Message-ID: <653f0438-c55a-02a5-dffb-2ee8e6d9ef4a@micronovasrl.com>
Date:   Wed, 7 Feb 2018 13:49:59 +0100
From:   Giulio Benetti <giulio.benetti@...ronovasrl.com>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
Cc:     airlied@...ux.ie, wens@...e.org, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] drm/sun4i: Handle DRM_MODE_FLAG_**SYNC_POSITIVE
 correctly

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

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

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

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.

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

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?

> 
> Maxime
> 

-- 
Giulio Benetti
R&D Manager &
Advanced Research

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