linux-kernel - Re: [PATCH v3 1/2] media: uvcvideo: Support partial control reads

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Message-ID: <CANiDSCvazZ4Y3OZ9X7chU-_N-4HbeQKUh23eOWkmkAxGaks2QA@mail.gmail.com>
Date: Wed, 20 Nov 2024 15:43:22 +0100
From: Ricardo Ribalda <ribalda@...omium.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Hans de Goede <hdegoede@...hat.com>, Mauro Carvalho Chehab <mchehab@...nel.org>, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Sakari Ailus <sakari.ailus@...ux.intel.com>, 
	stable@...r.kernel.org
Subject: Re: [PATCH v3 1/2] media: uvcvideo: Support partial control reads

Hi Laurent

On Wed, 20 Nov 2024 at 15:05, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Ricardo,
>
> Thank you for the patch.
>
> On Mon, Nov 18, 2024 at 05:16:51PM +0000, Ricardo Ribalda wrote:
> > Some cameras, like the ELMO MX-P3, do not return all the bytes
> > requested from a control if it can fit in less bytes.
> > Eg: Returning 0xab instead of 0x00ab.
> > usb 3-9: Failed to query (GET_DEF) UVC control 3 on unit 2: 1 (exp. 2).
> >
> > Extend the returned value from the camera and return it.
> >
> > Cc: stable@...r.kernel.org
> > Fixes: a763b9fb58be ("media: uvcvideo: Do not return positive errors in uvc_query_ctrl()")
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > ---
> >  drivers/media/usb/uvc/uvc_video.c | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > index cd9c29532fb0..e165850397a0 100644
> > --- a/drivers/media/usb/uvc/uvc_video.c
> > +++ b/drivers/media/usb/uvc/uvc_video.c
> > @@ -76,8 +76,22 @@ int uvc_query_ctrl(struct uvc_device *dev, u8 query, u8 unit,
> >
> >       ret = __uvc_query_ctrl(dev, query, unit, intfnum, cs, data, size,
> >                               UVC_CTRL_CONTROL_TIMEOUT);
> > -     if (likely(ret == size))
> > +     if (ret > 0) {
> > +             if (size == ret)
> > +                     return 0;
>
> Why is this within the ret > 0 block ? I would write
>
>         if (likely(ret == size))
>                 return 0;
>
>         if (ret > 0) {
>
> > +
> > +             /*
> > +              * In UVC the data is represented in little-endian by default.
>
> By default, or always ?
>
> > +              * Some devices return shorter control packages that expected
>
> What's a "control package" ?

usb control transfers.
>
> I think you meants "than expected", not "that expected".
>
> > +              * if the return value can fit in less bytes.
> > +              * Zero all the bytes that the device have not written.
> > +              */
>
> Do we want to apply this workaround to GET_INFO and GET_LEN, or can we
> restrict it to GET_CUR, GET_MIN, GET_MAX and GET_RES ?

I believe that the original behaviour before
a763b9fb58be ("media: uvcvideo: Do not return positive errors in
uvc_query_ctrl()")
was used for all types. I think the safest thing to do is to go back
to the old behaviour.

Let me know what you prefer.


>
> > +             memset(data + ret, 0, size - ret);
> > +             dev_warn_once(&dev->udev->dev,
> > +                           "UVC non compliance: %s control %u on unit %u returned %d bytes when we expected %u.\n",
> > +                           uvc_query_name(query), cs, unit, ret, size);
> >               return 0;
> > +     }
> >
> >       if (ret != -EPIPE) {
> >               dev_err(&dev->udev->dev,
> >
>
> --
> Regards,
>
> Laurent Pinchart



-- 
Ricardo Ribalda