Re: [PATCH net v2] net: macb: Shuffle the tx ring before enabling tx

[Simon Horman <horms@kernel.org>]

On Sat, Mar 07, 2026 at 03:08:54PM +0800, Kevin Hao wrote:
> 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: bf9cf80cab81 ("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
> ---
> Changes in v2:
> - Resolves the issue of incomplete copying of the tx descriptor, as identified by the AI [1].
> 
> - Resolves warnings for lines exceeding 80 columns.
> 
> - Link to v1: https://lore.kernel.org/r/20260305-zynqmp-v1-1-5de72254d56b@gmail.com
> 
> [1] https://netdev-ai.bots.linux.dev/ai-review.html?id=44318d8b-d8c3-42c8-8884-238421f708c5

Reviewed-by: Simon Horman <horms@kernel.org>