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

Hi

On Thu, 8 Dec 2022 at 21:59, Andrey Smirnov [off-list ref] wrote:

On Thu, Dec 8, 2022 at 7:46 AM David Rheinsberg
[off-list ref] wrote:

quoted

Hi

On Mon, 5 Dec 2022 at 22:04, Andrey Smirnov [off-list ref] wrote:

quoted

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