[PATCH] drm: lcdif: Use adjusted_mode .clock instead of .crtc_clock

[Marek Vasut]

On 10/21/24 1:10 PM, Kieran Bingham wrote:
> Quoting Marek Vasut (2024-10-20 03:49:29)
>> On 10/19/24 11:49 PM, Kieran Bingham wrote:
>>> Quoting Marek Vasut (2024-10-12 21:37:59)
>>>> On 10/11/24 5:10 AM, Liu Ying wrote:
>>>>
>>>> Hi,
>>>>
>>>>>>>> This Video PLL1 configuration since moved to &media_blk_ctrl {} , but it is still in the imx8mp.dtsi . Therefore, to make your panel work at the correct desired pixel clock frequency instead of some random one inherited from imx8mp.dtsi, add the following to the pollux DT, I believe that will fix the problem and is the correct fix:
>>>>>>>>
>>>>>>>> &media_blk_ctrl {
>>>>>>>>        // 506800000 = 72400000 * 7 (for single-link LVDS, this is enough)
>>>>>>>>        // there is no need to multiply the clock by * 2
>>>>>>>>        assigned-clock-rates = <500000000>, <200000000>, <0>, <0>, <500000000>, <506800000>;
>>>>>>>
>>>>>>> This assigns "video_pll1" clock rate to 506.8MHz which is currently not
>>>>>>> listed in imx_pll1443x_tbl[].
>>>>>>
>>>>>> Since commit b09c68dc57c9 ("clk: imx: pll14xx: Support dynamic rates") the 1443x PLLs can be configured to arbitrary rates which for video PLL is desirable as those should produce accurate clock.
>>>>>
>>>>> Ack.
>>>>>
>>>>>>
>>>>>>> Does the below patch[1] fix the regression issue? It explicitly sets
>>>>>>> the clock frequency of the panel timing to 74.25MHz.
>>>>>>>
>>>>>>> [1] https://patchwork.freedesktop.org/patch/616905/?series=139266&rev=1
>>>>>> That patch is wrong, there is an existing entry for this panel in panel-simple.c which is correct and precise, please do not add that kind of imprecise duplicate timings into DT.
>>>>>
>>>>> At least the patch[1] is legitimate now to override the display
>>>>> timing of the panel because the override mode is something
>>>>> panel-simple.c supports.
>>>>
>>>> It may be possible to override the mode, but why would this be the
>>>> desired if the panel-simple.c already contains valid accurate timings
>>>> for this particular panel ?
>>>
>>> I'm confused a little here. Why is it that setting the panel timings in
>>> the DT as per [1] make the display work, while the panel timeings in
>>> panel-simple alone are not enough?
>>>
>>> Is there some difference in code path for 'how' the panel timings are
>>> set as to whether they will apply fully or not ?
>> Because [1] sets inaccurate pixel clock of 74.25 MHz, which can be
>> divided down from random default Video PLL setting of 1039.5 MHz set in
>> imx8mp.dtsi media_blk_ctrl , 1039.5 / 74.25 = 14 .
>>
>> The panel-simple pixel clock are 72.4 MHz, to achieve that accurately,
>> it is necessary to reconfigure the Video PLL frequency to 506.8 MHz ,
>> which LCDIFv3 can do, but LDB can not, hence it has to be done in DT for
>> now.
> 
> Aha - right - Thanks, I'd missed the part that 74.25 MHz /is not/ the
> correct or supported pixel clock for the panel

It is inaccurate, it is still within the spec ...

> , so it just becomes
> 'close enough' and lucky that it works...

... but only by sheer chance, because the Video PLL in imx8mp.dtsi is 
accidentally set to frequency that is just close enough to be divisible 
to the inaccurate 74.25 MHz .

> Now I understand how your proposed :
> 
>>>>>>>> &media_blk_ctrl {
>>>>>>>>        // 506800000 = 72400000 * 7 (for single-link LVDS, this is enough)
>>>>>>>>        // there is no need to multiply the clock by * 2
>>>>>>>>        assigned-clock-rates = <500000000>, <200000000>, <0>, <0>, <500000000>, <506800000>;
>>>>>>>
>>>>>>> This assigns "video_pll1" clock rate to 506.8MHz which is currently not
>>>>>>> listed in imx_pll1443x_tbl[].
> 
> is more accurate. But is that acceptable for DT ? To just hardcode
> clocks like that?

I am not happy about it, but it is the best we can do right now.

See these two patches, they might address that part for next cycle:

clk: imx: clk-imx8mp: Allow LDB serializer clock reconfigure parent rate
drm: bridge: ldb: Configure LDB clock in .mode_set

> Or is this something we'll then remove when the additional patches make
> it upstream? (I'm curious on the basis that I thought we treat DT as
> 'firmware' so if we put this in we expect it to be there forever?)

It is already there in imx8mp.dtsi , so we are not making the situation 
any worse, rather the opposite.

> All of this seems like perhaps it should be in an overlay for the
> display too - but given this board comes with this panel as a kit - I
> suspect it's fine to keep it all there.
That's a separate topic, but yes, DTOs for displays are a good thing to 
have.