gpib_update_status(struct gpib_board *board,
					   unsigned int clear_mask)
{
	/* There is nothing we can do here, I guess */

	board->status &= ~clear_mask;

	DIA_LOG(1, "done with %x %lx\n", clear_mask, board->status);

	return board->status;
}

/* write */
/* beware: DLE characters are not escaped - can only send ASCII data */

static int usb_gpib_write(struct gpib_board *board,
			  u8 *buffer,
			  size_t length,
			  int send_eoi,
			  size_t *bytes_written)
{
	int retval;
	char *msg;

	DIA_LOG(1, "enter %p -> %zu\n", board, length);

	set_timeout(board);

	msg = kmalloc(length + 8, GFP_KERNEL);
	if (!msg)
		return -ENOMEM;

	memcpy(msg, "\nIB\020\002", 5);
	memcpy(msg + 5, buffer, length);
	memcpy(msg + 5 + length, "\020\003\n", 3);

	retval = send_command(board, msg, length + 8);
	kfree(msg);

	DIA_LOG(1, "<%.*s> -> %x\n", (int)length, buffer, retval);

	if (retval != ACK)
		return -EPIPE;

	*bytes_written = length;

	if (send_command(board, USB_GPIB_UNLISTEN, sizeof(USB_GPIB_UNLISTEN)) != 0x06)
		return -EPIPE;

	return length;
}

/*
 *  ***	 following functions not implemented yet  ***
 */

/* parallel_poll configure */

static void usb_gpib_parallel_poll_configure(struct gpib_board *board,
					     u8 configuration)
{
}

/* parallel_poll_response */

static void usb_gpib_parallel_poll_response(struct gpib_board *board, int ist)
{
}

/* primary_address */

static int  usb_gpib_primary_address(struct gpib_board *board, unsigned int address)
{
	return 0;
}

/* return_to_local */

static	void usb_gpib_return_to_local(struct gpib_board *board)
{
}

/* secondary_address */

static int usb_gpib_secondary_address(struct gpib_board *board,
				      unsigned int address,
				      int enable)
{
	return 0;
}

/* serial_poll_response */

static void usb_gpib_serial_poll_response(struct gpib_board *board, u8 status)
{
}

/* serial_poll_status */

static u8 usb_gpib_serial_poll_status(struct gpib_board *board)
{
	return 0;
}

/* t1_delay */

static int usb_gpib_t1_delay(struct gpib_board *board, unsigned int nano_sec)
{
	return 0;
}

/*
 *   ***  module dispatch table and init/exit functions	 ***
 */

static struct gpib_interface usb_gpib_interface = {
	.name = NAME,
	.attach = usb_gpib_attach,
	.detach = usb_gpib_detach,
	.read = usb_gpib_read,
	.write = usb_gpib_write,
	.command = usb_gpib_command,
	.take_control = usb_gpib_take_control,
	.go_to_standby = usb_gpib_go_to_standby,
	.request_system_control = usb_gpib_request_system_control,
	.interface_clear = usb_gpib_interface_clear,
	.remote_enable = usb_gpib_remote_enable,
	.enable_eos = usb_gpib_enable_eos,
	.disable_eos = usb_gpib_disable_eos,
	.parallel_poll = usb_gpib_parallel_poll,
	.parallel_poll_configure = usb_gpib_parallel_poll_configure,
	.parallel_poll_response = usb_gpib_parallel_poll_response,
	.local_parallel_poll_mode = NULL, // XXX
	.line_status = usb_gpib_line_status,
	.update_status = usb_gpib_update_status,
	.primary_address = usb_gpib_primary_address,
	.secondary_address = usb_gpib_secondary_address,
	.serial_poll_response = usb_gpib_serial_poll_response,
	.serial_poll_status = usb_gpib_serial_poll_status,
	.t1_delay = usb_gpib_t1_delay,
	.return_to_local = usb_gpib_return_to_local,
	.skip_check_for_command_acceptors = 1
};

