On Fri, Jul 19, 2013 at 5:20 AM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 18 Jul 2013, Clemens Ladisch wrote:
>
>> In any case, there must be _some_ mechanism to explicitly restart
>> a stream. I do not really care if this is some URB flag or some
>> function call.
>
> I prefer a function call over the flag. The function call can easily
> be issued just once, but the completion routine would have to clear the
> flag every time the URB gets used.
IMO, one good candidate is to do it in usb_set_interface(), and we may
avoid changes on most of drivers which are using isoc endpoints, also
it is reasonable, see blow:
>From "5.6.3 Isochronous Transfer Packet Size Constraints" of USB spec 2.0:
All device default interface settings must not include any isochronous
endpoints with non-zero data payload sizes (specified via
wMaxPacketSize
in the endpoint descriptor). Alternate interface settings may specify
non-zero data payload sizes for isochronous endpoints.
that said all drivers which are using isoc endpoints have to call
usb_set_interface(altsetting) explicitly before starting isoc schedule.
So for isoc drivers, it is very reasonable to call usb_set_interface(altsetting)
before starting streaming and call usb_set_interface(0) after stopping
streaming. I think we may document this usage, then use this info as
starting/stopping stream flag.
Looks both uvc and uac drivers support this way.
If one driver doesn't call usb_set_interface(0) after streaming off, we
can think it as buggy since the device still consumes bus bandwidth
unnecessary and may cause other device's bandwidth requirement not
satisfied on same bus.
>
> Maybe we can use usb_reset_endpoint() for this purpose after all. It
> is a perfect fit, because we want to tell the HCD to reset the
> isochronous endpoint back to the "start of stream" state.
I suggest not to introduce extra starting stream function to
usb_reset_endpoint(),
and if we have to do this, I suggest to add a new API for the purpose cleanly.
> A search under drivers/ shows that only a few HCDs other than ehci
> currently implement the endpoint_reset method: xhci, whci, dwc2, and
> ozwpan. It would not be hard to fix them up to ignore calls for
> isochronous endpoints.
>From xhci_endpoint_reset(), looks the hcd doesn't rule out isoc
endpoint.
Actually 'start of stream' should be done inside usbcore because it
is HCD-independent function, so it is better to provide the information
from usbcore and let HCD use it if HCD needs the flag.
Thanks,
--
Ming Lei
--
To unsubscribe from this list: send the line "unsubscribe linux-usb" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
