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SubjectRe: [PATCH v5 0/6] usb: gadget: add mechanism to asynchronously validate data stage of ctrl out request
On Fri, 11 Jan 2019, Paul Elder wrote:

> 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.

Okay; I will have to change the code in dummy-hcd to do this. You
might need to update musb as well.

> > 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.

The data stage of a control transfer cannot be larger than 64 KB.
Certainly a single usb_request can handle that; the question concerns
whether a function driver might want to split the data up among several
different requests just for convenience.

Felipe, any thoughts?

Alan Stern

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