Re: [PATCH 1/7] dma: fsl-edma: Add write barrier after TCD descriptor fill

Frank Li
2025-11-24T15:58:01Z

On Mon, Nov 24, 2025 at 01:50:22PM +0100, Jean-Michel Hautbois wrote:
> Add dma_wmb() barrier after filling TCD descriptors to ensure all
> descriptor writes are visible to the DMA engine before starting
> transfers. This prevents potential race conditions where the DMA
> hardware might read partially written descriptors.
>
> Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>
> ---
> drivers/dma/fsl-edma-common.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/dma/fsl-edma-common.c b/drivers/dma/fsl-edma-common.c
> index 4976d7dde08090d16277af4b9f784b9745485320..db36a6aafc910364d75ce6c5d334fd19d2120b6b 100644
> --- a/drivers/dma/fsl-edma-common.c
> +++ b/drivers/dma/fsl-edma-common.c
> @@ -553,6 +553,9 @@ void fsl_edma_fill_tcd(struct fsl_edma_chan *fsl_chan,
> fsl_edma_set_tcd_to_le(fsl_chan, tcd, csr, csr);
>
> trace_edma_fill_tcd(fsl_chan, tcd);
> +
> + /* Ensure descriptor writes are visible to DMA engine */
> + dma_wmb();

This is not necessary because there are writel() in
fsl_edma_issue_pending(), which will do memory barrier in writel().

currently, edma use vchan, descriptior have not dymantically to appending
to running queue. so writel() in fsl_edma_issue_pending() is enough.

Even though edma will support append to running queue in future, dma_wmd()
should be just before update csr.

dma_wmb(); // just before indicate TCD is ready to use.
fsl_edma_set_tcd_to_le(fsl_chan, tcd, csr, csr);

Frank
> }
>
> static struct fsl_edma_desc *fsl_edma_alloc_desc(struct fsl_edma_chan *fsl_chan,
>
> --
> 2.39.5
>