On Fri, Jan 15, 2016 at 10:57 AM, Nicolin Chen <nicoleotsuka@xxxxxxxxx> wrote: > On Fri, Jan 15, 2016 at 10:49:04AM -0800, Caleb Crome wrote: > >> > The watermark is merely a threshold to trigger a DMA request. The >> > only relationship with the burst size is that each burst transfer >> > should not carry more data than the number of empty slots; FIFO >> > under/overflow occurs otherwise. So it's just more efficient and >> > safer to set an identical value to both of them. I don't think >> > it will cause functional problems to set TFWM to 4 and burst size >> > to 1 -- It just lets DMA operate in a single data transfer mode. >> >> If there is no penalty for setting maxburst to 1 (or 2 in the case of >> dual fifo I think), then should we just set both the watermark and >> maxburst to 1? >> >> I guess the real difference would be when you're in FIQ mode. In FIQ >> mode, the penalty of an interrupt per word would be pretty bad, but in >> DMA mode, if we just set both to 1, we should be fine, right? > > There will be much more overhead drawn by frequent DMA transfers. > I believe you understand the idea -- less burst size then more DMA > request. Each DMA transfer contains a pair of handshaking overhead > according to the bus protocol. Apparently the bus will be wasted > with lots of handshaking section instead of keep dedicated to data > transfer truly. It might work if SSI is the only user of the bus, > which we shouldn't assume. Right, I knew there must be a trade off. So, how about I submit a patch with a fixed value of 4 for whichever platforms you think is correct. I see the 4 compatibles are: fsl,mpc8610-ssi, fsl,imx51-ssi, fsl,imx35-ssi, and fsl,imx21-ssi. Shall I submit a patch that sets the value to 4 for for any/all of the above? Should it be different based on whether fiq is enabled. Maybe set it to 4 for whenever DMA is used, and to 13 whenever FIQ is used? -Caleb -- 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
