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

From: [email-masked] (Laurent Pinchart)
Date: Thu, 18 Oct 2018 14:31:01 +0300

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 <[email-masked]>
> >>> 
> >>> 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.

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 ?

-- 
Regards,

Laurent Pinchart