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

[Sean Paul]

On Wed, Dec 07, 2016 at 11:48:55AM +0200, Laurent Pinchart wrote:
> Hello,
> 
> 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 :-)).

There is such thing as exact frequency when you have to worry about panel
warranty. We are fortunate, since we can talk to the panel manufacturer and
discuss which frequencies are tolerable inside the warranty. We usually hand
pick a rate/mode within these constraints.

Also, even though things *look* ok on the panel at a certain clock rate, running
outside the specified clock could damage the hardware.

I don't think it's unreasonable to add tolerances to drm_panel, since they will
differ in what is acceptable. The tricky part is teasing out what the tolerances
are.

Sean

> 
> 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 ?
> 
> > > 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.
> > 
> > I believe this should be handled by the bridge driver in the check
> > callback? The callback I'm changing is attached to the connector,
> > which I think doesn't get used if you have a bridge instead.
> > And this only checks the pre-registered display modes, such as
> > those specified in simple-panel or EDID.
> > 
> > > 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.
> > 
> > If you mean the get_timings callback, it's not very useful. Most of
> > the panels in simple-panel do not use the display_timings structure,
> > so they don't return anything. And I get that. The few datasheets
> > I found don't list min/max tolerances for the dotclock.
> > 
> > The ones that do have the min/max the same as the recommended value.
> > This may or may not be accurate. IIRC the one panel that had this
> > that I did check didn't list min/max values in its datasheet.
> > 
> > > 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?
> > 
> > Yes! Feedback please! Between Maxime and me I think we only have a
> > limited number of panels, with some overlap.
> 
> -- 
> Regards,
> 
> Laurent Pinchart
> 
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-- 
Sean Paul, Software Engineer, Google / Chromium OS