Re: [PATCH v2 05/19] dmaengine: dw-edma: Add LL interrupt placement policy

[Frank Li]

On Fri, Jul 24, 2026 at 02:46:23PM +0900, Koichiro Den wrote:
> On Thu, Jul 23, 2026 at 02:58:16PM -0500, Frank Li wrote:
> > On Thu, Jul 23, 2026 at 05:41:36PM +0900, Koichiro Den wrote:
> > > Move linked-list interrupt placement behind a core callback so eDMA and
> > > HDMA can use different policies.
> > >
> > > Keep the eDMA behavior. For HDMA, place watermarks at descriptor ends,
> > > before link entries, and every four entries when a descriptor exceeds the
> > > usable ring capacity or another issued descriptor follows. A later patch
> > > handles these watermarks as progress events.
> > >
> > > Four entries is an empirically chosen coalescing interval.
> > >
> > > Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
> > > ---
> > > Changes in v2:
> > >   - Fix the zero-based interval so the first watermark is on the fourth
> > >     entry, not the fifth.
> > >   - Revise the commit message and source comments.
> > >
> > >  drivers/dma/dw-edma/dw-edma-core.c    |  8 +++++++-
> > >  drivers/dma/dw-edma/dw-edma-core.h    |  2 ++
> > >  drivers/dma/dw-edma/dw-edma-v0-core.c | 10 ++++++++++
> > >  drivers/dma/dw-edma/dw-hdma-v0-core.c | 28 +++++++++++++++++++++++++++
> > >  4 files changed, 47 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/dma/dw-edma/dw-edma-core.c b/drivers/dma/dw-edma/dw-edma-core.c
> > > index a9f25a65294e..78d1bb6302fb 100644
> > > --- a/drivers/dma/dw-edma/dw-edma-core.c
> > > +++ b/drivers/dma/dw-edma/dw-edma-core.c
> > > @@ -98,6 +98,12 @@ static u32 dw_edma_core_get_free_num(struct dw_edma_chan *chan)
> > >  		chan->ll_max;
> > >  }
> > >
> > > +static bool dw_edma_core_enable_ll_irq(struct dw_edma_desc *desc, u32 i,
> > > +				       u32 free)
> > > +{
> > > +	return desc->chan->dw->core->ll_irq(desc, i, free);
> > > +}
> > > +
> > >  static void dw_edma_core_ll_start(struct dw_edma_desc *desc)
> > >  {
> > >  	struct dw_edma_chan *chan = desc->chan;
> > > @@ -120,7 +126,7 @@ static void dw_edma_core_ll_start(struct dw_edma_desc *desc)
> > >
> > >  		dw_edma_core_ll_data(chan, &desc->burst[i],
> > >  				     chan->ll_head, chan->cb,
> > > -				     i == desc->nburst - 1 || free == 1);
> > > +				     dw_edma_core_enable_ll_irq(desc, i, free));
> > >
> > >  		chan->ll_head++;
> > >
> > > diff --git a/drivers/dma/dw-edma/dw-edma-core.h b/drivers/dma/dw-edma/dw-edma-core.h
> > > index f2c2d5af1fff..8ed37e8a12cb 100644
> > > --- a/drivers/dma/dw-edma/dw-edma-core.h
> > > +++ b/drivers/dma/dw-edma/dw-edma-core.h
> > > @@ -162,6 +162,8 @@ struct dw_edma_core_ops {
> > >  	void (*ll_link)(struct dw_edma_chan *chan, u32 idx, bool cb, u64 addr);
> > >  	void (*ll_clear)(struct dw_edma_chan *chan, u32 idx);
> > >  	int (*ll_cur_idx)(struct dw_edma_chan *chan);
> > > +	/* Called with chan->vc.lock held. */
> > > +	bool (*ll_irq)(struct dw_edma_desc *desc, u32 i, u32 free);
> > >  	void (*ch_doorbell)(struct dw_edma_chan *chan);
> > >  	void (*ch_enable)(struct dw_edma_chan *chan);
> > >  	void (*ch_config)(struct dw_edma_chan *chan);
> > > diff --git a/drivers/dma/dw-edma/dw-edma-v0-core.c b/drivers/dma/dw-edma/dw-edma-v0-core.c
> > > index c31fff095b4f..62141aa32e50 100644
> > > --- a/drivers/dma/dw-edma/dw-edma-v0-core.c
> > > +++ b/drivers/dma/dw-edma/dw-edma-v0-core.c
> > > @@ -649,6 +649,15 @@ static int dw_edma_v0_core_ll_cur_idx(struct dw_edma_chan *chan)
> > >  	return (val - base) / EDMA_LL_SZ;
> > >  }
> > >
> > > +static bool dw_edma_v0_core_ll_irq(struct dw_edma_desc *desc, u32 i, u32 free)
> > > +{
> > > +	/*
> > > +	 * eDMA reports LL interrupts through DONE. Keep them at
> > > +	 * descriptor ends, plus the last free slot to refill the ring.
> > > +	 */
> > > +	return i == desc->nburst - 1 || free == 1;
> > > +}
> > > +
> > >  /* eDMA debugfs callbacks */
> > >  static void dw_edma_v0_core_debugfs_on(struct dw_edma *dw)
> > >  {
> > > @@ -685,6 +694,7 @@ static const struct dw_edma_core_ops dw_edma_v0_core = {
> > >  	.ll_link = dw_edma_v0_core_ll_link,
> > >  	.ll_clear = dw_edma_v0_core_ll_clear,
> > >  	.ll_cur_idx = dw_edma_v0_core_ll_cur_idx,
> > > +	.ll_irq = dw_edma_v0_core_ll_irq,
> > >  	.ch_doorbell = dw_edma_v0_core_ch_doorbell,
> > >  	.ch_enable = dw_edma_v0_core_ch_enable,
> > >  	.ch_config = dw_edma_v0_core_ch_config,
> > > diff --git a/drivers/dma/dw-edma/dw-hdma-v0-core.c b/drivers/dma/dw-edma/dw-hdma-v0-core.c
> > > index b2d35f0b7b6d..fb651a83f0f3 100644
> > > --- a/drivers/dma/dw-edma/dw-hdma-v0-core.c
> > > +++ b/drivers/dma/dw-edma/dw-hdma-v0-core.c
> > > @@ -13,6 +13,9 @@
> > >  #include "dw-hdma-v0-regs.h"
> > >  #include "dw-hdma-v0-debugfs.h"
> > >
> > > +/* Empirically chosen watermark interval. */
> > > +#define HDMA_V0_WATERMARK_INTERVAL			4
> > > +
> > >  enum dw_hdma_control {
> > >  	DW_HDMA_V0_CB					= BIT(0),
> > >  	DW_HDMA_V0_TCB					= BIT(1),
> > > @@ -417,6 +420,30 @@ static int dw_hdma_v0_core_ll_cur_idx(struct dw_edma_chan *chan)
> > >  	return (val - base) / EDMA_LL_SZ;
> > >  }
> > >
> > > +static bool dw_hdma_v0_core_ll_irq(struct dw_edma_desc *desc, u32 i, u32 free)
> > > +{
> > > +	struct dw_edma_chan *chan = desc->chan;
> > > +	bool needs_progress;
> > > +
> > > +	/*
> > > +	 * Always place a watermark at a descriptor end and before the link
> > > +	 * element. The first reports descriptor progress; the second provides
> > > +	 * one progress point per ring lap.
> > > +	 */
> > > +	if (i == desc->nburst - 1 || chan->ll_head == chan->ll_max - 1)
> > > +		return true;
> > > +
> > > +	/*
> > > +	 * Use periodic watermarks when a descriptor exceeds the usable ring or
> > > +	 * when more issued work follows it.
> > > +	 */
> > > +	needs_progress = desc->nburst > chan->ll_max - 1 ||
> > > +			 !list_is_last(&desc->vd.node, &chan->vc.desc_issued);
> > > +
> > > +	return needs_progress &&
> > > +	       (chan->ll_head + 1) % HDMA_V0_WATERMARK_INTERVAL == 0;
> >
> > This is not good layer design. edma/hdma supposed just do action to operate
> > register, less software logic.
>
> Indeed, thanks for pointing that out.
>
> >
> > Can we move this code to core layer?  introduce an 'interval' for difference
> > backend.
> >
> > eDMA: interval is number description.
> > hDMA: interval is HDMA_V0_WATERMARK_INTERVAL
>
> Sorry, by "eDMA: interval is number description", do you mean using desc->nburst
> as the "interval", so eDMA keeps one interrupt at the end of each descriptor?

