[PATCH] drm/mxsfb: fix pixel clock polarity

Stefan Agner stefan at agner.ch
Wed Dec 14 20:29:01 UTC 2016

On 2016-12-14 00:04, Marek Vasut wrote:
> On 12/14/2016 01:01 AM, Stefan Agner wrote:
>> 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?
> Which "version" do you have ? Probably not though.

I couldn't find it anymore, but I think it said Rev. 0

>> http://www.panelook.cn/COM43H4M85ULC_ORTUSTECH_4.3_LCM_overview_17296.html
>> 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...
> Merging which patch ?

This patch.

And I guess, to keep your display working, a patch which changes the
Ortustech pixel clock flag...


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