/*
 * usb_gpib_init_module(), usb_gpib_exit_module()
 *
 * This functions are called every time a new device is detected
 * and registered or is removed and unregistered.
 * We must take note of created and destroyed usb minors to be used
 * when usb_gpib_attach() and usb_gpib_detach() will be called on
 * request by gpib_config.
 */

static int usb_gpib_init_module(struct usb_interface *interface)
{
	int j, mask, rv;

	rv = mutex_lock_interruptible(&minors_lock);
	if (rv < 0)
		return rv;

	if (!assigned_usb_minors) {
		rv = gpib_register_driver(&usb_gpib_interface, THIS_MODULE);
		if (rv) {
			pr_err("gpib_register_driver failed: error = %d\n", rv);
			goto exit;
		}
	} else {
		/*
		 * check if minor is already registered - maybe useless, but if
		 * it happens the code is inconsistent somewhere
		 */

		for (j = 0 ; j < MAX_DEV ; j++) {
			if (usb_minors[j] == interface->minor && assigned_usb_minors & 1 << j) {
				pr_err("CODE BUG: USB minor %d registered at %d.\n",
				       interface->minor, j);
				rv = -1;
				goto exit;
			}
		}
	}

	/* find a free slot */

	for (j = 0 ; j < MAX_DEV ; j++) {
		mask = 1 << j;
		if ((assigned_usb_minors & mask) == 0) {
			usb_minors[j] = interface->minor;
			lpvo_usb_interfaces[j] = interface;
			assigned_usb_minors |= mask;
			rv = 0;
			goto exit;
		}
	}
	pr_err("No slot available for interface %p minor %d\n", interface, interface->minor);
	rv = -1;

exit:
	mutex_unlock(&minors_lock);
	return rv;
}

static void usb_gpib_exit_module(int minor)
{
	int j;

	mutex_lock(&minors_lock);
	for (j = 0 ; j < MAX_DEV ; j++) {
		if (usb_minors[j] == minor && assigned_usb_minors & 1 << j) {
			assigned_usb_minors &= ~(1 << j);
			usb_minors[j] = -1;
			if (assigned_usb_minors == 0)
				gpib_unregister_driver(&usb_gpib_interface);
			goto exit;
		}
	}
	pr_err("CODE BUG: USB minor %d not found.\n", minor);

exit:
	mutex_unlock(&minors_lock);
}

/*
 * Default latency time (16 msec) is too long.
 * We must use 1 msec (best); anyhow, no more than 5 msec.
 *
 * Defines and function taken and modified from the kernel tree
 * (see ftdi_sio.h and ftdi_sio.c).
 */

#define FTDI_SIO_SET_LATENCY_TIMER	9 /* Set the latency timer */
#define FTDI_SIO_SET_LATENCY_TIMER_REQUEST FTDI_SIO_SET_LATENCY_TIMER
#define FTDI_SIO_SET_LATENCY_TIMER_REQUEST_TYPE 0x40
#define WDR_TIMEOUT 5000 /* default urb timeout */
#define WDR_SHORT_TIMEOUT 1000	/* shorter urb timeout */

#define LATENCY_TIMER 1		   /* use a small latency timer: 1 ... 5 msec */
#define LATENCY_CHANNEL 0	   /* channel selection in multichannel devices */
static int write_latency_timer(struct usb_device *udev)
{
	int rv = usb_control_msg(udev,
				 usb_sndctrlpipe(udev, 0),
				 FTDI_SIO_SET_LATENCY_TIMER_REQUEST,
				 FTDI_SIO_SET_LATENCY_TIMER_REQUEST_TYPE,
				 LATENCY_TIMER, LATENCY_CHANNEL,
				 NULL, 0, WDR_TIMEOUT);
	if (rv < 0)
		dev_err(&udev->dev, "Unable to write latency timer: %i\n", rv);
	return rv;
}

