On Fri, Jan 15, 2016 at 5:46 AM, Timur Tabi <timur@xxxxxxxx> wrote: > Mark Brown wrote: >>> >>> >I admit it's a grey area, but the hardware doesn't work if you use the >>> > wrong >>> >value, and it is a fixed value per device. A p1022ds would use a >>> > different >>> >value than in in i.MX6, and once you pick a value, it's the same no >>> > matter >>> >which sample rate, buffer size, etc you choose. > > >> Caleb's original message suggested this was rate dependant. > > > Yeah, I just noticed that. In that case, I agree that a device tree > property is inappropriate, unless it's an array that contains tuples of > sample rates and watermark/maxburst settings. That would get unwieldy very > easily, though. The rate dependance is only a *potential* issue. I suspect that a value of 4 should be functional for all rates and chips. The only trade off is more DMA requests/bursts. In a typical 15 word fifo, 48kHz, stereo, single fifo DMA system, the old value was 15-2 = 13, which would mean 7385 13-word DMA bursts/second. A new value of 4 would mean 24,000 4-word DMA bursts/second. Is that consequential for anybody? It's about the same total bandwidth on the system, but just broken up into smaller chunks (I don't know what the overhead is for a DMA burst) In a high channel count system (16 channels @ 48kHz), the old value doesn't work, and the new value would mean 192,000 4-word DMA bursts/second, which works on my MX6. So given that 192000 works fine, I'm not sure that the difference in a typical system would matter at all. If nobody objects, we can just set the value to 4 and be done with it. Another question: is the watermark ever going to be different than maxburst? Is there any reason to have them different? -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
