[PATCH 2/2] drm/sun4i: Handle DRM_MODE_FLAG_**SYNC_POSITIVE correctly

[maxime.ripard@bootlin.com (Maxime Ripard)]

On Wed, Feb 07, 2018 at 01:49:59PM +0100, Giulio Benetti wrote:
> Hi,
> 
> Il 07/02/2018 11:39, Maxime Ripard ha scritto:
> > On Wed, Jan 24, 2018 at 08:37:28PM +0100, Giulio Benetti wrote:
> >>>>> Also, how was it tested? This seems quite weird that we haven't caught
> >>>>> that one sooner, and I'm a bit worried about the possible regressions
> >>>>> here.
> >>>>
> >>>> It sounds really strange to me too,
> >>>> because everybody under uboot use "sync:3"(HIGH).
> >>>> I will retry to measure,
> >>>> unfortunately at home I don't have a scope,
> >>>> but I think I'm going to have one soon, because of this. :)
> >>>
> >>> Here I am with scope captures and tcon0 registers dump:
> >>> tcon0_regs => https://pasteboard.co/H4r8Zcs.png
> >>> dclk_d0 => https://pasteboard.co/H4r8QRe.png
> >>> dclk_de => https://pasteboard.co/H4r8zh4R.png
> >>> dclk_vsnc => https://pasteboard.co/H4r8Hye.png
> >>>
> >>> As you can see circled in reg on registers,
> >>> TCON0_IO_POL_REG = 0x00000000.
> >>> But on all the waveforms you can see:
> >>> - dclk_d0: clock phase is 0, but it starts with falling edge, otherwise
> >>> the rising front overlaps dclk rising edge(not good), so to me this is
> >>> falling, then I mean it Negative.
> >>> - dclk_de: de pulse is clearly negative, even if register is 0 and its'
> >>> polarity bit is 0.
> >>> - dclk_vsnc: same as dclk_de
> >>> - dclk_hsync: I didn't take scope screenshot but I can assure you it's
> >>> negative.
> >>>
> >>> You can also check all the other registers about TCON0.
> >>>
> >>> Now I proceed testing it on A33, maybe the peripheral is slightly
> >>> different between Axx SoCs, if I find it that way,
> >>> it should be only a check about SoC or peripheral ID,
> >>> and treat polarity as it should be done.
> >>
> >> Here I am with A33 waveforms:
> >> tcon0_regs => https://pasteboard.co/H4rXfN0M.png
> >> dclk_d0 => https://pasteboard.co/H4rVXwy.png
> >> dclk_de => https://pasteboard.co/H4rWDt8.png
> >> dclk_vsnc => https://pasteboard.co/H4rWRACu.png
> >> dclk_hsync => https://pasteboard.co/H4rWK6I.png
> >>
> >> It behaves the same way as A20, so as I mean IO polarity,
> >> all signals(except D0-D23), are inverted.
> >> For A33 I've used A33-OLinuXino.
> >> For A20 our LiNova1.
> > 
> > If you have an A33 handy, you probably want to read that mail:
> > https://lists.freedesktop.org/archives/dri-devel/2017-July/147951.html
> > 
> > Especially the 90-phase part.
> 
> Here is a summary of different SoCs TCON:
> With DCLK_Sel:
> 0x0 => normal phase
> 0x1 => 1/3 phase
> 0x2 => 2/3 phase
> 
> A10, A20

Have you tested the option 4 and 5 there too?

> With DCLK_Sel:
> 0x0 => normal phase
> 0x1 => 1/3 phase
> 0x2 => 2/3 phase
> 0x5 => DCLK/2 phase 0
> 0x4 => DCLK/2 phase 90
> 
> A31, A31s, A33, A80, A83T

Ok, great, so Chen-Yu was right :)

I guess the option 5 (DCLK/2 phase 0) is still to early, just like
you've shown in the previous captures?

> Also I've found that TCON1 has not this feature,
> nor first, nor second case(at least is not described on user manuals).

At lot of things are not described, unfortunately...

> So I could handle differently according to SoC.
> Unfortunately there is not TCON register keeping IP version,
> so the only way I see is to create a long if-or statement to understand
> which kind of TCON we're using.

You can base that on the compatible, and add a field in the
sun4i_tcon_quirks structure, that will avoid the if statements you
mentionned.

> But what sounds not the best to me, is that DCLK is divided by 2 if
> using phase 90. So we need to reconsider also bitclock driver according
> to this.
> I don't know if it make sense.
> 
> IMHO, I would keep only:
> - As normal => "0x1 => 1/3 phase"

So that would mean sampling at raising edge on this one??

> - As inverted => "0x0 => normal phase"

And falling edge?

If so, and if remember the captures properly, the sampling would occur
right before the rise, and not really around the fall.

Would 2/3 be better here?

> According to scope captures above on both A20 and A33.
> Unfortunately I don't have other boards for the other SoCs to take captures.
> 
> What do you think?

I guess we can make that part applicable to all SoCs, we haven't seen
any significant differences on those part.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com
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