linux-kernel - Re: [PATCH v4 1/2] media: uvcvideo: Support partial control reads

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Message-ID: <20241127083444.GV5461@pendragon.ideasonboard.com>
Date: Wed, 27 Nov 2024 10:34:44 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
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 v4 1/2] media: uvcvideo: Support partial control reads

On Tue, Nov 26, 2024 at 07:12:53PM +0100, Ricardo Ribalda wrote:
> On Tue, 26 Nov 2024 at 19:06, Laurent Pinchart wrote:
> > On Wed, Nov 20, 2024 at 03:26:19PM +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()")
> > > Reviewed-by: Hans de Goede <hdegoede@...hat.com>
> > > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > > ---
> > >  drivers/media/usb/uvc/uvc_video.c | 16 ++++++++++++++++
> > >  1 file changed, 16 insertions(+)
> > >
> > > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > > index cd9c29532fb0..482c4ceceaac 100644
> > > --- a/drivers/media/usb/uvc/uvc_video.c
> > > +++ b/drivers/media/usb/uvc/uvc_video.c
> > > @@ -79,6 +79,22 @@ int uvc_query_ctrl(struct uvc_device *dev, u8 query, u8 unit,
> > >       if (likely(ret == size))
> > >               return 0;
> > >
> > > +     /*
> > > +      * In UVC the data is usually represented in little-endian.
> >
> > I had a comment about this in the previous version, did you ignore it on
> > purpose because you disagreed, or was it an oversight ?
> 
> I rephrased the comment. I added "usually" to make it clear that it
> might not be the case for all the data types. Like composed or xu.

Ah, that's what you meant by "usually". I read it as "usually in
little-endian, but could be big-endian too", which confused me.

Data types that are not integers will not work nicely with the
workaround below. How do you envision that being handled ? Do you
consider that the device will return too few bytes only for integer data
types, or that affected devices don't have controls that use compound
data types ? I don't see what else we could do so I'd be fine with such
a heuristic for this workaround, but it needs to be clearly explained.

> I also r/package/packet/
> 
> Did I miss another comment?
> 
> > > +      * Some devices return shorter USB control packets that expected if the
> > > +      * returned value can fit in less bytes. Zero all the bytes that the
> > > +      * device have not written.
> >
> > s/have/has/
> >
> > And if you meant to start a new paragraph here, a blank line is missing.
> > Otherwise, no need to break the line before 80 columns.
> 
> The patch is already in the uvc tree. How do you want to handle this?

The branch shared between Hans and me can be rebased, it's a staging
area.

> > > +      * We exclude UVC_GET_INFO from the quirk. UVC_GET_LEN does not need to
> > > +      * be excluded because its size is always 1.
> > > +      */
> > > +     if (ret > 0 && query != UVC_GET_INFO) {
> > > +             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,
> > >                       "Failed to query (%s) UVC control %u on unit %u: %d (exp. %u).\n",

-- 
Regards,

Laurent Pinchart