[RFC] dma: at_xdmac.c: enable descriptor reuse

[Vinod Koul]

On Tue, Nov 24, 2015 at 09:43:25AM +0100, Ludovic Desroches wrote:
> Hi David,
> 
> Sorry to answer a bit late.
> 
> I am interested if you have some benches about the benefit of reuse.
> 
> On Fri, Nov 13, 2015 at 01:39:16PM -0700, David Mosberger wrote:
> > Ludovic et al,
> > 
> > I'd be interested in comments on the patch below.  We'd like to be able to
> > issue a DMA transfer from time to time and want to avoid the overhead of
> > setting up a new DMA descriptor for each (identical) transfer.  The cyclic
> > DMA support doesn't quite work for us (I think) because, as I understand
> > it, that would continuously issue the DMA transfer.  From what I can see,
> > enabling descriptor-reuse just requires two lines of changes to the driver:
> > (a) mark the driver as capable of supporting descriptor_reuse and (b) skip
> > putting the descriptor on the free-list upon completion of a transfer, if
> > it is marked for reuse.
> > 
> > The patch also changes at_xdmac_remove_xfer() to clear the active_xfer
> > flag.  Without this change, at_xdmac_tasklet() may miss starting a pending
> > transfer on a re-used descriptor, because the active_xfer flag would not
> > have been cleared since that normally happens when a descriptor gets
> > reallocated and then reinitialized with at_xdmac_init_used_desc().
> > 
> > Like I said: at this point I'm just trying to understand if this patch is
> > going in the right direction and whether anybody sees any potential issues
> > with doing something along those lines.
> >
> 
> I have never thought about using reuse but I don't see any issue with your
> patch.

we do support desc reuse, it is in -next. Pls see patches in
topic/desc_reuse in my tree

>  
> > ​  --david​​
> > 
> > -- 
> > eGauge Systems LLC, http://egauge.net/, 1.877-EGAUGE1, fax 720.545.9768
> 
> > diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
> > index b5e132d..eea6b40 100644
> > --- a/drivers/dma/at_xdmac.c
> > +++ b/drivers/dma/at_xdmac.c
> > @@ -1455,7 +1510,16 @@ static void at_xdmac_remove_xfer(struct at_xdmac_chan *atchan,
> >  	 * descriptors into the free descriptors list.
> >  	 */
> >  	list_del(&desc->xfer_node);
> > -	list_splice_init(&desc->descs_list, &atchan->free_descs_list);
> > +	/*
> > +	 * Mark descriptor as inactive so new transfers can be
> > +	 * started.
> > +	 */
> > +	desc->active_xfer = false;
> > +	/*
> > +	 * Put descriptor on the free list unless it's marked to be reused.
> > +	 */
> > +	if (!dmaengine_desc_test_reuse(&desc->tx_dma_desc))
> > +		list_splice_init(&desc->descs_list, &atchan->free_descs_list);
> >  }
> 
> Since setting active_xfer to false is done in at_xdmac_init_used_desc(),
> I would write it in this to show that doing it here is only related to reuse
> case.
> 
> if (!dmaengine_desc_test_reuse(&desc->tx_dma_desc))
> 	list_splice_init(&desc->descs_list, &atchan->free_descs_list);
> else
> 	desc->active_xfer = false;
> 
> >  
> >  static void at_xdmac_advance_work(struct at_xdmac_chan *atchan)
> > @@ -1930,6 +2000,7 @@ static int at_xdmac_probe(struct platform_device *pdev)
> >  	atxdmac->dma.dst_addr_widths = AT_XDMAC_DMA_BUSWIDTHS;
> >  	atxdmac->dma.directions = BIT(DMA_DEV_TO_MEM) | BIT(DMA_MEM_TO_DEV);
> >  	atxdmac->dma.residue_granularity = DMA_RESIDUE_GRANULARITY_BURST;
> > +	atxdmac->dma.descriptor_reuse = true;
> >  
> >  	/* Disable all chans and interrupts. */
> >  	at_xdmac_off(atxdmac);
> 
> Regards
> 
> Ludovic
> --
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-- 
~Vinod