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

[Giulio Benetti]

Hi,

Il 08/02/2018 21:40, Maxime Ripard ha scritto:
> 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?

Exactly, it is like this:
https://pasteboard.co/H4r8QRe.png but with clock divided by 2.

> 
>> 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??

Exactly rising edge.

> 
>> - As inverted => "0x0 => normal phase"
> 
> And falling edge?

Yes.

> 
> 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?

Yes, you're right, 2/3 phase is better:

1/3 phase: https://pasteboard.co/H4VehON.png
2/3 phase: https://pasteboard.co/H4Veq8a.png

Take a look at the bit in middle(yellow) sampled by clock(blue).

Rising edge is almost in the middle of D0 bit.

> 
>> 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.

So let's keep:
- As normal(rising edge) => IO_POL_REG "0x2 => 2/3 phase"
- As inverted(falling edge) => IO_POL_REG "0x0 => normal phase"

Ok?

> 
> Maxime
> 


-- 
Giulio Benetti
CTO

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