Re: [RFC PATCH 0/2] Handling of non-numbered feature reports by hidraw

[David Rheinsberg]

Hi

On Thu, 8 Dec 2022 at 21:59, Andrey Smirnov <andrew.smirnov@xxxxxxxxx> wrote:
>
> On Thu, Dec 8, 2022 at 7:46 AM David Rheinsberg
> <david.rheinsberg@xxxxxxxxx> wrote:
> >
> > Hi
> >
> > On Mon, 5 Dec 2022 at 22:04, Andrey Smirnov <andrew.smirnov@xxxxxxxxx> wrote:
> > > I'm working on a firmware of a device that exposes a HID interface via
> > > USB and/or BLE and uses, among other things, non-numbered feature
> > > reports. Included in this series are two paches I had to create in
> > > order for hidraw devices created for aforementioned subsystems to
> > > behave in the same way when exerciesd by the same test tool.
> > >
> > > I don't know if the patches are acceptable as-is WRT to not breaking
> > > existing userspace, hence the RFC tag.
> >
> > Can you elaborate why you remove the special handling from USBHID but
> > add it to UHID? They both operate logically on the same level, so
> > shouldn't we simply adjust uhid to include the report-id in buf[0]?
> >
> > Also, you override buf[0] in UHID, so I wonder what UHID currently
> > returns there?
> >
> > IOW, can you elaborate a bit what the current behavior of each of the
> > involved modules is, and what behavior you would expect? This would
> > allow to better understand what you are trying to achieve. The more
> > context you can give, the easier it is to understand what happens
> > there.
> >
>
> Sorry it's not very clear, so the difference between the cases is that
> in the case of UHID the report ID ends up being included as a part of
> "SET_FEATURE", so BlueZ checks UHID_DEV_NUMBERED_FEATURE_REPORTS,
> which is not set (correctly) and tries to send the whole payload. This
> ends up as a maxlen + 1 (extra byte) write to a property that is
> maxlen long, which gets rejected by device's BLE stack.
>
> In the case of USBHID the problem happens in "GET_FEATURE" path. When
> userspace reads the expected data back it gets an extra 0 prepended to
> the payload, so all of the actual payload has an offset of 1. This
> doesn't happen with UHID, which I think is the correct behavior here.
>
> Hopefully that explains the difference, let me know if something is unclear

Yes, thanks, I completely missed that. Lets continue discussion on the patches.

Thanks!
David