Re: [alsa-devel] [PATCH 1/3 v1] ALSA: usb-audio: more tolerant packetsize

[Takashi Iwai <tiwai@suse.de>]

On Thu, 01 Dec 2016 12:16:47 +0100,
Clemens Ladisch wrote:
> 
> Takashi Iwai wrote:
> > Clemens Ladisch wrote:
> >> Jiada Wang wrote:
> >>> since commit 57e6dae1087bbaa6b33d3dd8a8e90b63888939a3 the expected packetsize is always limited to
> >>> nominal + 25%. It was discovered, that some devices
> >>
> >> Which devices?
> >>
> >>> have a much higher jitter in used packetsizes than 25%
> >>
> >> How high?  (Please note that the USB specification restricts the jitter
> >> to at most one frame in consecutive packets.)
> >>
> >>> which would result in BABBLE condition and dropping of packets.
> >>> A better solution is so assume the jitter to be the nominal packetsize
> >>
> >> This solution is better for this one particular device, but how does it
> >> affect normal devices, or the Scarlett 2i4 on EHCI affected?
> >
> > Actually, which value does this affected device in ep->maxpacksize?
> > In the commit mentioned above, we changed the logic to take +25%
> > frequency as the basis, and it my *reduce* if ep->maxpacksize is lower
> > than that.
> >
> > OTOH, if ep->maxpacksize is sane, we can rely on it rather than the
> > implicit +25% frequency.  That said, maybe we can check
> > ep->maxpacksize whether it fits within the expected range, then adapt
> > it, or take +25% freq as fallback?
> 
> You are describing how the current code behaves.  The +25% limit _is_
> what the code takes as the expected range.

Well, the question is what is the "sane" range.  +25% doesn't fit for
some devices. If maxpacksize fits without +100% as this patch
suggests, can we rely on it instead?


Takashi

> 
> 
> I'm wondering if that unknown device just declares a wrong interval value.
> 
> 
> Regards,
> Clemens
>