On Tue, 2016-03-08 at 09:46 +0100, Boris Brezillon wrote: > On Tue, 8 Mar 2016 08:51:31 +0100 > Maxime Ripard <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: > > > On Tue, Mar 08, 2016 at 08:25:47AM +0530, Vinod Koul wrote: > > > On Mon, Mar 07, 2016 at 09:30:24PM +0100, Maxime Ripard wrote: > > > > On Mon, Mar 07, 2016 at 04:08:57PM +0100, Boris Brezillon > > > > wrote: > > > > > Hi Vinod, > > > > > > > > > > On Mon, 7 Mar 2016 20:24:29 +0530 > > > > > Vinod Koul <vinod.koul@xxxxxxxxx> wrote: > > > > > > > > > > > On Mon, Mar 07, 2016 at 10:59:31AM +0100, Boris Brezillon > > > > > > wrote: > > > > > > > +/* Dedicated DMA parameter register layout */ > > > > > > > +#define SUN4I_DDMA_PARA_DST_DATA_BLK_SIZE(n) (((n > > > > > > > ) - 1) << 24) > > > > > > > +#define SUN4I_DDMA_PARA_DST_WAIT_CYCLES(n) (((n) > > > > > > > - 1) << 16) > > > > > > > +#define SUN4I_DDMA_PARA_SRC_DATA_BLK_SIZE(n) (((n > > > > > > > ) - 1) << 8) > > > > > > > +#define SUN4I_DDMA_PARA_SRC_WAIT_CYCLES(n) (((n) > > > > > > > - 1) << 0) > > > > > > > + > > > > > > > +/** > > > > > > > + * struct sun4i_dma_chan_config - DMA channel config > > > > > > > + * > > > > > > > + * @para: contains information about block size and time > > > > > > > before checking > > > > > > > + * DRQ line. This is device specific and only > > > > > > > applicable to dedicated > > > > > > > + * DMA channels > > > > > > > > > > > > What information, can you elobrate.. And why can't you use > > > > > > existing > > > > > > dma_slave_config for this? > > > > > > > > > > Block size is related to the device FIFO size. I guess it > > > > > allows the > > > > > DMA channel to launch a transfer of X bytes without having to > > > > > check the > > > > > DRQ line (the line telling the DMA engine it can transfer > > > > > more data > > > > > to/from the device). The wait cycles information is > > > > > apparently related > > > > > to the number of clks the engine should wait before > > > > > polling/checking > > > > > the DRQ line status between each block transfer. I'm not sure > > > > > what it > > > > > saves to put WAIT_CYCLES() to something != 1, but in their > > > > > BSP, > > > > > Allwinner tweak that depending on the device. > > > > > > we already have block size aka src/dst_maxburst, why not use that > > > one. > > > > I'm not sure it's really the same thing. The DMA controller also > > has a > > burst parameter, that is either 1 byte or 8 bytes, and ends up > > being > > different from this one. > > Well, that's what I understood to, but when reading more carefully > the > src/dst_maxburst description, it seems to match the block_size > concept > exposed by the sun4i dmaengine. But how should we choose the real > burst > size then. > IIRC, in most documentation/datasheets, burst size is referred as the > maximum number of words (word size depends on the selected width) a > master is allowed to transfer to a slave through the bus without > being interrupted by other master requests. > Am I correct? > > > > > > Why does dmaengine need to wait? Can you explain that > > > > We have no idea, we thought you might have one :) > > Yes, it's really unclear to us why this is needed. There might be > some > kind of contention, or maybe the slave device takes some time to put > DRQ line to low state, and without these wait_cycles the dmaengine > would assume some data are still available in the FIFO while there's > actually no more data to retrieve. > > > > > It doesn't really makes sense to us, but it does have a significant > > impact on the throughput. > > I wouldn't say significant impact, but tweaking those parameters has > some impact on the performances, and since it's not that complicated > to > implement, I thought it was worth a try, but maybe I'm wrong. Somewhat offtopic, but there was also another patchset a while ago for sun4i SPI that removed the 64 byte limit for SPI transfers (although this was DMA-less case) back then. http://lists.infradead.org/pipermail/linux-arm-kernel/2014-March/241392 .html (This patchset made it possible to use SPI-based small TFT displays via fbtft, when I tested it some time ago). As it currently stands, sun4i SPI is not using DMA and transfers are limited to 64 bytes. Päikest, Priit ;) -- 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
