[PATCH] drm/mxsfb: fix pixel clock polarity

Stefan Agner stefan at agner.ch
Wed Dec 14 00:01:59 UTC 2016

On 2016-12-08 15:38, Marek Vasut wrote:
> 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 at agner.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/

I looked at a old data sheet of that display and it seemed that
PIXDATA_POSEDGE is the right thing. Panelook.cn lists newer data sheets,
but I couldn't find them on the open internet, do you  have access to a
newer one?


I guess in the end it doesn't matter: Given that it is verified that the
controllers data sheet is right, I vote for merging that patch and fix
the displays polarity...


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