Yes, of cource, you also can periodic interval check eDMA if want.

Frank

>
> >
> > So core layer can use unified logic to handle for both eDMA and hDMA?
>
> I'll move as much of the software policy as possible into the common core. I
> think even the periodic interval and descriptor-end placement can be shared.
> I'll rework this part and verify it with performance tests.
>
> Thanks for the review and suggestion. Much appreciated.
> Koichiro
>
> >
> > Frank
> >
> > > +}
> > > +
> > >  /* HDMA debugfs callbacks */
> > >  static void dw_hdma_v0_core_debugfs_on(struct dw_edma *dw)
> > >  {
> > > @@ -441,6 +468,7 @@ static const struct dw_edma_core_ops dw_hdma_v0_core = {
> > >  	.ll_link = dw_hdma_v0_core_ll_link,
> > >  	.ll_clear = dw_hdma_v0_core_ll_clear,
> > >  	.ll_cur_idx = dw_hdma_v0_core_ll_cur_idx,
> > > +	.ll_irq = dw_hdma_v0_core_ll_irq,
> > >  	.ch_doorbell = dw_hdma_v0_core_ch_doorbell,
> > >  	.ch_enable = dw_hdma_v0_core_ch_enable,
> > >  	.ch_config = dw_hdma_v0_core_ch_config,
> > > --
> > > 2.51.0
> > >