On Tue, Nov 01, 2016 at 10:55:13PM +0800, Chen-Yu Tsai wrote: > >> * @src_maxburst: the maximum number of words (note: words, as in > >> * units of the src_addr_width member, not bytes) that can be sent > >> * in one burst to the device. Typically something like half the > >> * FIFO depth on I/O peripherals so you don't overflow it. This > >> * may or may not be applicable on memory sources. > >> * @dst_maxburst: same as src_maxburst but for destination target > >> * mutatis mutandis. > >> > >> The DMA engine driver should be free to select whatever burst size > >> that doesn't exceed this. So for max_burst = 4, the driver can select > >> burst = 4 for controllers that do support it, or burst = 1 for those > >> that don't, and do more bursts. > > > > Nope, the client configures these parameters and dmaengine driver > > validates and programs > > Shouldn't we just name it "burst_size" then if it's meant to be what > the client specifically asks for? Well if for some reason we program lesser than than max it would work technically. But a larger burst wont work at all, so thats why maxburst is significant. > My understanding is that the client configures its own parameters, > such as the trigger level for the DRQ, like raise DRQ when level < 1/4 > FIFO depth, request maxburst = 1/4 or 1/2 FIFO depth, so as not to > overrun the FIFO. When the DRQ is raised, the DMA engine will do a > burst, and after the burst the DRQ would be low again, so the DMA > engine will wait. So the DMA engine driver should be free to > program the actual burst size to something less than maxburst, shouldn't > it? Yup but not more that max.. > >> This also means we can increase max_burst for the audio codec, as > >> the FIFO is 64 samples deep for stereo, or 128 samples for mono. > > > > Beware that higher bursts means chance of underrun of FIFO. This value > > is selected with consideration of power and performance required. Lazy > > allocation would be half of FIFO size.. > > You mean underrun if its the source right? So the client setting maxburst > should take the DRQ trigger level into account for this. Yes -- ~Vinod -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
