Re: [alsa-devel] Fw: Isochronous transfer error on USB3

[Mauro Carvalho Chehab <m.chehab@samsung.com>]

Em Thu, 9 Jan 2014 09:29:57 -0200
Mauro Carvalho Chehab <m.chehab@samsung.com> escreveu:

> Em Thu, 09 Jan 2014 09:17:13 +0100
> Clemens Ladisch <clemens@ladisch.de> escreveu:
> 
> > Mauro Carvalho Chehab wrote:
> > > I'm getting an weird behavior with em28xx, especially when the device
> > > is connected into an audio port.
> > >
> > > 	http://git.linuxtv.org/mchehab/experimental.git/blob/refs/heads/em28xx-v4l2-v6:/drivers/media/usb/em28xx/em28xx-audio.c
> > >
> > > What happens is that, when I require xawtv3 to use any latency lower
> > > than 65 ms, the audio doesn't work, as it gets lots of underruns per
> > > second.
> > 
> > The driver uses five URBs with 64 frames each, so of course it
> > will not be able to properly handle periods smaller than that.
> > 
> > > FYI, em28xx works at a 48000 KHz sampling rate, and its PM capture Hw
> > > is described as:
> > >
> > > static struct snd_pcm_hardware snd_em28xx_hw_capture = {
> > > 	.info = SNDRV_PCM_INFO_BLOCK_TRANSFER |
> > > 		SNDRV_PCM_INFO_MMAP           |
> > > 		SNDRV_PCM_INFO_INTERLEAVED    |
> > > 		SNDRV_PCM_INFO_BATCH	      |
> > > 		SNDRV_PCM_INFO_MMAP_VALID,
> > >
> > > 	.formats = SNDRV_PCM_FMTBIT_S16_LE,
> > >
> > > 	.rates = SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_KNOT,
> > 
> > This should be just SNDRV_PCM_RATE_48000.
> 
> Ok.
> 
> > 
> > > 	.period_bytes_min = 64,		/* 12544/2, */
> > 
> > This is wrong (if the driver doesn't install other constraints on the
> > period length, like the USB audio class driver does).
> 
> Ok, how should it be estimated? Those values here were simply glued from
> the USB audio class driver a long time ago without a further analysis.
> 
> I changed it to 188 (the minimum URB size I experimentally noticed with
> the current settings) and it is now working fine with both xHCI and EHCI.

PS.: using 188 there, the URBs now have a total actual size of 24 bytes.

Regards,
-- 

Cheers,
Mauro