linux-kernel - Re: [PATCH 14/14] [media] fix warning on v4l2_subdev

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Message-ID: <CAK8P3a1zBW_QuPtRFNwuVyE_ziySoV9_ebz4sD7Bya3eRoo8SA@mail.gmail.com>
Date:   Mon, 17 Jul 2017 16:26:23 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Guenter Roeck <linux@...ck-us.net>,
        IDE-ML <linux-ide@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Niklas Söderlund <niklas.soderlund@...natech.se>,
        Robert Jarzmik <robert.jarzmik@...e.fr>,
        Daeseok Youn <daeseok.youn@...il.com>,
        Alan Cox <alan@...ux.intel.com>,
        adi-buildroot-devel@...ts.sourceforge.net,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        devel@...verdev.osuosl.org
Subject: Re: [PATCH 14/14] [media] fix warning on v4l2_subdev_call() result
 interpreted as bool

On Mon, Jul 17, 2017 at 3:45 PM, Hans Verkuil <hverkuil@...all.nl> wrote:
> On 14/07/17 11:36, Arnd Bergmann wrote:
>> @@ -201,8 +202,9 @@ static int cx18_g_fmt_sliced_vbi_cap(struct file *file, void *fh,
>>        * digitizer/slicer.  Note, cx18_av_vbi() wipes the passed in
>>        * fmt->fmt.sliced under valid calling conditions
>>        */
>> -     if (v4l2_subdev_call(cx->sd_av, vbi, g_sliced_fmt, &fmt->fmt.sliced))
>> -             return -EINVAL;
>> +     ret = v4l2_subdev_call(cx->sd_av, vbi, g_sliced_fmt, &fmt->fmt.sliced);
>> +     if (ret)
>> +             return ret;
>
> Please keep the -EINVAL here. I can't be 100% certain that returning 'ret' wouldn't
> break something.

I think Dan was recommending the opposite here, if I understood you
both correctly:
he said we should propagate the error code unless we know it's wrong, while you
want to keep the current behavior to avoid introducing changes ;-)

I guess in either case, looking at the callers more carefully would be
a good idea.

>> -     return 0;
>> +     return ret;
>>  }
>>
>>  int atomisp_flash_enable(struct atomisp_sub_device *asd, int num_frames)
>>
>
> This is all very hackish, though. I'm not terribly keen on this patch. It's not
> clear to me *why* these warnings appear in your setup.

it's possible that this only happened with 'ccache', which first preprocesses
the source and the passes it with v4l2_subdev_call expanded into the
compiler. This means the line looks like

        if ((!(cx->sd_av) ? -ENODEV :
            (((cx->sd_av)->ops->vbi && (cx->sd_av)->ops->vbi->g_sliced_fmt) ?
               (cx->sd_av)->ops->vbi->g_sliced_fmt(cx->sd_av)),
&fmt->fmt.sliced) :
               -ENOIOCTLCMD))

The compiler now complains about the sub-expression that it sees for
cx->sd_av==NULL:

   if (-ENODEV)

which it considers nonsense because it is always true and the value gets
ignored.

Let me try again without ccache for now and see what warnings remain.
We can find a solution for those first, and then decide how to deal with
ccache.

        Arnd