Date:   Mon, 19 Sep 2022 15:32:37 +0200
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 9/15/22 09:35, Benjamin Tissoires wrote:
> 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.

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

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