Re: [PATCH net v2] net: sparx5: fix tx DCB index drift in fdma xmit

[Simon Horman <horms@kernel.org>]

On Wed, Oct 21, 2026 at 04:04:59PM +0200, Daniel Machon wrote:
> sparx5_fdma_xmit() advances dcb_index before checking that the FDMA is
> done with the DCB, and does not undo the advance when the check fails.
> Before commit f4aa7e361ae2 ("net: sparx5: use FDMA library for adding
> DCB's in the tx path") the next DCB was always the one after last_dcb.
> Since then dcb_index drifts one DCB further away from last_dcb on every
> failed transmit.
> 
> While the ring is full, this drops frames even though completed DCBs
> sit behind dcb_index. After a run of failures, dcb_index can also land
> on last_dcb itself. Re-adding last_dcb loses the frame, because the
> FDMA has already passed the tail of the chain, and that DCB is never
> completed again.
> 
> Fix this by making dcb_index the next DCB to use, and advancing it only
> after the DCB has been added, as the rx path does. dcb_index and
> last_dcb now only move together, so dcb_index always points at the DCB
> after last_dcb.
> 
> The issue was found by an AI code review agent. It was reproduced on a
> sparx5 PCB135 board with a modified kernel that forces 63 consecutive
> transmit failures. Afterwards, about 1 in 64 frames is lost for good.
> With the fix, the same test loses no frames.
> 
> Fixes: f4aa7e361ae2 ("net: sparx5: use FDMA library for adding DCB's in the tx path")
> Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
> ---
> Changes in v2:
> - Advance dcb_index only after the DCB has been added, instead of
>   rejecting last_dcb. This fixes the drift itself (AI review).
> - Reword the subject and commit message to match.
> - Link to v1: https://lore.kernel.org/r/20261020-sparx5-fdma-last-dcb-v1-1-fb9a956196dd@microchip.com

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