Re: [PATCH v4 4/7] dmaengine: dw: calculate residue separately from completion

[Frank Li]

On Thu, Oct 29, 2026 at 04:29:38PM -0600, James Hilliard wrote:

dmaengine: dw: move residue calculation into helper dwc_get_hard_llp_desc_residue()

> From: Serge Semin <fancer.lancer@xxxxxxxxx>
>
> The descriptor scanner mixes transfer-complete handling with hardware-LLP
> residue calculation. A status query needs the latter for the requested
> cookie even when no transfer-complete event is pending.
>
> Move hardware-LLP residue calculation into its own helper and call it from
> the status getter under the channel lock. Keep software-LLP progress in
> the scanner and subtract the current block's progress when reporting its
> residue. The scanner now handles transfer-complete events only, retaining
> the distinction between zero source residue and destination completion.
>
> Also search the queued descriptors so a transfer waiting to start reports
> its full length instead of zero residue.
>
> Fixes: 3bfb1d20b547 ("dmaengine: Driver for the Synopsys DesignWare DMA controller")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Serge Semin <fancer.lancer@xxxxxxxxx>
> [james: retain polled completion and report queued residue]
> Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
>
> ---
>
> Changelog RFC:
> - Instead of just dropping the dwc_scan_descriptors() method invocation
>   calculate the residue in the Tx-status getter.
> ---
>  drivers/dma/dw/core.c | 166 +++++++++++++++++++++-----------------------------
>  1 file changed, 68 insertions(+), 98 deletions(-)
>
> diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c
> index 1f7afcd01565..1d6419e1cd3c 100644
> --- a/drivers/dma/dw/core.c
> +++ b/drivers/dma/dw/core.c
> @@ -274,123 +274,60 @@ static inline u32 dwc_get_sent(struct dw_dma_chan *dwc)
>
>  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;
>  	unsigned long flags;
>  	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) {
> -		/* Everything we've submitted is done */
> -		dma_writel(dw, CLEAR.XFER, dwc->mask);
> -		if (list_empty(&dwc->active_list)) {
> -			spin_unlock_irqrestore(&dwc->lock, flags);
> -			return;
> -		}
> -
> -		if (test_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags)) {
> -			struct list_head *head, *active = dwc->tx_node_active;
> -
> -			/*
> -			 * We are inside first active descriptor.
> -			 * Otherwise something is really wrong.
> -			 */
> -			desc = dwc_first_active(dwc);
> -
> -			head = &desc->tx_list;
> -			if (active != head) {
> -				/* Update residue to reflect last sent descriptor */
> -				if (active == head->next)
> -					desc->residue -= desc->len;
> -				else
> -					desc->residue -= to_dw_desc(active->prev)->len;
> -
> -				child = to_dw_desc(active);
> +	if (!(dma_readl(dw, RAW.XFER) & dwc->mask))
> +		goto unlock;
>
> -				/* Submit next block */
> -				dwc_do_single_block(dwc, child);
> +	dma_writel(dw, CLEAR.XFER, dwc->mask);
> +	if (list_empty(&dwc->active_list))
> +		goto unlock;
>
> -				spin_unlock_irqrestore(&dwc->lock, flags);
> -				return;
> -			}
> +	desc = dwc_first_active(dwc);
> +	if (test_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags)) {
> +		struct list_head *head = &desc->tx_list;
> +		struct list_head *active = dwc->tx_node_active;
>
> -			/* We are done here */
> -			clear_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags);
> -		}
> +		if (active != head) {
> +			/* Update residue to reflect last sent descriptor. */
> +			if (active == head->next)
> +				desc->residue -= desc->len;
> +			else
> +				desc->residue -= to_dw_desc(active->prev)->len;
>
> -		if (dma_readl(dw, CH_EN) & dwc->mask) {
> -			dev_err(chan2dev(&dwc->chan),
