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

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Message-ID: <aSVmVqRDhp3TRhKz@yoseli-yocto.yoseli.org>
Date: Tue, 25 Nov 2025 09:18:30 +0100
From: Jean-Michel Hautbois <jeanmichel.hautbois@...eli.org>
To: Frank Li <Frank.li@....com>
Cc: Vinod Koul <vkoul@...nel.org>, Greg Ungerer <gerg@...ux-m68k.org>,
	imx@...ts.linux.dev, dmaengine@...r.kernel.org,
	linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] dma: fsl-edma: Add write barrier after TCD
 descriptor fill

On Mon, Nov 24, 2025 at 10:57:49AM -0500, Frank Li wrote:
> 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@...eli.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);
> 

Thanks for you detailed explanation, I will remove this commit from v2
:-).

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