/*****************************************************************************
 *									     *
 *  The following code is a modified version of the USB Skeleton driver	     *
 *  written by Greg Kroah-Hartman and available in the kernel tree.	     *
 *									     *
 *  Functions skel_open() and skel_release() have been rewritten and named   *
 *  skel_do_open() and skel_do_release() to process the attach and detach    *
 *  requests coming from gpib_config.					     *
 *									     *
 *  Functions skel_read() and skel_write() have been split into a	     *
 *  skel_do_read() and skel_do_write(), that cover the kernel stuff of read  *
 *  and write operations, and the original skel_read() and skel_write(),     *
 *  that handle communication with user space and call their _do_ companion. *
 *									     *
 *  Only the _do_ versions are used by the lpvo_usb_gpib driver; other ones  *
 *  can be (optionally) maintained in the compilation to have direct access  *
 *  to a gpib controller for debug and diagnostics.			     *
 *									     *
 *  To avoid collisions in names, devices in user space have been renamed    *
 *  lpvo_raw1, lpvo_raw2 ....  and the usb driver has been renamed with the  *
 *  gpib module name.							     *
 *									     *
 *****************************************************************************/

/*
 * USB Skeleton driver - 2.2
 *
 * Copyright (C) 2001-2004 Greg Kroah-Hartman (greg@kroah.com)
 *
 * This driver is based on the 2.6.3 version of drivers/usb/usb-skeleton.c
 * but has been rewritten to be easier to read and use.
 */

#include <linux/errno.h>
#include <linux/kref.h>
#include <linux/uaccess.h>
#include <linux/mutex.h>

/* Get a minor range for your devices from the usb maintainer */
#define USB_SKEL_MINOR_BASE	   192

/*   private defines   */

#define MAX_TRANSFER		    (PAGE_SIZE - 512)
/*
 * MAX_TRANSFER is chosen so that the VM is not stressed by
 * allocations > PAGE_SIZE and the number of packets in a page
 * is an integer 512 is the largest possible packet on EHCI
 */

#define WRITES_IN_FLIGHT	1     /* we do not want more than one pending write */
#define USER_DEVICE 1		      /* compile for device(s) in user space */

/* Structure to hold all of our device specific stuff */
struct usb_skel {
	struct usb_device     *udev;		     /* the usb device for this device */
	struct usb_interface  *interface;	     /* the interface for this device */
	struct semaphore      limit_sem;	     /* limiting the number of writes in progress */
	struct usb_anchor     submitted;	     /* in case need to retract our submissions */
	struct urb	      *bulk_in_urb;	     /* the urb to read data with */
	unsigned char	      *bulk_in_buffer;	     /* the buffer to receive data */
	size_t		      bulk_in_size;	     /* the size of the receive buffer */
	size_t		      bulk_in_filled;	     /* number of bytes in the buffer */
	size_t		      bulk_in_copied;	     /* already copied to user space */
	__u8		      bulk_in_endpoint_addr;  /* the address of the bulk in endpoint */
	__u8		      bulk_out_endpoint_addr; /* the address of the bulk out endpoint */
	int		      errors;		     /* the last request tanked */
	bool		      ongoing_read;	     /* a read is going on */
	spinlock_t	      err_lock;		     /* lock for errors */
	struct kref	      kref;
	struct mutex	      io_mutex;		     /* synchronize I/O with disconnect */
	wait_queue_head_t     bulk_in_wait;	     /* to wait for an ongoing read */
};

#define to_skel_dev(d) container_of(d, struct usb_skel, kref)

static struct usb_driver skel_driver;
static void skel_draw_down(struct usb_skel *dev);

static void skel_delete(struct kref *kref)
{
	struct usb_skel *dev = to_skel_dev(kref);

	usb_free_urb(dev->bulk_in_urb);
	usb_put_dev(dev->udev);
	kfree(dev->bulk_in_buffer);
	kfree(dev);
}

/*
 * skel_do_open() - to be called by usb_gpib_attach
 */

static int skel_do_open(struct gpib_board *board, int subminor)
{
	struct usb_skel *dev;
	struct usb_interface *interface;
	int retval = 0;

	interface = usb_find_interface(&skel_driver, subminor);
	if (!interface) {
		dev_err(board->gpib_dev, "can't find device for minor %d\n", subminor);
		retval = -ENODEV;
		goto exit;
	}

	dev = usb_get_intfdata(interface);
	if (!dev) {
		retval = -ENODEV;
		goto exit;
	}

	retval = usb_autopm_get_interface(interface);
	if (retval)
		goto exit;

