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. > --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
