Re: [Xenomai] Write a hard real time usb driver

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2014-11-03 18:14, Gilles Chanteperdrix wrote:
> On Mon, Nov 03, 2014 at 06:12:11PM +0100, Jan Kiszka wrote:
>> On 2014-11-03 09:03, Jeroen Van den Keybus wrote:
>>>> I try to write a hard real time usb driver. I communicate my device  using
>>>> open(), read() etc. My device is a gpio module.
>>>
>>>
>>> I assume you want to control an USB host controller using a GPIO interface ?
>>>
>>>
>>>> I am coding a kernel module
>>>> to communicate the device using xenomai on real time. But I am not sure
>>>> that this way is reasonable. If I should going on this way, what should I
>>>> implement? Could you give me some example or link to write hard real time
>>>> usb driver for* physical* device?
>>>
>>>
>>> I cannot help you with that, but one important issue to consider is that
>>> the USB protocol itself operates with small, hardware controlled time slots
>>> of 1 ms (USB1.x) or 125 usec (USB2.0). You may therefore need a significant
>>> amount of software to properly distribute real-time traffic in these -
>>> essentially asynchronous - slots. The latency of a real-time process
>>> waiting for USB data can therefore also be as large as the time slot
>>> duration. If your real-time requirements are modest, you may be able to
>>> live with that. Still, count on a major software effort: just have a look
>>> at the Linux USB driver framework.
>>>
>>> Alternatively, you could check if you can synchronize your real-time
>>> process with the USB controller frame rate.
>>>
>>
>> FWIW, we still don't have USB 2.0 support (though I vaguely remember
>> previous efforts in that direction),
> 
> There is one here:
> http://sourceforge.net/projects/usb20rt.berlios/
> 
> Have not looked at the code though.

Yeah, that's the one I recalled. Seems dead, unfortunately.

Jan

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