	/* increment our usage count for the device */
	kref_get(&dev->kref);

	/* save our object in the file's private structure */
	GPIB_DEV = dev;

exit:
	return retval;
}

/*
 * skel_do_release() - to be called by usb_gpib_detach
 */

static int skel_do_release(struct gpib_board *board)
{
	struct usb_skel *dev;

	dev = GPIB_DEV;
	if (!dev)
		return -ENODEV;

	/* allow the device to be autosuspended */
	mutex_lock(&dev->io_mutex);
	if (dev->interface)
		usb_autopm_put_interface(dev->interface);
	mutex_unlock(&dev->io_mutex);

	/* decrement the count on our device */
	kref_put(&dev->kref, skel_delete);
	return 0;
}

/*
 * read functions
 */

static void skel_read_bulk_callback(struct urb *urb)
{
	struct usb_skel *dev;
	unsigned long flags;

	dev = urb->context;

	spin_lock_irqsave(&dev->err_lock, flags);
	/* sync/async unlink faults aren't errors */
	if (urb->status) {
		if (!(urb->status == -ENOENT ||
		      urb->status == -ECONNRESET ||
		      urb->status == -ESHUTDOWN))
			dev_err(&dev->interface->dev, "nonzero read bulk status received: %d\n",
				urb->status);

		dev->errors = urb->status;
	} else {
		dev->bulk_in_filled = urb->actual_length;
	}
	dev->ongoing_read = 0;
	spin_unlock_irqrestore(&dev->err_lock, flags);

	wake_up_interruptible(&dev->bulk_in_wait);
}

static int skel_do_read_io(struct usb_skel *dev, size_t count)
{
	int rv;

	/* prepare a read */
	usb_fill_bulk_urb(dev->bulk_in_urb,
			  dev->udev,
			  usb_rcvbulkpipe(dev->udev,
					  dev->bulk_in_endpoint_addr),
			  dev->bulk_in_buffer,
			  min(dev->bulk_in_size, count),
			  skel_read_bulk_callback,
			  dev);
	/* tell everybody to leave the URB alone */
	spin_lock_irq(&dev->err_lock);
	dev->ongoing_read = 1;
	spin_unlock_irq(&dev->err_lock);

	/* submit bulk in urb, which means no data to deliver */
	dev->bulk_in_filled = 0;
	dev->bulk_in_copied = 0;

	/* do it */
	rv = usb_submit_urb(dev->bulk_in_urb, GFP_KERNEL);
	if (rv < 0) {
		dev_err(&dev->interface->dev, "failed submitting read urb, error %d\n", rv);
		rv = (rv == -ENOMEM) ? rv : -EIO;
		spin_lock_irq(&dev->err_lock);
		dev->ongoing_read = 0;
		spin_unlock_irq(&dev->err_lock);
	}

	return rv;
}

/*
 * skel_do_read() - read operations from lpvo_usb_gpib
 */

static ssize_t skel_do_read(struct usb_skel *dev, char *buffer, size_t count)
{
	int rv;
	bool ongoing_io;

	/* if we cannot read at all, return EOF */

	if (!dev->bulk_in_urb || !count)
		return 0;

restart:  /* added to comply with ftdi timeout technique */

	/* no concurrent readers */

	rv = mutex_lock_interruptible(&dev->io_mutex);
	if (rv < 0)
		return rv;

	if (!dev->interface) {		      /* disconnect() was called */
		rv = -ENODEV;
		goto exit;
	}

retry:
	/* if IO is under way, we must not touch things */
	spin_lock_irq(&dev->err_lock);
	ongoing_io = dev->ongoing_read;
	spin_unlock_irq(&dev->err_lock);

	if (ongoing_io) {
//		  /* nonblocking IO shall not wait */
//		  /* no file, no O_NONBLOCK; maybe provide when from user space */
//		  if (file->f_flags & O_NONBLOCK) {
//			  rv = -EAGAIN;
//			  goto exit;
//		  }