> -				"BUG: XFER bit set, but channel not idle!\n");
> +			child = to_dw_desc(active);
>
> -			/* Try to continue after resetting the channel... */
> -			dwc_chan_disable(dw, dwc);
> +			/* Submit next block, including for polled transfers. */
> +			dwc_do_single_block(dwc, child);
> +			goto unlock;
>  		}
>
> -		/* Detach the completed transfer before starting queued work. */
> -		list_splice_init(&dwc->active_list, &list);
> -		list_for_each_entry(desc, &list, desc_node)
> -			dma_cookie_complete(&desc->txd);
> -		dwc_dostart_first_queued(dwc);
> -		spin_unlock_irqrestore(&dwc->lock, flags);
> -
> -		list_for_each_entry_safe(desc, _desc, &list, desc_node) {
> -			list_del_init(&desc->desc_node);
> -			dwc_descriptor_free(dwc, desc, true);
> -		}
> -		return;
> +		clear_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags);
>  	}
>
> -	if (list_empty(&dwc->active_list)) {
> -		spin_unlock_irqrestore(&dwc->lock, flags);
> -		return;
> -	}
> +	if (dma_readl(dw, CH_EN) & dwc->mask) {
> +		dev_err(chan2dev(&dwc->chan),
> +			"BUG: XFER bit set, but channel not idle!\n");
>
> -	if (test_bit(DW_DMA_IS_SOFT_LLP, &dwc->flags)) {
> -		dev_vdbg(chan2dev(&dwc->chan), "%s: soft LLP mode\n", __func__);
> -		spin_unlock_irqrestore(&dwc->lock, flags);
> -		return;
> +		/* Try to continue after resetting the channel... */
> +		dwc_chan_disable(dw, dwc);
>  	}
>
> -	dev_vdbg(chan2dev(&dwc->chan), "%s: llp=%pad\n", __func__, &llp);
> -
> -	list_for_each_entry_safe(desc, _desc, &dwc->active_list, desc_node) {
> -		/* Initial residue value */
> -		desc->residue = desc->total_len;
> -
> -		/* Check first descriptors addr */
> -		if (desc->txd.phys == DWC_LLP_LOC(llp)) {
> -			spin_unlock_irqrestore(&dwc->lock, flags);
> -			return;
> -		}
> -
> -		/* Check first descriptors llp */
> -		if (lli_read(desc, llp) == llp) {
> -			/* This one is currently in progress */
> -			desc->residue -= dwc_get_sent(dwc);
> -			spin_unlock_irqrestore(&dwc->lock, flags);
> -			return;
> -		}
> -
> -		desc->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);
> -				spin_unlock_irqrestore(&dwc->lock, flags);
> -				return;
> -			}
> -			desc->residue -= child->len;
> -		}
> +	dma_cookie_complete(&desc->txd);
> +	list_move_tail(&desc->desc_node, &list);
> +	dwc_dostart_first_queued(dwc);
> +unlock:
> +	spin_unlock_irqrestore(&dwc->lock, flags);
>
> +	list_for_each_entry_safe(desc, _desc, &list, desc_node) {
> +		list_del_init(&desc->desc_node);
> +		dwc_descriptor_free(dwc, desc, true);
>  	}
> -
> -	/* Residue describes source reads; only RAW.XFER completes a transfer. */
> -	spin_unlock_irqrestore(&dwc->lock, flags);
>  }
>
>  static inline void dwc_dump_lli(struct dw_dma_chan *dwc, struct dw_desc *desc)
> @@ -969,9 +906,39 @@ static struct dw_desc *dwc_find_desc(struct dw_dma_chan *dwc, dma_cookie_t c)
>  		if (desc->txd.cookie == c)
>  			return desc;
>
> +	list_for_each_entry(desc, &dwc->queue, desc_node)
> +		if (desc->txd.cookie == c)
> +			return desc;
> +
>  	return NULL;
>  }
>
> +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;
> +
> +	llp = channel_readl(dwc, LLP);
> +
> +	/* The first descriptor may still be waiting to be fetched. */
> +	if (desc->txd.phys == DWC_LLP_LOC(llp))
> +		return residue;
> +
> +	if (lli_read(desc, llp) == llp)
> +		return residue - dwc_get_sent(dwc);
> +
> +	residue -= desc->len;
> +	list_for_each_entry(child, &desc->tx_list, desc_node) {
> +		if (lli_read(child, llp) == llp)
> +			return residue - dwc_get_sent(dwc);
> +
> +		residue -= child->len;
> +	}
> +
> +	return residue;
> +}

This patch still too big. You can create new helper()
dwc_get_hard_llp_desc_residue() and replace orginal code exactly, then
change gradulatly.

Frank