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

Maxime Ripard maxime.ripard at free-electrons.com
Fri Dec 9 08:57:57 UTC 2016


On Wed, Dec 07, 2016 at 11:48:55AM +0200, Laurent Pinchart wrote:
> On Wednesday 07 Dec 2016 10:26:25 Chen-Yu Tsai wrote:
> > On Wed, Dec 7, 2016 at 1:29 AM, Maxime Ripard wrote:
> > > 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.
> > 
> > Yes. I agree. This patch is mainly to give something that works for
> > people who don't care about the details, and to get some feedback
> > from people that do.
> > 
> > > Some panels require an exact frequency,
> There's no such thing as an exact frequency, there will always be some 
> tolerance (and if your display controller can really generate an exact 
> frequency I'd be very interested in that hardware :-)).
> This is something that has been bugging me for some time now. The problem has 
> been mostly ignored, or worked around in different ways by different drivers. 
> I'm afraid I have no generic solution available, but I think we should try to 
> agree on a common behaviour.
> I don't believe it would be reasonable to request each panel to report a 
> tolerance, as the value is most of the time not available from the 
> documentation (when documentation is available). Worse, I'm pretty sure that 
> most panels documented as fixed timing can actually accept a wide range of 
> timings. The timings reported in the datasheet are just the nominal values.
> Panels that don't support multiple resolutions obviously require fixed active 
> h/v values. Even if they can tolerate some departure from the nominal timings 
> for the sync and porches lengths, it might not be very useful to support that 
> as I don't expect the display controllers and encoders to be a limiting factor 
> by not supporting the particular timings that a panel considers as nominal. On 
> the other hand, departing from the nominal pixel clock frequency is needed as 
> we can't achieve an exact match, and even possibly to have some control over 
> the frame rate (although that might also require changing the sync and porches 
> timings). Without specific information about panel tolerance, do we have any 
> option other than picking an arbitrary value ?

If you consider only panels, yes, chances are the EE picked a panel
that has a decent chance to work (especially since most of the boards
we support are consumer electronics products, and people like to have
a panel that works on their tablet).

However, bridges are a different story, and provide on some SoCs modes
that are way out of reach for our pixel clock, which is why we had
that test in the first place.


Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
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