		/*
		 * IO may take forever
		 * hence wait in an interruptible state
		 */
		rv = wait_event_interruptible(dev->bulk_in_wait, (!dev->ongoing_read));
		if (rv < 0)
			goto exit;
	}

	/* errors must be reported */
	rv = dev->errors;
	if (rv < 0) {
		/* any error is reported once */
		dev->errors = 0;
		/* to preserve notifications about reset */
		rv = (rv == -EPIPE) ? rv : -EIO;
		/* report it */
		goto exit;
	}

	/*
	 * if the buffer is filled we may satisfy the read
	 * else we need to start IO
	 */

	if (dev->bulk_in_filled) {
		/* we had read data */

		size_t available = dev->bulk_in_filled - dev->bulk_in_copied;
//		  size_t chunk = min(available, count);	 /* compute chunk later */
		size_t chunk;

		if (!available) {
			/*
			 * all data has been used
			 * actual IO needs to be done
			 */
			/*
			 * it seems that requests for less than dev->bulk_in_size
			 *  are not accepted
			 */
			rv = skel_do_read_io(dev, dev->bulk_in_size);
			if (rv < 0)
				goto exit;
			else
				goto retry;
		}

		/*
		 * data is available - chunk tells us how much shall be copied
		 */

		/*
		 * Condition dev->bulk_in_copied > 0 maybe will never happen. In case,
		 * signal the event and copy using the original procedure, i.e., copy
		 * first two bytes also
		 */

		if (dev->bulk_in_copied) {
			chunk = min(available, count);
			memcpy(buffer, dev->bulk_in_buffer + dev->bulk_in_copied, chunk);
			rv = chunk;
			dev->bulk_in_copied += chunk;

			/* copy discarding first two bytes that contain ftdi chip status */

		} else {
			/* account for two bytes to be discarded */
			chunk = min(available, count + 2);
			if (chunk < 2) {
				dev_err(&dev->udev->dev, "BAD READ - chunk: %zu\n", chunk);
				rv = -EIO;
				goto exit;
			}

			memcpy(buffer, dev->bulk_in_buffer + 2, chunk - 2);
			rv = chunk;
			dev->bulk_in_copied += chunk;
		}

		/*
		 * if we are asked for more than we have,
		 * we start IO but don't wait
		 *
		 * No, no read ahead allowed; if the case, more data will be
		 * asked for by the lpvo_usb_gpib layer.
		 */
//		  if (available < count)
//			  skel_do_read_io(dev, dev->bulk_in_size);
	} else {
		/* no data in the buffer */
		rv = skel_do_read_io(dev, dev->bulk_in_size);
		if (rv < 0)
			goto exit;
		else
			goto retry;
	}
exit:
	mutex_unlock(&dev->io_mutex);
	if (rv == 2)
		goto restart;	/* ftdi chip returns two status bytes after a latency anyhow */

	if (rv > 0)
		return rv - 2;	/* account for 2 discarded bytes in a valid buffer */
	return rv;
}

/*
 * write functions
 */

static void skel_write_bulk_callback(struct urb *urb)
{
	struct usb_skel *dev;
	unsigned long flags;

	dev = urb->context;

	/* sync/async unlink faults aren't errors */
	if (urb->status) {
		if (!(urb->status == -ENOENT ||
		      urb->status == -ECONNRESET ||
		      urb->status == -ESHUTDOWN))
			dev_err(&dev->interface->dev,
				"nonzero write bulk status received: %d\n", urb->status);

		spin_lock_irqsave(&dev->err_lock, flags);
		dev->errors = urb->status;
		spin_unlock_irqrestore(&dev->err_lock, flags);
	}

	/* free up our allocated buffer */
	usb_free_coherent(urb->dev, urb->transfer_buffer_length,
			  urb->transfer_buffer, urb->transfer_dma);
	up(&dev->limit_sem);
}

