[PATCH v4 06/13] drm/bridge: lvds-codec: Add "lvds-decoder" support

[Geert Uytterhoeven]

Hi Fabrizio,

On Tue, Dec 17, 2019 at 1:31 PM Fabrizio Castro
<fabrizio.castro at bp.renesas.com> wrote:
> > From: linux-renesas-soc-owner at vger.kernel.org <linux-renesas-soc-owner at vger.kernel.org> On Behalf Of Geert Uytterhoeven
> > Sent: 17 December 2019 12:21
> > Subject: Re: [PATCH v4 06/13] drm/bridge: lvds-codec: Add "lvds-decoder" support
> >
> > On Tue, Dec 17, 2019 at 12:03 PM Fabrizio Castro
> > <fabrizio.castro at bp.renesas.com> wrote:
> > > > From: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > > > Sent: 13 December 2019 17:11
> > > > Subject: Re: [PATCH v4 06/13] drm/bridge: lvds-codec: Add "lvds-decoder" support
> > > >
> > > > On Wed, Nov 13, 2019 at 03:51:25PM +0000, Fabrizio Castro wrote:
> > > > > Add support for transparent LVDS decoders by adding a new
> > > > > compatible string ("lvds-decoder") to the driver.
> > > > > This patch also adds member connector_type to struct lvds_codec,
> > > > > and that's because LVDS decoders have a different connector type
> > > > > from LVDS encoders. We fill this new member up with the data
> > > > > matching the compatible string.
> > > > >
> > > > > Signed-off-by: Fabrizio Castro <fabrizio.castro at bp.renesas.com>

> > > > > --- a/drivers/gpu/drm/bridge/lvds-codec.c
> > > > > +++ b/drivers/gpu/drm/bridge/lvds-codec.c

> > > > > @@ -65,6 +67,7 @@ static int lvds_codec_probe(struct platform_device *pdev)
> > > > >     if (!lvds_codec)
> > > > >             return -ENOMEM;
> > > > >
> > > > > +   lvds_codec->connector_type = (u32)of_device_get_match_data(&pdev->dev);
> > > >
> > > > I'm now getting a compilation failure here:
> > > >
> > > > drivers/gpu/drm/bridge/lvds-codec.c: In function ‘lvds_codec_probe’:
> > > > drivers/gpu/drm/bridge/lvds-codec.c:68:31: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
> > > >   lvds_codec->connector_type = (u32)of_device_get_match_data(&pdev->dev);
> > > >                                ^
> > > >
> > > > The fix should be simple:
> > > >
> > > >       lvds_codec->connector_type = (uintptr_t)of_device_get_match_data(dev);
> > > >
> > > > I'm bothered by the fact that I've compiled this before without any
> > > > issue, so this really puzzles me. Do you get the same warning ?
> > >
> > > The warning appears when compiling for arm64, understandably so.
> > > We must have compiled this for arm only the first time around.
> > >
> > > I think the right way to solve this is to either cast to (u32)(uintptr_t) or (u32)(unsigned long).
> >
> > Just casting to uintptr_t should be sufficient.
>
> It should be sufficient for the compiler, but I have seen examples where people
> preferred to be explicit, like in:
> drivers/mailbox/mtk-cmdq-mailbox.c
> drivers/leds/leds-pm8058.c
>
> Since the kernel is increasing its tightness with respect to warnings, I personally prefer
> (u32)(uintptr_t), even though not strictly necessary, but I am fine with (uintptr_t) if you
> don't like (u32)(uintptr_t).

It depends. I try to have as few casts as possible ("casts are evil").

While adding the extra (u32) cast makes it clearer that the intended
result is a u32 (for now), it will cause silent truncation on 64-bit if
connector_type is ever enlarged to unsigned long, and larger values are
used.

In this particular case this is unlikely, though, as unsigned long would
still be 32-bit on 32-bit platforms, so the larger values cannot be
used.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds