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Quanyang observed that when using an NFS rootfs on an AMD ZynqMp board,
the rootfs may take an extended time to recover after a suspend.
Upon investigation, it was determined that the issue originates from a
problem in the macb driver.
According to the Zynq UltraScale TRM [1], when transmit is disabled,
the transmit buffer queue pointer resets to point to the address
specified by the transmit buffer queue base address register.
In the current implementation, the code merely resets `queue->tx_head`
and `queue->tx_tail` to '0'. This approach presents several issues:
- Packets already queued in the tx ring are silently lost,
leading to memory leaks since the associated skbs cannot be released.
- Concurrent write access to `queue->tx_head` and `queue->tx_tail` may
occur from `macb_tx_poll()` or `macb_start_xmit()` when these values
are reset to '0'.
- The transmission may become stuck on a packet that has already been sent
out, with its 'TX_USED' bit set, but has not yet been processed. However,
due to the manipulation of 'queue->tx_head' and 'queue->tx_tail',
`macb_tx_poll()` incorrectly assumes there are no packets to handle
because `queue->tx_head == queue->tx_tail`. This issue is only resolved
when a new packet is placed at this position. This is the root cause of
the prolonged recovery time observed for the NFS root filesystem.
To resolve this issue, shuffle the tx ring and tx skb array so that
the first unsent packet is positioned at the start of the tx ring.
Additionally, ensure that updates to `queue->tx_head` and
`queue->tx_tail` are properly protected with the appropriate lock.
[1] https://docs.amd.com/v/u/en-US/ug1085-zynq-ultrascale-trm
Fixes: bf9cf80cab ("net: macb: Fix tx/rx malfunction after phy link down and up")
Reported-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Cc: stable@vger.kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260307-zynqmp-v2-1-6ef98a70e1d0@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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