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

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Date:   Thu, 1 Dec 2016 12:16:47 +0100
From:   Clemens Ladisch <clemens@...isch.de>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Jiada Wang <jiada_wang@...tor.com>, alsa-devel@...a-project.org,
        apape@...adit-jv.com, linux-kernel@...r.kernel.org,
        Mark_Craske@...tor.com
Subject: Re: [alsa-devel] [PATCH 1/3 v1] ALSA: usb-audio: more tolerant
 packetsize

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.


I'm wondering if that unknown device just declares a wrong interval value.


Regards,
Clemens