linux-kernel - Re: [PATCH RFC] drm/sun4i: rgb: Add 5% tolerance to dot clock frequency check

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Message-ID: <20161206172911.z6sbjzgqv3vfcrfh@lukather>
Date:   Tue, 6 Dec 2016 18:29:11 +0100
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     David Airlie <airlied@...ux.ie>, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...glegroups.com,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sean Paul <seanpaul@...omium.org>,
        Eric Anholt <eric@...olt.net>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [PATCH RFC] drm/sun4i: rgb: Add 5% tolerance to dot clock
 frequency check

On Thu, Nov 24, 2016 at 07:22:31PM +0800, Chen-Yu Tsai wrote:
> 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.

As we discussed already, I really believe this is just as arbitrary as
the current behaviour.

Some panels require an exact frequency, some have a minimal frequency
but no maximum, some have a maximum frequency but no minimal, and I
guess most of them deviates by how much exactly they can take (and
possibly can take more easily a higher frequency, but are less
tolerant if you take a frequency lower than the nominal.

And we cannot remove that check entirely, since some bridges will
report out of range frequencies for higher modes that we know we
cannot reach.

We could just try to see if the screen pixel clock frequency is out of
the pixel clock range we can generate, but then we will loop back on
how much out of range is it exactly, and is it within the screen
tolerancy.

We have an API to deal with the panel tolerancies in the DRM panel
framework, we can (and should) use it.

I'm not sure how others usually deal with this though. I think I
remember Eric telling me that for the RPi they just adjusted the
timings a bit, but they only really had a single panel to deal with.

Daniel, Eric, Laurent, Sean? Any ideas?

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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