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

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 <[email protected]>
---
 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 <[email protected]>