Re: [PATCH] USB: note that usb_fill_int_urb() can be used used for ISOC urbs.

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On Wed, 20 Jun 2018, Sebastian Andrzej Siewior wrote:

> Laurent suggested that the kerneldoc documentation could state that
> usb_fill_int_urb() can also be used for the initialisation of an
> isochronous urb. The USB documentation in
> Documentation/driver-api/usb/URB.rst already mentions this, some drivers
> do so and there is no explicit usb_fill_iso_urb().
> 
> Suggested-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> On 2018-06-20 17:14:53 [+0300], Laurent Pinchart wrote:
> > > So you simply asking that the kerneldoc of usb_fill_int_urb() is
> > > extended to mention isoc, too?
> > 
> > That would be nice I think.
> 
> here it is.
> 
>  include/linux/usb.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/usb.h b/include/linux/usb.h
> index 4cdd515a4385..c3a8bd586121 100644
> --- a/include/linux/usb.h
> +++ b/include/linux/usb.h
> @@ -1657,7 +1657,8 @@ static inline void usb_fill_bulk_urb(struct urb *urb,
>   *	the endpoint descriptor's bInterval value.
>   *
>   * Initializes a interrupt urb with the proper information needed to submit
> - * it to a device.
> + * it to a device. This function can also be used to initialize an isochronous
> + * urb.

No, no!  This function can _help_ initialize an isochronous URB, but
that's all.  It would be better to create an explicit 
usb_fill_isoc_urb() routine, and even that would have to be incomplete.

There are two problems with using usb_fill_int_urb() to initialize an 
isochronous URB:

	The calculation of the interval value is wrong for full-speed
	devices.

	The routine does not set urb->number_of_packets or
	urb->iso_frame_desc[].

Alan Stern

>   *
>   * Note that High Speed and SuperSpeed(+) interrupt endpoints use a logarithmic
>   * encoding of the endpoint interval, and express polling intervals in
> 




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