[PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to dts

Tero Kristo t-kristo at ti.com
Mon Sep 30 12:47:52 UTC 2019

On 30/09/2019 15:41, Adam Ford wrote:
> On Mon, Sep 30, 2019 at 3:53 AM Tero Kristo <t-kristo at ti.com> wrote:
>> On 30/09/2019 09:45, Tomi Valkeinen wrote:
>>> Hi,
>>> On 27/09/2019 18:47, Tomi Valkeinen wrote:
>>>> On 27/09/2019 18:37, Tero Kristo wrote:
>>>>> If you can provide details about what clock framework / driver does
>>>>> wrong (sample clk_set_xyz call sequence, expected results via
>>>>> clk_get_xyz, and what fails), I can take a look at it. Just reporting
>>>>> arbitrary display driver issues I won't be able to debug at all (I
>>>>> don't have access to any of the displays, nor do I want to waste time
>>>>> debugging them without absolutely no knowledge whatsoever.)
>>>> I used your hack patches to allow changing rates via debugfs. And set
>>>> dss1_alwon_fck_3430es2 to 27000000 or 27870967. The end result was
>>>> that DSS gets some very high clock from dss1_alwon_fck_3430es2, as the
>>>> frame rate jumps to many hundreds fps.
>>>> So, these numbers are not real, but to give the idea what I saw.
>>>> Running first with 50 MHz, I can see, say, 40 fps. Then I set the
>>>> clock to 30 MHz, and fps dropped to, say, 30fps, as expected with
>>>> lower clock. Then I set the clock to 27MHz (or the other one),
>>>> expecting a bit lower fps, but instead I saw hundreds of fps.
>>>> I don't know if there's any other way to observe the wrong clock rate
>>>> but have the dss enabled and running kmstest or similar. I can help
>>>> you set that up next week, should be trivial. You don't need a display
>>>> for that.
>>> Here's how to reproduce. I have the attached patches. Three of them are
>>> the clk-debug ones, and one of mine to make it easy to test without a
>>> display, and without underflow flood halting the device. There are on
>>> top of v5.3. Kernel config also attached.
>>> kmstest is from kms++ project (https://github.com/tomba/kmsxx). It
>>> should be straightforward to compile, but kmstest binary is also
>>> included in TI's rootfs.
>> Ok, I ignored all your test code and just fiddled with my trusty clk
>> debugfs patches. I don't like debugging with test code I have no
>> experience with. :)
>> Anyways, it seems the dpll4_m4_ck max divider value is wrong, it only
>> accepts values upto 16 at least on my board. The setting for this in DT
>> is 32, and it is most likely SoC specific what happens if you write an
>> invalid value to the divider.
>> The best action here is probably to drop the max-div value for this
>> clock to 16. Can someone check this with their display setup and see
>> what happens? Attached patch should do the trick.
> I tried your attached patch on my dm3730 and that seems to make it
> somewhat better in that it doesn't hang anymore, so that leads me to
> believe that your comment about the divider being only valid on the
> omap36 may not be true. I do think it solves the hanging issue that i
> was seeing, but I now see a new one now which is dumping a backtrace.
> It looks like it's unhappy that its trying to get one frequency and
> getting something different instead.
> [   10.014099] WARNING: CPU: 0 PID: 111 at
> drivers/gpu/drm/omapdrm/dss/dss.c:655 dss_set_fck_rate+0x70/0x90
> [omapdss]
> [   10.014129] clk rate mismatch: 27870968 != 27000000

I believe this one is for Tomi to comment, his driver does some magic 
compares for the requested vs. actual received clock rates. If I am not 
mistaken, we are only modifying an integer divider here, and thus it is 
physically impossible to get accurate 27MHz rate to display.


> See attached log for the full dump.
> Either way, I think you've identified the main issue.  I just think we
> may have uncovered another one in the process.
> For what it's worth, the video looks good.  :-)
> adam
>> -Tero
>> --

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