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

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Message-ID: <CANiDSCvvCtkiHHPCj0trox-oeWeh_rks3Cqm+kS9Hvtp9QC6Yg@mail.gmail.com>
Date: Wed, 27 Nov 2024 09:58:21 +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 v4 1/2] media: uvcvideo: Support partial control reads

On Wed, 27 Nov 2024 at 09:34, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> 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.

Non integer datatypes might work if the last part of the data is
expected to be zero.
I do not think that we can find a heuristic that can work for all the cases.

For years we have ignored partial reads and it has never been an
issue. I vote for not adding any heuristics, the logging should help
identify future issues (if there is any).

>
> > 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.

I will send a new version, fixing the typo. and the missing new line.
I will also remove the sentence
`* In UVC the data is usually represented in little-endian.`
It is confusing.


>
> > > > +      * 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



-- 
Ricardo Ribalda