[alsa-devel] A problem with USB audio devices and the UHCI scheduler

[Monty Montgomery]

A question for Clemens et al...

Background: I can run a USB audio device at 6ch/24 bit single duplex,
but I cannot run the same device at 16 bit stereo full duplex.  UHCI
and EHCI both say 'not enough bandwidth' (OHCI works).

EHCI's scheduling issues are well known, but I was surprised UHCI
could not schedule this either.  The cause is simple; UHCI (and EHCI)
use the endpoint's wMaxPacketSize for all bandwidth allocation.  My
device supports all of its sample rates on the same altset, with a
wMaxPacketSize matching the highest (96kHz) rate.  So, the HCD is
reserving most of the USB bus's bandwidth regardless of the actual
requirements.

How should I approach fixing this in the driver? There's currently no
means to communicate a desired bandwidth reservation to the HCD from
USB audio, so I'd have to extend even the quirks.  I was considering
shadowing the endpoint's original wMaxPacketSize in the substream
struct, and replacing the value in the endpoint's native descriptor
while the stream is running.  I have no idea how much trouble that would
cause (eg, corruption elsewhere, or middle finger from Linus).

Suggestions? For now I have things working by disabling UHCI's initial
bandwidth checks (as there is in fact more than enough bandwidth to
schedule what I'm doing) but I'd like a more permanent solution.

Monty