On Fri, Jul 26, 2013 at 11:46 PM, James Stone <jamesmstone@xxxxxxxxx> wrote: > On Fri, Jul 26, 2013 at 11:43 PM, James Stone <jamesmstone@xxxxxxxxx> wrote: >> On Fri, Jul 26, 2013 at 7:54 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: >>> On Fri, 26 Jul 2013, Clemens Ladisch wrote: >>> >>>> Alan Stern wrote: >>>> > On Thu, 25 Jul 2013, James Stone wrote: >>>> >> The only slight difference I can see is that maybe the 3.10 uses >>>> >> slightly higher CPU load than 3.5 at the ridiculously low latency of >>>> >> 64 frames/period duplex. >>>> > >>>> > With the new patch, what you actually get is 44.1 frames/period (on >>>> > average). >>>> >>>> In ALSA, the number of frames per period is a constant integer, and Jack >>>> requires it to be a power of two. (Where "frame" is an audio frame, and >>>> "period" is the interval between interrupts reported to user space.) >>>> >>>> With a sample rate of 44100 Hz and a packet rate of 8000 Hz, there >>>> should be about 5.5 samples per packet. With a period size of 64 audio >>>> frames, this results in about 11.6 packets per period. >>>> >>>> The driver does not completely fill URBs to ensure that interrupts >>>> happen at period boundaries. >>> >>> Oho! I missed all that period_elapsed stuff in prepare_playback_urb()! >>> But you don't do the same thing for recording URBs -- presumably >>> because you can't tell in advance how many samples the device will >>> send. >>> >>> This makes the calculation of the number of URBs more complicated. >>> Revised patch below. >>> >>> James, can you try this out and send me the usbmon trace? At 64 >>> frames/period this should end up using 4 URBs, which is the minimum >>> requirement if there are no more than 8 packets per URB and an >>> incompletely filled URB can contain as few as 1 packets. With this >>> patch, there should be no difficulty going down to 32 or maybe even 16 >>> frames/period. >>> >>> >> >> Here you go.. (attached). This is at 64 frames/period. >> >> James > > Apologies - that was with 3 periods/buffer. 2 periods/buffer attached. > > James OK. So this seems to have solved the starting jack at low latencies problem, but I am still getting sporadic cannot submit urb (err = -18) under normal use. Will try to add some more info to the #1191603 report if I can get it to happen while logging IRQ. James -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
