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From:   Doug Anderson <dianders@chromium.org>
Date:   Wed, 10 Jul 2019 15:39:52 -0700
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Subject: Re: [PATCH v5 2/7] drm/panel: simple: Add ability to override typical timing
To:     Sam Ravnborg <sam@ravnborg.org>
Cc:     Thierry Reding <thierry.reding@gmail.com>,
        Heiko Stuebner <heiko@sntech.de>,
        Sean Paul <seanpaul@chromium.org>, devicetree@vger.kernel.org,
        Rob Herring <robh+dt@kernel.org>,
        David Airlie <airlied@linux.ie>,
        Jeffy Chen <jeffy.chen@rock-chips.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@lists.infradead.org>,
        Boris Brezillon <boris.brezillon@collabora.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        =?UTF-8?Q?Enric_Balletb=C3=B2?= <enric.balletbo@collabora.com>,
        =?UTF-8?Q?St=C3=A9phane_Marchesin?= <marcheu@chromium.org>,
        Ezequiel Garcia <ezequiel@collabora.com>,
        Matthias Kaehlcke <mka@chromium.org>
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Sam,

On Mon, Jul 8, 2019 at 10:50 AM Sam Ravnborg <sam@ravnborg.org> wrote:
>
> Hi Dough.
>
> On Mon, Jul 01, 2019 at 09:39:24AM -0700, Doug Anderson wrote:
> > Hi,
> >
> > On Sun, Jun 30, 2019 at 1:22 PM Sam Ravnborg <sam@ravnborg.org> wrote:
> > >
> > > > @@ -91,6 +92,8 @@ struct panel_simple {
> > > >       struct i2c_adapter *ddc;
> > > >
> > > >       struct gpio_desc *enable_gpio;
> > > > +
> > > > +     struct drm_display_mode override_mode;
> > > I fail to see where this poiter is assigned.
> >
> > In panel_simple_parse_override_mode().  Specifically:
> >
> > drm_display_mode_from_videomode(&vm, &panel->override_mode);
>
> The above code-snippet is only called in the panel has specified display
> timings using display_timings - it is not called when display_mode is
> used.
> So override_mode is only assigned in some cases and not all cases.
> This needs to be fixed so we do not reference override_mode unless
> it is set.

I'm afraid I'm not following you here.

* override_mode is a structure that's directly part of "struct panel_simple".

* The panel is allocated in panel_simple_probe() with devm_kzalloc().

* The "z" in kzalloc means that this memory will be zero-initialized.

From the points above, "override_mode" will always be set to
something.  If we didn't run "drm_display_mode_from_videomode(&vm,
&panel->override_mode);" then we know the entire override_mode
structure will be zero.

While it took a while for me to get used to it, the kernel convention
is to rely on zero-initialization and not to explicitly init things to
zero.  As an example of this being codified in the source, you can see
that "checkpatch.pl" will yell at you for a similar thing: "do not
initialise globals to 0".


> > > @@ -152,6 +162,44 @@ static int panel_simple_get_fixed_modes(struct panel_simple *panel)
> > > >               num++;
> > > >       }
> > > >
> > > > +     return num;
> > > > +}
> > > > +
> > > > +static int panel_simple_get_non_edid_modes(struct panel_simple *panel)
> > > > +{
> > > > +     struct drm_connector *connector = panel->base.connector;
> > > > +     struct drm_device *drm = panel->base.drm;
> > > > +     struct drm_display_mode *mode;
> > > > +     bool has_override = panel->override_mode.type;
> > > This looks suspicious.
> > > panel->override_mode.type is an unsigned int that may have a number of
> > > bits set.
> > > So the above code implicitly convert a .type != 0 to a true.
> > > This can be expressed in a much more reader friendly way.
> >
> > You would suggest that I add a boolean field to a structure to
> > indicate whether an override mode is present?
> A simple  bool has_override = panel->override_mode.type != 0;
> would do the trick here.
> Then there is no hidden conversion from int to a bool.

I will change this to "panel->override_mode.type != 0" if you're
really sure, but this seems both against the general Linux style
feedback I've received over the years (though there is definitely not
100% consistency) and also against the local convention in this file.
Examples in this file of treating ints as bools without an explicit
"!= 0":

* panel_simple_get_fixed_modes checks "if (panel->desc->bus_format)"
* panel_simple_disable checks "if (p->desc->delay.disable)"
* panel_simple_unprepare checks "if (p->desc->delay.unprepare)"
* panel_simple_prepare checks "if (delay)"
* panel_simple_enable checks "if (p->desc->delay.enable)"

...and, although slightly different, pointers in this file are checked
for NULL vs. non-NULL without an explicit "== NULL".

Of course just because all the other examples in the file do it one
way doesn't mean that new code has to do it another way, but I wanted
to be really sure you wanted me to go against the existing convention
before changing this.


> But as override_mode can be NULL something more needs to be done.

I'm afraid I don't understand how override_mode can be NULL since it's
not a pointer.  Can you clarify?


-Doug