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 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.

Thanks!
Maxime

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