Re: Feedback endpoint poll rate

Hi guys,

any input appreciated on how the ep poll rate is set in a libusbK.sys driver.

Thanks,
Borge

On Thu, Apr 10, 2014 at 10:32 AM, Børge Strand-Bergesen
<[email protected]> wrote:
> Hi guys,
>
> I'm maintaining an audio ASIO driver for USB Audio Class 2 (i.e.
> Hi-Speed).  It uses libusbK. Descriptor dump from tdd.exe attached.
> Code: https://github.com/nikkov/Win-Widget/tree/exp_asio It builds
> with VC++08.
>
> Most of it works quite well, but I have trouble setting the poll rate
> for a feedback endpoint. This ep lets the Device tell the Host if the
> latter needs to adjust the audio sample rate. That's at the core of
> asynchronous USB audio.
>
> In line 220 bInterval = 0x04. The Device uses 250us microframes.
> Regardless of what I set this bInterval to, the Windows driver polls
> the endpoint at a 1ms rate. OS X and Linux support it in built-in
> class drivers. OS X polls the ep at quite a different rate. (4 or 8ms
> if I remember correctly.) The 0x04 value is a compromize where both
> libusbK and OS X are functional. In Linux I haven't yet correlated
> bInterval to the actual poll rate.
>
> I'd like to significantly slow down the poll rate. A 32 or 64ms
> interval will probably work just fine in the application and reduce
> the load on both Host and Device.
>
> I don't know if there is a bug in the descriptor.
>
> Question: What part of the chain (OS, low-level USB driver,
> libusbK.sys, libusbk.dll, user mode application) does the actual ep
> poll timing? When parsing this value from the descriptor, how is the
> poll timing set up?
>
> I have control over the user mode application and Device firmware. And
> in both I see that correct feedback data is received and generated,
> respectively. I just don't know how to trace it through the Host and
> back to the Device.
>
>
> Thanks,
>
> Borge

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