[alsa-devel] [PATCH 4/9 v2] ALSA: usb-audio: use usb_fill_int_urb()

[Sebastian Andrzej Siewior]

On 2018-06-20 15:21:49 [+0200], Takashi Iwai wrote:
> > usb_fill_int_urb() ensures that syncinterval is within the allowed range
> > on HS/SS. The interval value seems to come from
> > snd_usb_parse_datainterval() which is bInterval - 1 and only in the rage
> > 1 … 4. So in order to keep the magic working I pass datainterval + 1.
> 
> This needs more explanation.  By this conversion, the interval
> computation becomes really tricky.
> 
> There are two interval calculations.  The first one is
> fp->datainterval and it's from snd_usb_parse_datainterval() as you
> mentioned.  And a tricky part is that fp->datainterval is 0 on
> FULL_SPEED.  Casually, fp->datainterval+1 will be translated to the
> correct value since (0 + 1) == (1 << 0).
> 
> OTOH, for the sync EP, it's taken from ep->syncinterval, which is set
> in snd_usb_add_endpoint().  Luckily, again, ep->syncinterval + 1 works
> for FULL_SPEED as well, as (1 + 1) == (1 << 1).

Do you want me to add your additional explanation to the description?
 
> thanks,
> 
> Takashi

Sebastian