linux-kernel - Re: [PATCH v5 2/7] drm/panel: simple: Add ability to override typical timing

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Message-ID: <20190708175606.GB3511@ravnborg.org>
Date:   Mon, 8 Jul 2019 19:56:06 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     devicetree@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        Jeffy Chen <jeffy.chen@...k-chips.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Boris Brezillon <boris.brezillon@...labora.com>,
        Enric Balletbò <enric.balletbo@...labora.com>,
        Stéphane Marchesin <marcheu@...omium.org>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v5 2/7] drm/panel: simple: Add ability to override
 typical timing

On Mon, Jul 01, 2019 at 09:39:06AM -0700, Doug Anderson wrote:
> Hi,
> 
> On Sun, Jun 30, 2019 at 1:55 PM Sam Ravnborg <sam@...nborg.org> wrote:
> >
> > Hi Douglas.
> >
> > > > +
> > > > +   /* Only add timings if override was not there or failed to validate */
> > > > +   if (num == 0 && panel->desc->num_timings)
> > > > +           num = panel_simple_get_timings_modes(panel);
> > > > +
> > > > +   /*
> > > > +    * Only add fixed modes if timings/override added no mode.
> > >
> > > This part I fail to understand.
> > > If we have a panel where we in panel-simple have specified the timings,
> > > and done so using display_timing so with proper {min, typ, max} then it
> > > should be perfectly legal to specify a more precise variant in the DT
> > > file.
> > > Or what did I miss here?
> >
> > Got it now.
> > If display_mode is used for timings this is what you call "fixed mode".
> > Hmm, if I got confused someone else may also be confused by this naming.
> 
> The name "fixed mode" comes from the old code, though I guess in the
> old code it used to refer to a mode that came from either the
> display_timing or the display_mode.
> 
> How about if I call it "panel_simple_get_from_fixed_display_mode"?
> ...or if you have another suggestion feel free to chime in.
What we really want to distingush here is the use of display_mode
and display_timings (if I got the names right).
That display_mode specify a fixed timing and display_timing specify
a valid range is something in the semantics of the two types.
So naming that refer to display_mode versus display_timing will make the
code simpler to understand. and then a nice comment that when
display_mode
is used one looses the possibility to use override_mode.
That would be fine to have in the struct in the driver.

> NOTE: Since this feedback is minor and this patch has been outstanding
> for a while (and is blocking other work), I am assuming that the best
> path forward is for Heiko to land this patch with Thierry's Ack and
> I'll send a follow-up.  Please yell if you disagree.
Let's give the patches a spin more as we have passed the possibility for
the current merge window.

I am on vacation at the moment and thus slow in responses, but will be back
at the home office next week and will be more responsive again.

	Sam - who is enjoying the alps in Austria