linux-kernel - Re: [RESEND PATCH 1/2] HID: Add driver for RC Simulator Controllers

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Date:   Thu, 15 Sep 2022 08:35:25 +0100
From:   Benjamin Tissoires <benjamin.tissoires@...hat.com>
To:     Marcus Folkesson <marcus.folkesson@...il.com>
Cc:     Jonathan Corbet <corbet@....net>, Jiri Kosina <jikos@...nel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>
Subject: Re: [RESEND PATCH 1/2] HID: Add driver for RC Simulator Controllers

On Thu, Sep 15, 2022 at 8:28 AM Marcus Folkesson
<marcus.folkesson@...il.com> wrote:
>
> Hi Benjamin,
>
> On Tue, Aug 30, 2022 at 02:45:11PM +0200, Benjamin Tissoires wrote:
> > On Thu, Aug 25, 2022 at 8:44 AM Marcus Folkesson
> > <marcus.folkesson@...il.com> wrote:
> > >
>
>
> [...]
> > >
> > >
> > > Is the fact that more than one button share the same
> > > byte hard to describe in the report?
> >
> > No, this is actually easy to describe. You say that there is one usage
> > of "something" which has a report size of 1 bit, and then you have
> > another usage of "something else" with the same report size.
> >
> > But usually you have to add padding after to make up to 8 bits (so 6
> > bits in that case).
> >
> > I was referring to the case  where you are parsing the same bit on the
> > wire, and give a different usage based if you have received an odd or
> > an even number of reports. In that case, we probably need to use move
> > this bit to a const field in the original report descriptor and say
> > that the data is now not const:
> >
> > - initial report (completely random example):
> >   X (2 bytes) | Y (2 bytes) | button this_or_that (1 bit, depending of
> > odd or even received reports) | 7 bits of padding
> > - we can declare it as:
> >   X (2 bytes) | Y (2 bytes) | button this (1 bit) | button that (1
> > bit) | 6 bits of padding
>
> How about if there is no unused bytes?
>
> The XTRG2FMS has 8 10-bit channels and use every byte in the report.
> Should I specify 8 8-bit channels instead and fix that in raw_event?
> If so, should I only use 8bit values then?

If I am not wrong, you should be able to add another byte in the
report descriptor, as long as your raw_event function always adds it.
Though now that I am typing it, I am actually wondering if this will
work. You can always try, there is a chance it'll work, but I can't
remember if it'll result in a timeout on the USB front because it'll
expect one more byte that will never arrive.

>
> (Are you at the ELCE conference btw?)

I was at Plumbers this week, but got an extra day today. But yeah, I'm
in Dublin today.

Cheers,
Benjamin

>
> Best regards
> Marcus Folkesson