/*
 * skel_do_write() - write operations from lpvo_usb_gpib
 */

static ssize_t skel_do_write(struct usb_skel *dev, const char *buffer, size_t count)
{
	int retval = 0;
	struct urb *urb = NULL;
	char *buf = NULL;
	size_t writesize = min_t(size_t, count, (size_t)MAX_TRANSFER);

	/* verify that we actually have some data to write */
	if (count == 0)
		goto exit;

	/*
	 * limit the number of URBs in flight to stop a user from using up all
	 * RAM
	 */
	/* Only one URB is used, because we can't have a pending write() and go on */

//	  if (!(file->f_flags & O_NONBLOCK)) {	/* no NONBLOCK provided */
	if (down_interruptible(&dev->limit_sem)) {
		retval = -ERESTARTSYS;
		goto exit;
	}
//	  } else {
//		  if (down_trylock(&dev->limit_sem)) {
//			  retval = -EAGAIN;
//			  goto exit;
//		  }
//	  }

	spin_lock_irq(&dev->err_lock);
	retval = dev->errors;
	if (retval < 0) {
		/* any error is reported once */
		dev->errors = 0;
		/* to preserve notifications about reset */
		retval = (retval == -EPIPE) ? retval : -EIO;
	}
	spin_unlock_irq(&dev->err_lock);
	if (retval < 0)
		goto error;

	/* create a urb, and a buffer for it, and copy the data to the urb */
	urb = usb_alloc_urb(0, GFP_KERNEL);
	if (!urb) {
		retval = -ENOMEM;
		goto error;
	}

	buf = usb_alloc_coherent(dev->udev, writesize, GFP_KERNEL,
				 &urb->transfer_dma);
	if (!buf) {
		retval = -ENOMEM;
		goto error;
	}

	memcpy(buf, buffer, count);

	/* this lock makes sure we don't submit URBs to gone devices */
	mutex_lock(&dev->io_mutex);
	if (!dev->interface) {		      /* disconnect() was called */
		mutex_unlock(&dev->io_mutex);
		retval = -ENODEV;
		goto error;
	}

	/* initialize the urb properly */
	usb_fill_bulk_urb(urb, dev->udev,
			  usb_sndbulkpipe(dev->udev, dev->bulk_out_endpoint_addr),
			  buf, writesize, skel_write_bulk_callback, dev);
	urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;
	usb_anchor_urb(urb, &dev->submitted);

	/* send the data out the bulk port */
	retval = usb_submit_urb(urb, GFP_KERNEL);
	mutex_unlock(&dev->io_mutex);
	if (retval) {
		dev_err(&dev->interface->dev, "failed submitting write urb, error %d\n", retval);
		goto error_unanchor;
	}

	/*
	 * release our reference to this urb, the USB core will eventually free
	 * it entirely
	 */
	usb_free_urb(urb);

	return writesize;

error_unanchor:
	usb_unanchor_urb(urb);
error:
	if (urb) {
		usb_free_coherent(dev->udev, writesize, buf, urb->transfer_dma);
		usb_free_urb(urb);
	}
	up(&dev->limit_sem);

exit:
	return retval;
}

/*
 * services for the user space devices
 */

#if USER_DEVICE	 /* conditional compilation of user space device */

static int skel_flush(struct file *file, fl_owner_t id)
{
	struct usb_skel *dev;
	int res;

	dev = file->private_data;
	if (!dev)
		return -ENODEV;

	/* wait for io to stop */
	mutex_lock(&dev->io_mutex);
	skel_draw_down(dev);

	/* read out errors, leave subsequent opens a clean slate */
	spin_lock_irq(&dev->err_lock);
	res = dev->errors ? (dev->errors == -EPIPE ? -EPIPE : -EIO) : 0;
	dev->errors = 0;
	spin_unlock_irq(&dev->err_lock);

	mutex_unlock(&dev->io_mutex);

	return res;
}

static int skel_open(struct inode *inode, struct file *file)
{
	struct usb_skel *dev;
	struct usb_interface *interface;
	int subminor;
	int retval = 0;

	subminor = iminor(inode);

	interface = usb_find_interface(&skel_driver, subminor);
	if (!interface) {
		pr_err("can't find device for minor %d\n", subminor);
		retval = -ENODEV;
		goto exit;
	}

	dev = usb_get_intfdata(interface);
	if (!dev) {
		retval = -ENODEV;
		goto exit;
	}

	retval = usb_autopm_get_interface(interface);
	if (retval)
		goto exit;

