linux-kernel - Re: [PATCH 2/2] drm/sun4i: Handle DRM_MODE_FLAG_**SYNC

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Date:   Wed, 28 Feb 2018 16:56:56 +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 16/02/2018 16:50, Maxime Ripard ha scritto:
> On Thu, Feb 15, 2018 at 07:05:56PM +0100, Giulio Benetti wrote:
>>> 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"
> 
> I was actually thinking 1/3 for rising, 2/3 for falling.

1/3 is almost the same waveform as D0,
having rising an falling edges almost in sync with D0.
It's not so clear because DCLK has a bad figure,
but it is that way.

2/3 instead is almost in the middle of D0 as rising.

Summarizing:
- use 0/3 as falling, then DRM_BUS_FLAG_PIXDATA_NEGEDGE
- use 2/3 as rising, then DRM_BUS_FLAG_PIXDATA_POSEDGE

I follow with a new patch using clk_set_phase function.

> 
> Maxime
> 


-- 
Giulio Benetti
CTO

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