These days, it's fairly common to see panels that have touchscreens
attached to them. The panel and the touchscreen can somewhat be
thought of as totally separate devices and, historically, this is how
Linux has treated them. However, treating them as separate isn't
necessarily the best way to model the two devices, it was just that
there was no better way. Specifically, there is little practical
reason to have the touchscreen powered on when the panel is turned
off, but if we model the devices separately we have no way to keep the
two devices' power states in sync with each other.
The issue described above makes it sound as if the problem here is
just about efficiency. We're wasting power keeping the touchscreen
powered up when the screen is off. While that's true, the problem can
go deeper. Specifically, hardware designers see that there's no reason
to have the touchscreen on while the screen is off and then build
hardware assuming that software would never turn the touchscreen on
while the screen is off.
In the very simplest case of hardware designs like this, the
touchscreen and the panel share some power rails. In most cases, this
turns out not to be terrible and is, again, just a little less
efficient. Specifically if we tell Linux that the touchscreen and the
panel are using the same rails then Linux will keep the rails on when
_either_ device is turned on. That ends to work OK-ish, but now if you
turn the panel off not only will the touchscreen remain powered, but
the power rails for the panel itself won't be switched off, burning
extra power.
The above two inefficiencies are _extra_ minor when you consider the
fact that laptops rarely spend much time with the screen off. The main
use case would be when an external screen (and presumably a power
supply) is attached.
Unfortunately, it gets worse from here. On sc7180-trogdor-homestar,
for instance, the display's TCON (timing controller) sometimes crashes
if you don't power cycle it whenever you stop and restart the video
stream (like during a modeset). The touchscreen keeping the power
rails on causes real problems. One proposal in the homestar timeframe
was to move the touchscreen to an always-on rail, dedicating the main
power rail to the panel. That caused _different_ problems as talked
about in commit 557e05fa9f ("HID: i2c-hid: goodix: Stop tying the
reset line to the regulator"). The end result of all of this was to
add an extra regulator to the board, increasing cost.
Recently, Cong Yang posted a patch [1] where things are even worse.
The panel and touch controller on that system seem even more
intimately tied together and really can't be thought of separately.
To address this issue, let's start allowing devices to register
themselves as "panel followers". These devices will get called after a
panel has been powered on and before a panel is powered off. This
makes the panel the primary device in charge of the power state, which
matches how userspace uses it.
The panel follower API should be fairly straightforward to use. The
current code assumes that panel followers are using device tree and
have a "panel" property pointing to the panel to follow. More
flexibility and non-DT implementations could be added as needed.
Right now, panel followers can follow the prepare/unprepare functions.
There could be arguments made that, instead, they should follow
enable/disable. I've chosen prepare/unprepare for now since those
functions are guaranteed to power up/power down the panel and it seems
better to start the process earlier.
A bit of explaining about why this is a roll-your-own API instead of
using something more standard:
1. In standard APIs in Linux, parent devices are automatically powered
on when a child needs power. Applying that here, it would mean that
we'd force the panel on any time someone was listening to the
touchscreen. That, unfortunately, would have broken homestar's need
(if we hadn't changed the hardware, as per above) where the panel
absolutely needs to be able to power cycle itself. While one could
argue that homestar is broken hardware and we shouldn't have the
API do backflips for it, _officially_ the eDP timing guidelines
agree with homestar's needs and the panel power sequencing diagrams
show power going off. It's nice to be able to support this.
2. We could, conceibably, try to add a new flag to device_link causing
the parent to be in charge of power. Then we could at least use
normal pm_runtime APIs. This sounds great, except that we run into
problems with initial probe. As talked about in the later patch
("HID: i2c-hid: Support being a panel follower") the initial power
on of a panel follower might need to do things (like add
sub-devices) that aren't allowed in a runtime_resume function.
The above complexities explain why this API isn't using common
functions. That being said, this patch is very small and
self-contained, so if someone was later able to adapt it to using more
common APIs while solving the above issues then that could happen in
the future.
[1] https://lore.kernel.org/r/20230519032316.3464732-1-yangcong5@huaqin.corp-partner.google.com
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230727101636.v4.3.Icd5f96342d2242051c754364f4bee13ef2b986d4@changeid
In a whole pile of panel drivers, we have code to make the
prepare/unprepare/enable/disable callbacks behave as no-ops if they've
already been called. It's silly to have this code duplicated
everywhere. Add it to the core instead so that we can eventually
delete it from all the drivers. Note: to get some idea of the
duplicated code, try:
git grep 'if.*>prepared' -- drivers/gpu/drm/panel
git grep 'if.*>enabled' -- drivers/gpu/drm/panel
NOTE: arguably, the right thing to do here is actually to skip this
patch and simply remove all the extra checks from the individual
drivers. Perhaps the checks were needed at some point in time in the
past but maybe they no longer are? Certainly as we continue
transitioning over to "panel_bridge" then we expect there to be much
less variety in how these calls are made. When we're called as part of
the bridge chain, things should be pretty simple. In fact, there was
some discussion in the past about these checks [1], including a
discussion about whether the checks were needed and whether the calls
ought to be refcounted. At the time, I decided not to mess with it
because it felt too risky.