	/* increment our usage count for the device */
	kref_get(&dev->kref);

	/* save our object in the file's private structure */
	file->private_data = dev;

exit:
	return retval;
}

static int skel_release(struct inode *inode, struct file *file)
{
	struct usb_skel *dev;

	dev = file->private_data;
	if (!dev)
		return -ENODEV;

	/* allow the device to be autosuspended */
	mutex_lock(&dev->io_mutex);
	if (dev->interface)
		usb_autopm_put_interface(dev->interface);
	mutex_unlock(&dev->io_mutex);

	/* decrement the count on our device */
	kref_put(&dev->kref, skel_delete);
	return 0;
}

/*
 * user space access to read function
 */

static ssize_t skel_read(struct file *file, char __user *buffer, size_t count,
			 loff_t *ppos)
{
	struct usb_skel *dev;
	char *buf;
	ssize_t rv;

	dev = file->private_data;

	buf = kmalloc(count, GFP_KERNEL);
	if (!buf)
		return -ENOMEM;

	rv = skel_do_read(dev, buf, count);

	if (rv > 0) {
		if (copy_to_user(buffer, buf, rv)) {
			kfree(buf);
			return -EFAULT;
		}
	}
	kfree(buf);
	return rv;
}

/*
 * user space access to write function
 */

static ssize_t skel_write(struct file *file, const char __user *user_buffer,
			  size_t count, loff_t *ppos)
{
	struct usb_skel *dev;
	char *buf;
	ssize_t rv;

	dev = file->private_data;

	buf = kmalloc(count, GFP_KERNEL);
	if (!buf)
		return -ENOMEM;

	if (copy_from_user(buf, user_buffer, count)) {
		kfree(buf);
		return -EFAULT;
	}

	rv = skel_do_write(dev, buf, count);
	kfree(buf);
	return rv;
}
#endif

static const struct file_operations skel_fops = {
	.owner =	THIS_MODULE,
#if USER_DEVICE
	.read =	   skel_read,
	.write =   skel_write,
	.open =	   skel_open,
	.release = skel_release,
	.flush =   skel_flush,
	.llseek =  noop_llseek,
#endif
};

/*
 * usb class driver info in order to get a minor number from the usb core,
 * and to have the device registered with the driver core
 */
#if USER_DEVICE
static struct usb_class_driver skel_class = {
	.name =		       "lpvo_raw%d",
	.fops =		       &skel_fops,
	.minor_base =	     USB_SKEL_MINOR_BASE,
};
#endif

static int skel_probe(struct usb_interface *interface,
		      const struct usb_device_id *id)
{
	struct usb_skel *dev;
	struct usb_endpoint_descriptor *bulk_in, *bulk_out;
	int retval;
	char *device_path;

	mutex_init(&minors_lock);   /* required for handling minor numbers table */

	/* allocate memory for our device state and initialize it */
	dev = kzalloc_obj(*dev);
	if (!dev)
		return -ENOMEM;

	kref_init(&dev->kref);
	sema_init(&dev->limit_sem, WRITES_IN_FLIGHT);
	mutex_init(&dev->io_mutex);
	spin_lock_init(&dev->err_lock);
	init_usb_anchor(&dev->submitted);
	init_waitqueue_head(&dev->bulk_in_wait);

	dev->udev = usb_get_dev(interface_to_usbdev(interface));
	dev->interface = interface;

