Re: [PATCH v6 05/10] dmaengine: dw: calculate residue in the status getter

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

On Tue, Nov 03, 2026 at 03:26:07PM -0700, James Hilliard wrote:
> From: Serge Semin <fancer.lancer@gmail.com>
>
> Calculate hardware-LLP residue while looking up the requested cookie under
> the channel lock. The scanner and the status getter currently take that
> lock separately, allowing a descriptor to start between the scanner's
> residue update and the status query.
>
> Have dwc_get_hard_llp_desc_residue() return a local residue value from a
> fresh LLP snapshot. Keep the persistent residue field for software-LLP
> block accounting, subtracting the current block's progress when reporting
> status. The descriptor scanner continues to advance software-LLP blocks
> and complete cookies from RAW.XFER, including for polled transfers.
>
> Fixes: 3bfb1d20b547 ("dmaengine: Driver for the Synopsys DesignWare DMA controller")
> Cc: stable@vger.kernel.org
> Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
> [james: retain polled completion and software-LLP progress]
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
> ---

Reviewed-by: Frank Li <Frank.Li@nxp.com>

>  drivers/dma/dw/core.c | 64 +++++++++++++++------------------------------------
>  1 file changed, 19 insertions(+), 45 deletions(-)
>
> diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c
> index 5a1caca7ad68..d9127337c849 100644
> --- a/drivers/dma/dw/core.c
> +++ b/drivers/dma/dw/core.c
> @@ -267,41 +267,34 @@ static inline u32 dwc_get_sent(struct dw_dma_chan *dwc)
>  	return dw->block2bytes(dwc, ctlhi, ctllo >> 4 & 7);
>  }
>
> -static bool dwc_get_hard_llp_desc_residue(struct dw_dma_chan *dwc,
> -				       struct dw_desc *desc, dma_addr_t llp)
> +static u32 dwc_get_hard_llp_desc_residue(struct dw_dma_chan *dwc, struct dw_desc *desc)
>  {
> +	u32 residue = desc->total_len;
>  	struct dw_desc *child;
> +	dma_addr_t llp;
>
> -	/* Initial residue value */
> -	desc->residue = desc->total_len;
> +	llp = channel_readl(dwc, LLP);
>
> -	/* Check first descriptors addr */
> +	/* The first descriptor may still be waiting to be fetched. */
>  	if (desc->txd.phys == DWC_LLP_LOC(llp))
> -		return true;
> +		return residue;
>
> -	/* Check first descriptors llp */
> -	if (lli_read(desc, llp) == llp) {
> -		/* This one is currently in progress */
> -		desc->residue -= dwc_get_sent(dwc);
> -		return true;
> -	}
> +	if (lli_read(desc, llp) == llp)
> +		return residue - dwc_get_sent(dwc);
>
> -	desc->residue -= desc->len;
> +	residue -= desc->len;
>  	list_for_each_entry(child, &desc->tx_list, desc_node) {
> -		if (lli_read(child, llp) == llp) {
> -			/* Currently in progress */
> -			desc->residue -= dwc_get_sent(dwc);
> -			return true;
> -		}
> -		desc->residue -= child->len;
> +		if (lli_read(child, llp) == llp)
> +			return residue - dwc_get_sent(dwc);
> +
> +		residue -= child->len;
>  	}
>
> -	return false;
> +	return residue;
>  }
>
>  static void dwc_scan_descriptors(struct dw_dma *dw, struct dw_dma_chan *dwc)
>  {
> -	dma_addr_t llp;
>  	struct dw_desc *desc, *_desc;
>  	struct dw_desc *child;
>  	u32 status_xfer;
> @@ -309,7 +302,6 @@ static void dwc_scan_descriptors(struct dw_dma *dw, struct dw_dma_chan *dwc)
>  	LIST_HEAD(list);
>
>  	spin_lock_irqsave(&dwc->lock, flags);
> -	llp = channel_readl(dwc, LLP);
>  	status_xfer = dma_readl(dw, RAW.XFER);
>
>  	if (status_xfer & dwc->mask) {
> @@ -372,27 +364,6 @@ static void dwc_scan_descriptors(struct dw_dma *dw, struct dw_dma_chan *dwc)
>  		return;
>  	}
>
> -	if (list_empty(&dwc->active_list)) {
> -		spin_unlock_irqrestore(&dwc->lock, flags);
> -		return;
> -	}
> -
> -	if (test_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags)) {
> -		dev_vdbg(dmaengine_chan_dev(&dwc->chan), "%s: soft LLP mode\n", __func__);
> -		spin_unlock_irqrestore(&dwc->lock, flags);
> -		return;
> -	}
> -
> -	dev_vdbg(dmaengine_chan_dev(&dwc->chan), "%s: llp=%pad\n", __func__, &llp);
> -
> -	list_for_each_entry_safe(desc, _desc, &dwc->active_list, desc_node) {
> -		if (dwc_get_hard_llp_desc_residue(dwc, desc, llp)) {
> -			spin_unlock_irqrestore(&dwc->lock, flags);
> -			return;
> -		}
> -	}
> -
> -	/* Residue describes source reads; only RAW.XFER completes a transfer. */
>  	spin_unlock_irqrestore(&dwc->lock, flags);
>  }
>
> @@ -987,9 +958,12 @@ static u32 dwc_get_residue_and_status(struct dw_dma_chan *dwc, dma_cookie_t cook
>  	desc = dwc_find_desc(dwc, cookie);
>  	if (desc) {
>  		if (desc == dwc_first_active(dwc)) {
> -			residue = desc->residue;
> -			if (test_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags) && residue)
> +			if (test_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags)) {
> +				residue = desc->residue;
>  				residue -= dwc_get_sent(dwc);
> +			} else {
> +				residue = dwc_get_hard_llp_desc_residue(dwc, desc);
> +			}
>  			if (test_bit(DW_DMA_IS_PAUSED, &dwc->flags))
>  				*status = DMA_PAUSED;
>  		} else {
>
> --
> 2.53.0
>