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SubjectRe: [PATCH v5 0/6] usb: gadget: add mechanism to asynchronously validate data stage of ctrl out request
On Thu, Jan 10, 2019 at 03:39:25PM -0500, Alan Stern wrote:
> On Wed, 9 Jan 2019, Paul Elder wrote:
> > This patch series adds a mechanism to allow asynchronously validating
> > the data stage of a control OUT request, and for stalling or suceeding
> > the request accordingly.
> One thing we haven't mentioned explicitly: What should happen when the
> time for the status stage rolls around if the gadget driver queues a
> non-zero length request?

Ah, yeah, I missed that.

> This can happen in a few different ways. One obvious possibility is
> that the gadget driver sets the explicit_status flag and then submits a
> non-zero length request. Another is that the gadget driver submits
> _two_ requests during the data stage (the second would be interpreted
> as the status-stage request). A third is that the gadget driver
> submits a data-stage request that is too long and the excess portion is
> used for the status stage.
> My feeling is that the behavior in these cases should officially be
> undefined. Almost anything could happen: the status stage could STALL,
> it could succeed, it could NAK, or it could send a non-zero packet to
> the host. The request could return with 0 status or an error status,
> and req->actual could take on any reasonable value.
> Alternatively, the UDC driver could detect these errors and report them
> somehow. Maybe STALL the status stage and complete the request with
> -EPIPE status or some such thing.
> Any preferences or other ideas?

I think error detection and reporting would be useful. The question is
what action to take after that; either leave it undefined or STALL. I
think STALL would be fine, since if a non-zero length request is
submitted for a status stage, intentionally or not, it isn't part of
proper behavior and should count as an error.

> One other thing: Some UDC drivers may assume that the data stage of a
> control transfer never spans more than a single usb_request. Should
> this become an official requirement?

Would the data stage of a control transfer ever need more space than a
single usb_request can contain? I know UVC doesn't; that's why we pack
it together with the setup stage data in 3/6. If so, I would think we
can make it a requirement.


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