Re: [PATCH 2/4] dmaengine: loongson: loongson2-apb-cmc-dma: Fix residue for queued descriptors

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

On Mon, Oct 26, 2026 at 12:31:21PM +0800, Binbin Zhou wrote:
> tx_status() computes residue for both the active and the queued
> descriptors through loongson2_cmc_dma_desc_residue(), which reads the
> hardware CNDTR register. For a descriptor that has not started yet that
> register does not describe its transfer, so clients can observe a
> residue that does not match the amount of data still to be moved.

why this happen, suppose write CNDTR at loongson2_cmc_dma_start_transfer()
when issue pending.

Maybe there are raise condition, between send to issue list and start_transfer()

suppose loongson2_cmc_dma_tx_status() should not find vdesc by cookie id
if issue pending have not complete yet.

Maybe move loongson2_cmc_dma_start_transfer() into lchan->vchan.lock, you
will not see this problem.

Frank
>
> Report residue from the descriptor length for queued transfers, and keep
> the hardware-based calculation only for the active descriptor.
>
> Cc: stable@vger.kernel.org
> Fixes: 1c0028e725f1 ("dmaengine: loongson: New driver for the Loongson Multi-Channel DMA controller")
> Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
> ---
>  drivers/dma/loongson/loongson2-apb-cmc-dma.c | 23 ++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/dma/loongson/loongson2-apb-cmc-dma.c b/drivers/dma/loongson/loongson2-apb-cmc-dma.c
> index f090fbe0b8a1..8b8d530d60c3 100644
> --- a/drivers/dma/loongson/loongson2-apb-cmc-dma.c
> +++ b/drivers/dma/loongson/loongson2-apb-cmc-dma.c
> @@ -472,6 +472,17 @@ loongson2_cmc_dma_prep_dma_cyclic(struct dma_chan *chan, dma_addr_t buf_addr, si
>  	return vchan_tx_prep(&lchan->vchan, &desc->vdesc, flags);
>  }
>
> +static size_t loongson2_cmc_dma_desc_total_len(struct loongson2_cmc_dma_desc *desc)
> +{
> +	size_t total = 0;
> +	u32 i;
> +
> +	for (i = 0; i < desc->num_sgs; i++)
> +		total += desc->sg_req[i].len;
> +
> +	return total;
> +}
> +
>  static size_t loongson2_cmc_dma_desc_residue(struct loongson2_cmc_dma_chan *lchan,
>  					     struct loongson2_cmc_dma_desc *desc, u32 next_sg)
>  {
> @@ -518,12 +529,16 @@ static enum dma_status loongson2_cmc_dma_tx_status(struct dma_chan *chan, dma_co
>
>  	scoped_guard(spinlock_irqsave, &lchan->vchan.lock) {
>  		vdesc = vchan_find_desc(&lchan->vchan, cookie);
> -		if (lchan->desc && cookie == lchan->desc->vdesc.tx.cookie)
> +		if (lchan->desc && cookie == lchan->desc->vdesc.tx.cookie) {
> +			/* Active descriptor: compute residue from hardware progress */
>  			state->residue = loongson2_cmc_dma_desc_residue(lchan, lchan->desc,
>  									lchan->next_sg);
> -		else if (vdesc)
> -			state->residue = loongson2_cmc_dma_desc_residue(lchan,
> -									to_lmdma_desc(vdesc), 0);
> +		} else if (vdesc) {
> +			/* Queued descriptor not yet started: residue is the total length */
> +			struct loongson2_cmc_dma_desc *ldesc = to_lmdma_desc(vdesc);
> +
> +			state->residue = loongson2_cmc_dma_desc_total_len(ldesc);
> +		}
>  	}
>
>  	return status;
> --
> 2.52.0
>