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

[Benjamin Tissoires]

On 9/15/22 09:35, Benjamin Tissoires wrote:
> On Thu, Sep 15, 2022 at 8:28 AM Marcus Folkesson
> <marcus.folkesson@xxxxxxxxx> 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@xxxxxxxxx> 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.

I am back home, and I just tested that. I had a doubt, and it is indeed
failing. You need the following change for this to be working (I need to
send it as a proper patch after assessing it hasn't side effects)

---

diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
index 13cce286247e..f37ffe2bd488 100644
--- a/drivers/hid/usbhid/hid-core.c
+++ b/drivers/hid/usbhid/hid-core.c
@@ -275,6 +275,7 @@ static void hid_irq_in(struct urb *urb)
        int                     status;
 
        switch (urb->status) {
+       case -EOVERFLOW:        /* happens with modified report descriptors */
        case 0:                 /* success */
                usbhid->retry_delay = 0;
                if (!test_bit(HID_OPENED, &usbhid->iofl))
---

Cheers,
Benjamin

> > (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