Re: [PATCH 1/3] drm: panel: simple: Allow timing constraints, not fixed delays

From: Sam Ravnborg <sam@ravnborg.org>
To: Thierry Reding <thierry.reding@gmail.com>,
	Douglas Anderson <dianders@chromium.org>,
	robdclark@chromium.org, Rob Herring <robh+dt@kernel.org>,
	dri-devel@lists.freedesktop.org, David Airlie <airlied@linux.ie>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] drm: panel: simple: Allow timing constraints, not fixed delays
Date: Tue, 27 Oct 2020 20:51:52 +0100	[thread overview]
Message-ID: <20201027195152.GA457661@ravnborg.org> (raw)
In-Reply-To: <20201027192318.GR401619@phenom.ffwll.local>

On Tue, Oct 27, 2020 at 08:23:18PM +0100, Daniel Vetter wrote:
> On Tue, Oct 27, 2020 at 06:14:59PM +0100, Thierry Reding wrote:
> > On Tue, Oct 27, 2020 at 09:45:54AM -0700, Douglas Anderson wrote:
> > > The simple panel code currently allows panels to define fixed delays
> > > at certain stages of initialization.  These work OK, but they don't
> > > really map all that clearly to the requirements presented in many
> > > panel datasheets.  Instead of defining a fixed delay, those datasheets
> > > provide a timing diagram and specify a minimum amount of time that
> > > needs to pass from event A to event B.
> > > 
> > > Because of the way things are currently defined, most panels end up
> > > over-delaying.  One prime example here is that a number of panels I've
> > > looked at define the amount of time that must pass between turning a
> > > panel off and turning it back on again.  Since there is no way to
> > > specify this, many developers have listed this as the "unprepare"
> > > delay.  However, if nobody ever tried to turn the panel on again in
> > > the next 500 ms (or whatever the delay was) then this delay was
> > > pointless.  It's better to do the delay only in the case that someone
> > > tried to turn the panel on too quickly.
> > > 
> > > Let's support specifying delays as constraints.  We'll start with the
> > > one above and also a second one: the minimum time between prepare
> > > being done and doing the enable.  On the panel I'm looking at, there's
> > > an 80 ms minimum time between HPD being asserted by the panel and
> > > setting the backlight enable GPIO.  By specifying as a constraint we
> > > can enforce this without over-delaying.  Specifically the link
> > > training is allowed to happen in parallel with this delay so adding a
> > > fixed 80 ms delay isn't ideal.
> > > 
> > > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > > ---
> > > 
> > >  drivers/gpu/drm/panel/panel-simple.c | 51 ++++++++++++++++++++++++----
> > >  1 file changed, 44 insertions(+), 7 deletions(-)
> > 
> > This has always been bugging me a bit about the current setup, so I very
> > much like this idea.
> > 
> > > 
> > > diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> > > index 2be358fb46f7..cbbe71a2a940 100644
> > > --- a/drivers/gpu/drm/panel/panel-simple.c
> > > +++ b/drivers/gpu/drm/panel/panel-simple.c
> > > @@ -92,6 +92,19 @@ struct panel_desc {
> > >  		unsigned int unprepare;
> > >  	} delay;
> > >  
> > > +	/**
> > > +	 * @prepare_to_enable_ms: If this many milliseconds hasn't passed after
> > > +	 *                        prepare finished, add a delay to the start
> > > +	 *                        of enable.
> > > +	 * @unprepare_to_prepare_ms: If this many milliseconds hasn't passed
> > > +	 *                           unprepare finished, add a delay to the
> > > +	 *                           start of prepare.
> > 
> > I find this very difficult to understand and it's also not clear from
> > this what exactly the delay is. Perhaps this can be somewhat clarified
> > Something like the below perhaps?
> > 
> > 	@prepare_to_enable_ms: The minimum time, in milliseconds, that
> > 	    needs to have passed between when prepare finished and enable
> > 	    may begin. If at enable time less time has passed since
> > 	    prepare finished, the driver waits for the remaining time.
> 
> Also maybe split the kerneldoc into the sub-structure (this should work I
> think), so that you can go really wild on formatting :-)
I have a patch somewhere where I inlined all the comments and polished
them a bit. Will try to dig it up in the weekend.
It was motivated by a small W=1 detour.

	Sam