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 > -- > To unsubscribe from this list: send the line "unsubscribe dmaengine" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- ~Vinod -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
