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Subject: Re: [PATCH 9/9] [DO NOT MERGE] drm/sun4i: rgb: Add 5% tolerance to
 dot clock frequency check
From:   Icenowy Zheng <icenowy@aosc.io>
To:     Maxime Ripard <maxime.ripard@bootlin.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     devicetree@vger.kernel.org, Dave Airlie <airlied@linux.ie>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linux-sunxi <linux-sunxi@googlegroups.com>,
        Rob Herring <robh+dt@kernel.org>,
        Daniel Vetter <daniel@ffwll.ch>, Chen-Yu Tsai <wens@csie.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
Date:   Thu, 18 Oct 2018 20:50:15 +0800
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         <20181018094258.oahlopafvm2udvoe@flea> <1585874.9P5eR8PMbl@avalon>
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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?

> 
> Thanks!
> Maxime
>