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

From: Marek Vasut
Date: Wed Dec 14 2016 - 15:38:49 EST

On 12/14/2016 09:29 PM, Stefan Agner wrote:
> 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@xxxxxxxx>
>>>>>>>>>> ---
>>>>>>>>>> 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:
>>>>> 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]
>>> I looked at a old data sheet of that display and it seemed that
>>> PIXDATA_POSEDGE is the right thing. 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
>>> 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...

Well, let's do that. Btw can you send a V2 with s/edige/edge/ ? ;-)


Best regards,
Marek Vasut