linux-kernel - Re: [PATCH] drm/mxsfb: fix pixel clock polarity

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Message-ID: <1da939e2-f616-e60c-416b-a71e6ecd1dd4@denx.de>
Date:   Fri, 9 Dec 2016 00:38:37 +0100
From:   Marek Vasut <marex@...x.de>
To:     Stefan Agner <stefan@...er.ch>
Cc:     airlied@...ux.ie, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Philipp Zabel <p.zabel@...gutronix.de>
Subject: Re: [PATCH] drm/mxsfb: fix pixel clock polarity

On 12/08/2016 09:46 PM, Stefan Agner wrote:
> On 2016-12-07 18:37, Marek Vasut wrote:
>> On 12/08/2016 02:26 AM, Stefan Agner wrote:
>>> On 2016-12-07 16:59, Stefan Agner wrote:
>>>> On 2016-12-07 16:49, Marek Vasut wrote:
>>>>> On 12/08/2016 01:27 AM, Stefan Agner wrote:
>>>>>> The DRM subsystem specifies the pixel clock polarity from a
>>>>>> controllers perspective: DRM_BUS_FLAG_PIXDATA_NEGEDGE means
>>>>>> the controller drives the data on pixel clocks falling edge.
>>>>>> That is the controllers DOTCLK_POL=0 (Default is data launched
>>>>>> at negative edge).
>>>>>>
>>>>>> Also change the data enable logic to be high active by default
>>>>>> and only change if explicitly requested via bus_flags. With
>>>>>> that defaults are:
>>>>>> - Data enable: high active
>>>>>> - Pixel clock polarity: controller drives data on negative edge
>>>>>>
>>>>>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>>>>>> ---
>>>>>> Hi Marek,
>>>>>
>>>>> Hi, that was quick, thanks for checking this.
>>>>
>>>> Yeah, I couldn't wait seeing it flying :-)
>>>>
>>>>>
>>>>>> I discovered this while testing on a i.MX 7 eLCDIF IP. Particularly the
>>>>>> non-standard DE polarity was causing issues. I was using a EDT display
>>>>>> which is part of simple panel driver since a while now and does not
>>>>>> specify any bus_flags currently... Of course I could (and probably should)
>>>>>> add the proper bus_flags there too, but there are several displays
>>>>>> which do not specify any polarity and likely rely on sensible driver
>>>>>> standards (which is afact high active for the DE signal).
>>>>>
>>>>> I actually use a panel which requires correct settings of the flags, see
>>>>> e0932f9d7ba9a16f99a84943b720f109de8e3e06 in mainline , so this patch
>>>>> would break things for me. So I wonder whether you should fix the panel
>>>>> driver or whether the mxsfb should be fixed ?
>>>>
>>>> If you ask me, mxsfb.
>>>>
>>>> Ok, there are actually two things, one is a bug, one is a default
>>>> change.
>>>>
>>>> The bug: Pixel clock polarity is clearly defined to be controller
>>>> centric (see comments around DRM_BUS_FLAG_PIXDATA_*EDGE in
>>>> include/drm/drm_connector.h). The driver does it wrong currently.
>>>>
>>>> This might affect your display, and if it does, it is actually wrong
>>>> also in your display... However, since it is a bug, I think it is not
>>>> really a debate, it should be fixed...
>>>
>>> FWIW, it seems that Ortustech com43h4m85ulc samples on falling edge, so
>>> DRM_BUS_FLAG_PIXDATA_POSEDGE seems right. And it means that DOTCLK_POL
>>> should be 1 (inverted), so with this patch the polarity should actually
>>> be correct for that panel.
>>
>> Well, if I apply this patch, my image is shifted by 1 px to the left and
>> there is a 1px white bar on the right side, so I think I have some
>> polarity problem now ?
> 
> Ok, lets create facts here:
> 1. SoloX Refrence Manual, Figure 37-13. shows DOTCLK_POL=0, and it shows
> that the controller drives signals on falling edge of the pixel clock.
> The i.MX 7 has the same figure.
> 2. Just to verify, I hooked up an oscilloscope on my i.MX 7: It shows
> that with DOTCLK_POL=0 the controller drives on falling edge:
> http://imgur.com/a/2f2Xt 
> 
> So my measurements verify what is in the i.MX data sheets.

Good

> The current code sets the bit when negative edge (falling edge) is
> requested, which is wrong:
> #define VDCTRL0_DOTCLK_ACT_FALLING	(1 << 25)
> ...
> if (bus_flags & DRM_BUS_FLAG_PIXDATA_NEGEDGE)
>      vdctrl0 |= VDCTRL0_DOTCLK_ACT_FALLING;
> 
> Not sure what is going on with your display, maybe the datasheet is just
> wrong (it requires DRM_BUS_FLAG_PIXDATA_NEGEDGE in fact) or it is some
> other artifact.

This is probably where the problem crept in [1], droping PIXDATA_POSEDGE
actually makes this patch work for me. CCing Philipp.

[1] https://patchwork.kernel.org/patch/9301517/

-- 
Best regards,
Marek Vasut