Re: [PATCH v3 1/2] dmaengine: sun6i: fix non-atomic read of DMA position registers

On Mon, Aug 17, 2026 at 03:51:22PM +0200, Christian Lugnberg wrote:
> sun6i_get_chan_size() reads DMA_CHAN_LLI_ADDR and DMA_CHAN_CUR_CNT in two
> separate readl() calls with no synchronisation between them:
>
>     pos   = readl(pchan->base + DMA_CHAN_LLI_ADDR);
>     bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);
>
> DMA_CHAN_LLI_ADDR holds the physical address of the *next* descriptor the
> engine will load once the current one completes. DMA_CHAN_CUR_CNT holds the
> remaining byte count for the *current* descriptor. If the DMA engine
> advances to the next LLI entry between the two reads, pos becomes stale: it
> still points to what was the next descriptor at the time of the first read,
> but that descriptor is now the current one and CUR_CNT reflects its initial
> (full) byte count. The subsequent virtual-chain walk starts one entry too
> early and accumulates an extra full period's worth of bytes into the
> residue estimate.
>
> Fix this by re-reading DMA_CHAN_LLI_ADDR after DMA_CHAN_CUR_CNT and
> retrying if the value changed. This double-read pattern guarantees that
> both registers were sampled during the same descriptor interval. The cost
> is at most one extra readl() pair per call in the racy case, which occurs
> only at descriptor boundaries (~every 2 ms) and is negligible.
>
> Fixes: a90e173f3faf ("dmaengine: sun6i: Add cyclic capability")
> Cc: [email protected]
> Assisted-by: Claude:claude-sonnet-4-6
> Signed-off-by: Christian Lugnberg <[email protected]>
> ---

Reviewed-by: Frank Li <[email protected]>

>  drivers/dma/sun6i-dma.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma/sun6i-dma.c b/drivers/dma/sun6i-dma.c
> index f47a326dd7ff..04fe1f5042e9 100644
> --- a/drivers/dma/sun6i-dma.c
> +++ b/drivers/dma/sun6i-dma.c
> @@ -354,8 +354,10 @@ static size_t sun6i_get_chan_size(struct sun6i_pchan *pchan)
>  	size_t bytes;
>  	dma_addr_t pos;
>
> -	pos = readl(pchan->base + DMA_CHAN_LLI_ADDR);
> -	bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);
> +	do {
> +		pos = readl(pchan->base + DMA_CHAN_LLI_ADDR);
> +		bytes = readl(pchan->base + DMA_CHAN_CUR_CNT);
> +	} while (pos != readl(pchan->base + DMA_CHAN_LLI_ADDR));
>
>  	if (pos == LLI_LAST_ITEM)
>  		return bytes;
> --
> 2.54.0 (Apple Git-156)
>