Re: [PATCH 9/9] [DO NOT MERGE] drm/sun4i: rgb: Add 5% tolerance to dot clock frequency check

[Maxime Ripard <maxime.ripard@bootlin.com>]

On Thu, Oct 18, 2018 at 08:50:15PM +0800, Icenowy Zheng wrote:
> 在 2018-10-18四的 14:18 +0200，Maxime Ripard写道：
> > On Thu, Oct 18, 2018 at 02:31:01PM +0300, Laurent Pinchart wrote:
> > > Hello,
> > > 
> > > On Thursday, 18 October 2018 12:42:58 EEST Maxime Ripard wrote:
> > > > On Thu, Oct 18, 2018 at 11:18:06AM +0200, Daniel Vetter wrote:
> > > > > On Thu, Oct 18, 2018 at 10:55 AM Laurent Pinchart wrote:
> > > > > > On Thursday, 18 October 2018 10:33:27 EEST Icenowy Zheng
> > > > > > wrote:
> > > > > > > From: Chen-Yu Tsai <wens@csie.org>
> > > > > > > 
> > > > > > > The panels shipped with Allwinner devices are very
> > > > > > > "generic", i.e.
> > > > > > > they do not have model numbers or reliable sources of
> > > > > > > information
> > > > > > > for the timings (that we know of) other than the fex files
> > > > > > > shipped
> > > > > > > on them. The dot clock frequency provided in the fex files
> > > > > > > have all
> > > > > > > been rounded to the nearest MHz, as that is the unit used
> > > > > > > in them.
> > > > > > > 
> > > > > > > We were using the simple panel "urt,umsh-8596md-t" as a
> > > > > > > substitute
> > > > > > > for the A13 Q8 tablets in the absence of a specific model
> > > > > > > for what
> > > > > > > may be many different but otherwise timing compatible
> > > > > > > panels. This
> > > > > > > was usable without any visual artifacts or side effects,
> > > > > > > until the
> > > > > > > dot clock rate check was added in commit bb43d40d7c83
> > > > > > > ("drm/sun4i:
> > > > > > > rgb: Validate the clock rate").
> > > > > > > 
> > > > > > > The reason this check fails is because the dotclock
> > > > > > > frequency for
> > > > > > > this model is 33.26 MHz, which is not achievable with our
> > > > > > > dot clock
> > > > > > > hardware, and the rate returned by clk_round_rate deviates
> > > > > > > slightly,
> > > > > > > causing the driver to reject the display mode.
> > > > > > > 
> > > > > > > The LCD panels have some tolerance on the dot clock
> > > > > > > frequency, even
> > > > > > > if it's not specified in their datasheets.
> > > > > > > 
> > > > > > > This patch adds a 5% tolerence to the dot clock check.
> > > > > > 
> > > > > > Why do you think this shouldn't be merged ?
> > > > > 
> > > > > It pisses of a lot of people who really insist upon accurate
> > > > > timing.
> > > > 
> > > > It's not just about accurate timings. That 5% is a made-up limit,
> > > > that
> > > > never have really been confirmed by looking at the typical
> > > > tolerancies
> > > > of panels.
> > > > 
> > > > And while being to relaxed might make some panels work that are
> > > > not
> > > > working currently, it might also break some panels that currently
> > > > work
> > > > and won't now, and ideally, we should really be able to let those
> > > > panels work if they can, and filter out resolutions if they
> > > > can't.
> > > > 
> > > > > I think a better fix would be to have a dotclock range in
> > > > > drm_panel,
> > > > > and some magic to figure out which one of these we can actually
> > > > > do. Then tell userspace that this is the mode is should
> > > > > request. That way userspace knows what the actual
> > > > > dotclock/refresh
> > > > > rate is, and the panel still works.
> > > > 
> > > > It's not just about panels, but also bridges with EDID where the
> > > > tolerancy is not exposed.
> > > 
> > > Given that the tolerance is a property of the panel or bridge, I
> > > agree with 
> > > Daniel that it should be implemented there, or at least in
> > > cooperation with 
> > > drm_panel and drm_bridge.
> > 
> > How are we supposed to deal with panels without any documentation
> > then?
> > 
> > > Semi-related information, I think the CEA and VESA standards allow
> > > a 0.5% 
> > > clock tolerance. What is the maximum clock frequency deviation
> > > required for 
> > > this platform ?
> > 
> > Looks like it does indeed. That's definetely good to know.
> 
> Then maybe we can choose to allow 0.5% error when a bridge is attached?

It doesn't fix the panel case, but that sounds reasonable yes

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com