Looking closer at it now, I'm fairly certain that nothing in the
existing codebase is expecting these calls to be refcounted. The only
real question is whether someone is already doing something to ensure
prepare()/unprepare() match and enabled()/disable() match. I would say
that, even if there is something else ensuring that things match,
there's enough complexity that adding an extra bool and an extra
double-check here is a good idea. Let's add a drm_warn() to let people
know that it's considered a minor error to take advantage of
drm_panel's double-checking but we'll still make things work fine.
We'll also add an entry to the official DRM todo list to remove the
now pointless check from the panels after this patch lands and,
eventually, fixup anyone who is triggering the new warning.
[1] https://lore.kernel.org/r/20210416153909.v4.27.I502f2a92ddd36c3d28d014dd75e170c2d405a0a5@changeid
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230727101636.v4.2.I59b417d4c29151cc2eff053369ec4822b606f375@changeid
As talked about in the patch ("drm/panel: Add a way for other devices
to follow panel state"), touchscreens that are connected to panels are
generally expected to be power sequenced together with the panel
they're attached to. Today, nothing provides information allowing you
to find out that a touchscreen is connected to a panel. Let's add a
phandle for this.
The proerty is added to the generic touchscreen bindings and then
enabled in the bindings for the i2c-hid backed devices. This can and
should be added for other touchscreens in the future, but for now
let's start small.
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Benjamin Tissoires <bentiss@kernel.org>
Acked-by: Benjamin Tissoires <bentiss@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230727101636.v4.1.Id68e30343bb1e11470582a9078b086176cfec46b@changeid
A very basic debugging rule when a device is connected for the first
time is to access a read-only register which contains known data in
order to ensure the communication protocol is properly working. This
driver lacked any read helper which is often a critical piece for
speeding-up bring-ups.
Add a read helper and use it to verify the communication with the panel
is working as soon as possible in order to inform the user early if this
is not the case.
As this panel may work with no MISO line, the check is discarded in this
case. Upon error, we do not fail probing but just warn the user, in case
the DT description would be lacking the Rx bus width (which is likely on
old descriptions) in order to avoid breaking existing devices.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Tested-by: Sebastian Reichel <sre@kernel.org> # no MISO line
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714013756.1546769-20-sre@kernel.org
This panel from Emerging Display Technologies Corporation features an
ST7789V2 LCD controller panel inside which is almost identical to what
the Sitronix panel driver supports.
In practice, the module physical size is specific, and experiments show
that the display will malfunction if any of the following situation
occurs:
* Pixel clock is above 3MHz
* Pixel clock is not inverted
I could not properly identify the reasons behind these failures, scope
captures show valid input signals.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714013756.1546769-19-sre@kernel.org
The newly added driver only builds when DRM_DISPLAY_DP_HELPER is enabled:
x86_64-linux-ld: drivers/gpu/drm/panel/panel-visionox-r66451.o: in function `visionox_r66451_enable':
panel-visionox-r66451.c:(.text+0x105): undefined reference to `drm_dsc_pps_payload_pack'
Select both CONFIG_DRM_DISPLAY_DP_HELPER and CONFIG_DRM_DISPLAY_HELPER to
ensure the helper function is always available.
Fixes: a6dfab2738 ("drm/panel: Add driver for Visionox r66451 panel")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230719130940.659837-1-arnd@kernel.org
When building with W=1, the following warning occurs.
drivers/gpu/drm/bridge/analogix/analogix-anx78xx.c:48:17: warning: ‘anx781x_i2c_addresses’ defined but not used [-Wunused-const-variable=] static const u8 anx781x_i2c_addresses[] = {
^~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/bridge/analogix/analogix-anx78xx.c:40:17: warning: ‘anx7808_i2c_addresses’ defined but not used [-Wunused-const-variable=] static const u8 anx7808_i2c_addresses[] = {
When CONFIG_IO is disabled, above two variables are not used,
since the place where it is used is inclueded in the macro
CONFIG_OF.
Even for drivers that do not depend on CONFIG_OF, it's almost
always better to leave out the of_match_ptr(), since the only
thing it can possibly do is to save a few bytes of .text if a
driver can be used both with and without it. Hence we remove
all of_match_ptr() used in other places.
Fixes: 0647e7dd3f ("drm/bridge: Add Analogix anx78xx support")
Signed-off-by: Zhu Wang <wangzhu9@huawei.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230731021345.219588-1-wangzhu9@huawei.com
