Re: [RFC PATCH 1/2] HID: uhid: Don't send the report ID if it's zero

[David Rheinsberg <david.rheinsberg@gmail.com>]

Hi

On Mon, 5 Dec 2022 at 22:04, Andrey Smirnov <andrew.smirnov@gmail.com> wrote:
>
> Report ID of zero is a special case handling ID-less reports and in
> that case we should omit report ID from the payload being sent to the
> backend.
>
> Without this change UHID_DEV_NUMBERED_{FEATURE,OUTPUT}_REPORTS doesn't
> represent a semantical difference.
>
> Cc: David Rheinsberg <david.rheinsberg@gmail.com>
> Cc: Jiri Kosina <jikos@kernel.org>
> Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
> Cc: linux-input@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-usb@vger.kernel.org
> Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
> ---
>  drivers/hid/uhid.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 2a918aeb0af1..7551120215e8 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -273,11 +273,11 @@ static int uhid_hid_get_report(struct hid_device *hid, unsigned char rnum,
>  }
>
>  static int uhid_hid_set_report(struct hid_device *hid, unsigned char rnum,
> -                              const u8 *buf, size_t count, u8 rtype)
> +                              u8 *buf, size_t count, u8 rtype)
>  {
>         struct uhid_device *uhid = hid->driver_data;
>         struct uhid_event *ev;
> -       int ret;
> +       int ret, skipped_report_id = 0;
>
>         if (!READ_ONCE(uhid->running) || count > UHID_DATA_MAX)
>                 return -EIO;
> @@ -286,6 +286,15 @@ static int uhid_hid_set_report(struct hid_device *hid, unsigned char rnum,
>         if (!ev)
>                 return -ENOMEM;
>
> +       /* Byte 0 is the report number. Report data starts at byte 1.*/
> +       buf[0] = rnum;
> +       if (buf[0] == 0x0) {
> +               /* Don't send the Report ID */
> +               buf++;
> +               count--;
> +               skipped_report_id = 1;
> +       }
> +

In HID core, the buffer is filled by a call to hid_output_report() in
__hid_request(). And hid_output_report() only writes the ID if it is
non-zero. So your patch looks like it is duplicating this logic? In
which scenario is the report-ID not skipped exactly?

Regardless, if you want to mess with the buffer, you should do that
after the memcpy(). I don't see why we should mess with the buffer
from HID core, when we have our own, anyway.

David