Re: [PATCH v3 31/34] dmaengine: dw: defer callbacks via channel BH

[Frank Li]

On Mon, Aug 10, 2026 at 11:09:32AM -0700, Allen Pais wrote:
> Queue completed descriptors on the channel and invoke their callbacks
> from per-channel BH work instead of the controller tasklet. Add a
> synchronize hook so termination can wait for deferred callbacks before
> their descriptor storage is released.
>
> Signed-off-by: Allen Pais <allen.lkml@xxxxxxxxx>
> ---

Reviewed-by: Frank Li <Frank.Li@xxxxxxx>

>  drivers/dma/dw/core.c | 42 ++++++++++++++++++++++++++++++++++--------
>  drivers/dma/dw/regs.h |  1 +
>  2 files changed, 35 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/dma/dw/core.c b/drivers/dma/dw/core.c
> index dd75f97a33b3..2765acfe5644 100644
> --- a/drivers/dma/dw/core.c
> +++ b/drivers/dma/dw/core.c
> @@ -236,25 +236,24 @@ dwc_descriptor_complete(struct dw_dma_chan *dwc, struct dw_desc *desc,
>  	struct dma_async_tx_descriptor	*txd = &desc->txd;
>  	struct dw_desc			*child;
>  	unsigned long			flags;
> -	struct dmaengine_desc_callback	cb;
>
>  	dev_vdbg(chan2dev(&dwc->chan), "descriptor %u complete\n", txd->cookie);
>
>  	spin_lock_irqsave(&dwc->lock, flags);
>  	dma_cookie_complete(txd);
> -	if (callback_required)
> -		dmaengine_desc_get_callback(txd, &cb);
> -	else
> -		memset(&cb, 0, sizeof(cb));
> +	list_del_init(&desc->desc_node);
>
>  	/* async_tx_ack */
>  	list_for_each_entry(child, &desc->tx_list, desc_node)
>  		async_tx_ack(&child->txd);
>  	async_tx_ack(&desc->txd);
> -	dwc_desc_put(dwc, desc);
> +	if (callback_required) {
> +		list_add_tail(&desc->desc_node, &dwc->completed_list);
> +		dmaengine_schedule_bh(&dwc->chan);
> +	} else {
> +		dwc_desc_put(dwc, desc);
> +	}
>  	spin_unlock_irqrestore(&dwc->lock, flags);
> -
> -	dmaengine_desc_callback_invoke(&cb, NULL);
>  }
>
>  static void dwc_complete_all(struct dw_dma *dw, struct dw_dma_chan *dwc)
> @@ -285,6 +284,24 @@ static void dwc_complete_all(struct dw_dma *dw, struct dw_dma_chan *dwc)
>  		dwc_descriptor_complete(dwc, desc, true);
>  }
>
> +static void dwc_chan_bh(struct dma_chan *chan)
> +{
> +	struct dw_dma_chan *dwc = to_dw_dma_chan(chan);
> +	struct dw_desc *desc, *_desc;
> +	struct dmaengine_desc_callback cb;
> +	LIST_HEAD(list);
> +
> +	spin_lock_irq(&dwc->lock);
> +	list_splice_init(&dwc->completed_list, &list);
> +	spin_unlock_irq(&dwc->lock);
> +
> +	list_for_each_entry_safe(desc, _desc, &list, desc_node) {
> +		dmaengine_desc_get_callback(&desc->txd, &cb);
> +		dmaengine_desc_callback_invoke(&cb, NULL);
> +		dwc_desc_put(dwc, desc);
> +	}
> +}
> +
>  /* Returns how many bytes were already received from source */
>  static inline u32 dwc_get_sent(struct dw_dma_chan *dwc)
>  {
> @@ -965,6 +982,11 @@ static int dwc_terminate_all(struct dma_chan *chan)
>  	return 0;
>  }
>
> +static void dwc_synchronize(struct dma_chan *chan)
> +{
> +	dmaengine_kill_bh(chan);
> +}
> +
>  static struct dw_desc *dwc_find_desc(struct dw_dma_chan *dwc, dma_cookie_t c)
>  {
>  	struct dw_desc *desc;
> @@ -1256,6 +1278,8 @@ int do_dma_probe(struct dw_dma_chip *chip)
>
>  		INIT_LIST_HEAD(&dwc->active_list);
>  		INIT_LIST_HEAD(&dwc->queue);
> +		INIT_LIST_HEAD(&dwc->completed_list);
> +		dmaengine_init_bh(&dwc->chan, dwc_chan_bh);
>
>  		channel_clear_bit(dw, CH_EN, dwc->mask);
>
> @@ -1321,6 +1345,7 @@ int do_dma_probe(struct dw_dma_chip *chip)
>  	dw->dma.device_pause = dwc_pause;
>  	dw->dma.device_resume = dwc_resume;
>  	dw->dma.device_terminate_all = dwc_terminate_all;
> +	dw->dma.device_synchronize = dwc_synchronize;
>
>  	dw->dma.device_tx_status = dwc_tx_status;
>  	dw->dma.device_issue_pending = dwc_issue_pending;
> @@ -1374,6 +1399,7 @@ int do_dma_remove(struct dw_dma_chip *chip)
>
>  	list_for_each_entry_safe(dwc, _dwc, &dw->dma.channels,
>  			chan.device_node) {
> +		dmaengine_kill_bh(&dwc->chan);
>  		list_del(&dwc->chan.device_node);
>  		channel_clear_bit(dw, CH_EN, dwc->mask);
>  	}
> diff --git a/drivers/dma/dw/regs.h b/drivers/dma/dw/regs.h
> index 5969d9cc8d7a..c9e32bd6110a 100644
> --- a/drivers/dma/dw/regs.h
> +++ b/drivers/dma/dw/regs.h
> @@ -279,6 +279,7 @@ struct dw_dma_chan {
>  	unsigned long		flags;
>  	struct list_head	active_list;
>  	struct list_head	queue;
> +	struct list_head	completed_list;
>
>  	unsigned int		descs_allocated;
>
> --
> 2.43.0
>