linux-kernel - Re: [PATCH 07/10] media: uvcvideo: set error

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Date:   Thu, 11 Mar 2021 22:51:18 +0100
From:   Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Hans Verkuil <hverkuil@...all.nl>
Cc:     Ricardo Ribalda <ribalda@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tomasz Figa <tfiga@...omium.org>,
        linux-media <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, senozhatsky@...omium.org
Subject: Re: [PATCH 07/10] media: uvcvideo: set error_idx to count on EACCESS

Hi Laurent

On Thu, Mar 11, 2021 at 5:20 PM Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Ricardo,
>
> Thank you for the patch.

Thank you for the review :)

>
> On Thu, Mar 11, 2021 at 01:20:37PM +0100, Ricardo Ribalda wrote:
> > According to the doc:
>
> The previous paragraph states:
>
> This check is done to avoid leaving the hardware in an inconsistent
> state due to easy-to-avoid problems. But it leads to another problem:
> the application needs to know whether an error came from the validation
> step (meaning that the hardware was not touched) or from an error during
> the actual reading from/writing to hardware.

I think we wrote his standard when we were young and naive ;)

>
> > The, in hindsight quite poor, solution for that is to set error_idx to
> > count if the validation failed.
> >
> > Fixes v4l2-compliance:
> > Control ioctls (Input 0):
> >                 fail: v4l2-test-controls.cpp(645): invalid error index write only control
> >         test VIDIOC_G/S/TRY_EXT_CTRLS: FAIL
> >
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > ---
> >  drivers/media/usb/uvc/uvc_v4l2.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_v4l2.c b/drivers/media/usb/uvc/uvc_v4l2.c
> > index 625c216c46b5..9b6454bb2f28 100644
> > --- a/drivers/media/usb/uvc/uvc_v4l2.c
> > +++ b/drivers/media/usb/uvc/uvc_v4l2.c
> > @@ -1076,7 +1076,8 @@ static int uvc_ioctl_g_ext_ctrls(struct file *file, void *fh,
> >               ret = uvc_ctrl_get(chain, ctrl);
> >               if (ret < 0) {
> >                       uvc_ctrl_rollback(handle);
> > -                     ctrls->error_idx = i;
> > +                     ctrls->error_idx = (ret == -EACCES) ?
> > +                                             ctrls->count : i;
>
> No need for parentheses.

I really like my parenthesis before the ? :. Can I leave it? :)

If it bothers you a lot I remove it, but I like to delimit where the
ternary operators end.
>
> I'm not sure this is correct though. -EACCES is returned by
> __uvc_ctrl_get() when the control is found and is a write-only control.
> The uvc_ctrl_get() calls for the previous controls will have potentially
> touched the device to read the current control value if it wasn't cached
> already, to this contradicts the rationale from the specification.
>
> I understand the need for this when setting controls, but when reading
> them, it's more puzzling, as the interactions with the hardware to read
> the controls are not supposed to affect the hardware state in a way that
> applications should care about. It may be an issue in the V4L2
> specification.

I have no strong opinions in both directions. If v4l2-compliance is
the de-facto standard I believe we should keep this patch or change
the compliance test.

Hans, what do think?

>
> >                       return ret;
> >               }
> >       }
>
> --
> Regards,
>
> Laurent Pinchart



-- 
Ricardo Ribalda