Re: [PATCH] dmaengine: xilinx_dma: Fix hardware buffer descriptor reuse order

[Frank Li <Frank.li@oss.nxp.com>]

On Mon, Aug 17, 2026 at 11:23:55AM +0200, Alex Bereza wrote:
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>
> xilinx_dma_alloc_chan_resources() builds a static ring of hardware
> buffer descriptors once and the driver uses this ring throughout the
> lifetime of a channel. This requires the allocation order of hardware
> buffer descriptors from chan->free_seg_list to stay in sync with the
> hardware buffer descriptor ring built at channel allocation time by
> returning oldest descriptors to chan->free_seg_list first.
>
> When chan->pending_list is not empty e.g. during
> xilinx_dma_terminate_all() the chan->free_seg_list and the order of the
> static hardware buffer descriptor ring get out of sync. Descriptors age
> in this order: pending -> active -> done. So freeing pending_list first
> returns the newest buffer descriptors to the chan->free_seg_list first
> and thus breaks the order required by the static hardware buffer
> descriptor ring. Then when the channel is reused, after a wrap around of
> the free_seg_list the DMA will find a hardware buffer descriptor with a
> length field that is still zeroed and stop with something like this:
>
>   xilinx-vdma 86000000.dma: Channel 000000003a21d7b8 has errors 10, cdr 6de4c000 tdr 6de4c000
>
> After this no more descriptors are completed and a consumer potentially
> blocks and waits forever. The only way to get out of this error state is
> to rebuild the static hardware buffer descriptor ring and the
> free_seg_list by releasing and re-acquiring the channel.
>
> Fix the order in which hardware buffer descriptors are returned to
> free_seg_list to ensure the mentioned requirement holds.
>
> Fixes: 23059408b6a3 ("dmaengine: xilinx_dma: Fix race condition in the driver for multiple descriptor scenario")
> Signed-off-by: Alex Bereza <alex@bereza.email>
> ---

Reviewed-by: Frank Li <Frank.Li@nxp.com>

>  drivers/dma/xilinx/xilinx_dma.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
> index bef2b031dba1..0817b74f7450 100644
> --- a/drivers/dma/xilinx/xilinx_dma.c
> +++ b/drivers/dma/xilinx/xilinx_dma.c
> @@ -920,9 +920,9 @@ static void xilinx_dma_free_descriptors(struct xilinx_dma_chan *chan)
>
>         spin_lock_irqsave(&chan->lock, flags);
>
> -       xilinx_dma_free_desc_list(chan, &chan->pending_list);
>         xilinx_dma_free_desc_list(chan, &chan->done_list);
>         xilinx_dma_free_desc_list(chan, &chan->active_list);
> +       xilinx_dma_free_desc_list(chan, &chan->pending_list);
>
>         spin_unlock_irqrestore(&chan->lock, flags);
>  }
>
> ---
> base-commit: 0d995da5fb97e8c312834575604d4423eb6225b7
> change-id: 20260817-fix-hw-buf-desc-reuse-b730e9e02185
>
> Best regards,
> --
> Alex Bereza <alex@bereza.email>
>