	/* set up the endpoint information */
	/* use only the first bulk-in and bulk-out endpoints */
	retval = usb_find_common_endpoints(interface->cur_altsetting,
					   &bulk_in, &bulk_out, NULL, NULL);
	if (retval) {
		dev_err(&interface->dev,
			"Could not find both bulk-in and bulk-out endpoints\n");
		goto error;
	}

	dev->bulk_in_size = usb_endpoint_maxp(bulk_in);
	dev->bulk_in_endpoint_addr = bulk_in->bEndpointAddress;
	dev->bulk_in_buffer = kmalloc(dev->bulk_in_size, GFP_KERNEL);
	if (!dev->bulk_in_buffer) {
		retval = -ENOMEM;
		goto error;
	}
	dev->bulk_in_urb = usb_alloc_urb(0, GFP_KERNEL);
	if (!dev->bulk_in_urb) {
		retval = -ENOMEM;
		goto error;
	}

	dev->bulk_out_endpoint_addr = bulk_out->bEndpointAddress;

	/* save our data pointer in this interface device */
	usb_set_intfdata(interface, dev);

	/* let the world know */

	device_path = kobject_get_path(&dev->udev->dev.kobj, GFP_KERNEL);
	dev_dbg(&interface->dev, "New lpvo_usb_device -> bus: %d  dev: %d  path: %s\n",
		dev->udev->bus->busnum, dev->udev->devnum, device_path);
	kfree(device_path);

#if USER_DEVICE
	/* we can register the device now, as it is ready */
	retval = usb_register_dev(interface, &skel_class);
	if (retval) {
		/* something prevented us from registering this driver */
		dev_err(&interface->dev,
			"Not able to get a minor for this device.\n");
		usb_set_intfdata(interface, NULL);
		goto error;
	}
#endif

	write_latency_timer(dev->udev);	    /* adjust the latency timer */

	usb_gpib_init_module(interface);    /* last, init the lpvo for this minor */

	return 0;

error:
	/* this frees allocated memory */
	kref_put(&dev->kref, skel_delete);

	return retval;
}

static void skel_disconnect(struct usb_interface *interface)
{
	struct usb_skel *dev;
	int minor = interface->minor;

	usb_gpib_exit_module(minor);	  /* first, disactivate the lpvo */

	dev = usb_get_intfdata(interface);
	usb_set_intfdata(interface, NULL);

#if USER_DEVICE
	/* give back our minor */
	usb_deregister_dev(interface, &skel_class);
#endif

	/* prevent more I/O from starting */
	mutex_lock(&dev->io_mutex);
	dev->interface = NULL;
	mutex_unlock(&dev->io_mutex);

	usb_kill_anchored_urbs(&dev->submitted);

	/* decrement our usage count */
	kref_put(&dev->kref, skel_delete);
}

static void skel_draw_down(struct usb_skel *dev)
{
	int time;

	time = usb_wait_anchor_empty_timeout(&dev->submitted, 1000);
	if (!time)
		usb_kill_anchored_urbs(&dev->submitted);
	usb_kill_urb(dev->bulk_in_urb);
}

static int skel_suspend(struct usb_interface *intf, pm_message_t message)
{
	struct usb_skel *dev = usb_get_intfdata(intf);

	if (!dev)
		return 0;
	skel_draw_down(dev);
	return 0;
}

static int skel_resume(struct usb_interface *intf)
{
	return 0;
}

static int skel_pre_reset(struct usb_interface *intf)
{
	struct usb_skel *dev = usb_get_intfdata(intf);

	mutex_lock(&dev->io_mutex);
	skel_draw_down(dev);

	return 0;
}

static int skel_post_reset(struct usb_interface *intf)
{
	struct usb_skel *dev = usb_get_intfdata(intf);

	/* we are sure no URBs are active - no locking needed */
	dev->errors = -EPIPE;
	mutex_unlock(&dev->io_mutex);

	return 0;
}

static struct usb_driver skel_driver = {
	.name =			NAME,
	.probe =		skel_probe,
	.disconnect =		skel_disconnect,
	.suspend =		skel_suspend,
	.resume =		skel_resume,
	.pre_reset =		skel_pre_reset,
	.post_reset =		skel_post_reset,
	.id_table =		skel_table,
	.supports_autosuspend = 1,
};

module_usb_driver(skel_driver);