Re: Feedback endpoint poll rate

Greetings,

I have never worked with ISO feedback endpoints.

libusbK doesn't do anything special for a ISO feedback endpoint nor does 
it manage the polling period (bInterval). I *beleive* the polling period 
is managed by usbhub.sys but I'm not certain.

That said, you may experiment with the following ISO pipe policies:
#define ISO_START_LATENCY        0x20
#define ISO_ALWAYS_START_ASAP    0x21

..but I don't think these are going to help you.

Here is a windows specific link for dealing with ISO that might be of 
some use:
http://msdn.microsoft.com/en-us/library/windows/hardware/hh406225%28v=vs.85%29.aspx

Regards,
Travis

On 4/11/2014 5:30 AM, Børge Strand-Bergesen wrote:
> 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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