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

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

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.

Regards,
Mauro