Re: [PATCH 4/5] Revert "drm/atmel-hlcdc: allow selecting a higher pixel-clock than requested"

[Peter Rosin]

On 2019-12-13 10:28, Claudiu.Beznea@xxxxxxxxxxxxx wrote:
> 
> 
> On 11.12.2019 15:28, Peter Rosin wrote:
>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>
>> On 2019-12-11 12:45, Claudiu.Beznea@xxxxxxxxxxxxx wrote:
>>>
>>>
>>> On 10.12.2019 19:22, Peter Rosin wrote:
>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>>
>>>> On 2019-12-10 15:59, Claudiu.Beznea@xxxxxxxxxxxxx wrote:
>>>>>
>>>>>
>>>>> On 10.12.2019 16:11, Peter Rosin wrote:
>>>>>> On 2019-12-10 14:24, Claudiu Beznea wrote:
>>>>>>> This reverts commit f6f7ad3234613f6f7f27c25036aaf078de07e9b0.
>>>>>>> ("drm/atmel-hlcdc: allow selecting a higher pixel-clock than requested")
>>>>>>> because allowing selecting a higher pixel clock may overclock
>>>>>>> LCD devices, not all of them being capable of this.
>>>>>>
>>>>>> Without this patch, there are panels that are *severly* underclocked (on the
>>>>>> magnitude of 40MHz instead of 65MHz or something like that, I don't remember
>>>>>> the exact figures).
>>>>>
>>>>> With patch that switches by default to 2xsystem clock for pixel clock, if
>>>>> using 133MHz system clock (as you specified in the patch I proposed for
>>>>> revert here) that would go, without this patch at 53MHz if 65MHz is
>>>>> requested. Correct me if I'm wrong.
>>>>
>>>> It might have been 53MHz, whatever it was it was too low for things to work.
>>>>
>>>>>> And they are of course not capable of that. All panels
>>>>>> have *some* slack as to what frequencies are supported, and the patch was
>>>>>> written under the assumption that the preferred frequency of the panel was
>>>>>> requested, which should leave at least a *little* headroom.
>>>>>
>>>>> I see, but from my point of view, the upper layers should decide what
>>>>> frequency settings should be done on the LCD controller and not let this at
>>>>>  the driver's latitude.
>>>>
>>>> Right, but the upper layers do not support negotiating a frequency from
>>>> ranges. At least the didn't when the patch was written, and implementing
>>>> *that* seemed like a huge undertaking.
>>>>
>>>>>>
>>>>>> So, I'm curious as to what panel regressed. Or rather, what pixel-clock it needs
>>>>>> and what it gets with/without the patch?
>>>>>
>>>>> I have 2 use cases:
>>>>> 1/ system clock = 200MHz and requested pixel clock (mode_rate) ~71MHz. With
>>>>> the reverted patch the resulted computed pixel clock would be 80MHz.
>>>>> Previously it was at 66MHz
>>>>
>>>> I don't see how that's possible.
>>>>
>>>> [doing some calculation by hand]
>>>>
>>>> Arrgh. *blush*
>>>>
>>>> The code does not do what I intended for it to do.
>>>> Can you please try this instead of reverting?
>>>>
>>>> Cheers,
>>>> Peter
>>>>
>>>> From b3e86d55b8d107a5c07e98f879c67f67120c87a6 Mon Sep 17 00:00:00 2001
>>>> From: Peter Rosin <peda@xxxxxxxxxx>
>>>> Date: Tue, 10 Dec 2019 18:11:28 +0100
>>>> Subject: [PATCH] drm/atmel-hlcdc: prefer a lower pixel-clock than requested
>>>>
>>>> The intention was to only select a higher pixel-clock rate than the
>>>> requested, if a slight overclocking would result in a rate significantly
>>>> closer to the requested rate than if the conservative lower pixel-clock
>>>> rate is selected. The fixed patch has the logic the other way around and
>>>> actually prefers the higher frequency. Fix that.
>>>>
>>>> Fixes: f6f7ad323461 ("drm/atmel-hlcdc: allow selecting a higher pixel-clock than requested")
>>>> Reported-by: Claudiu Beznea <claudiu.beznea@xxxxxxxxxxxxx>
>>>> Signed-off-by: Peter Rosin <peda@xxxxxxxxxx>
>>>> ---
>>>>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
>>>> index 9e34bce089d0..03691845d37a 100644
>>>> --- a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
>>>> +++ b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
>>>> @@ -120,8 +120,8 @@ static void atmel_hlcdc_crtc_mode_set_nofb(struct drm_crtc *c)
>>>>                 int div_low = prate / mode_rate;
>>>>
>>>>                 if (div_low >= 2 &&
>>>> -                   ((prate / div_low - mode_rate) <
>>>> -                    10 * (mode_rate - prate / div)))
>>>> +                   (10 * (prate / div_low - mode_rate) <
>>>> +                    (mode_rate - prate / div)))
>>>
>>> I tested it on my setup (I have only one of those specified above) and it
>>> is OK. Doing some math for the other setup it should also be OK.
>>
>> Glad to hear it, and thanks for testing/verifying!
>>
>>> As a whole, I'm OK with this at the moment (let's hope it will work for all
>>> use-cases) but still I am not OK with selecting here, in the driver,
>>> something that might work.
>>
>> The driver has to select *something*. If it can deliver the exact requested
>> frequency, fine. Otherwise? What should it do? Bail out? Why is 53MHz better
>> and more likely to produce a picture than 66MHz, when 65MHz is requested?
>> That's of course an impossible question for the driver to answer.
>>
>> So, if you are not ok with that, you need to implement something that uses
>> the min/max fields from the various fields inside struct display_timing
>> instead of only looking at the typ field. E.g. the panel_lvds driver calls
>> videomode_from_timings() and the result is a single possible mode with only
>> the typical timings, with no negotiation of the best option within the
>> given ranges with the other drivers involved with the pipe. I think the
>> panel-simple driver also makes this one-sided decision of only making use
>> of the typ field for each given timing range. Having dabbled a bit in what
>> the sound stack does to negotiate the sample rate, sample format and
>> channel count etc, I can only predict that retrofitting something like that
>> for video modes will be ... interesting. Which is probably why it's not
>> done at all, at least not in the general case.
>>
>> And yes, I agree, the current mechanics are less than ideal. But I have no
>> time to do anything about it.
>>
>>>                            Although I am not familiar with how other DRM
>>> drivers are handling this kind of scenarios. Maybe you and/or other DRM
>>> guys knows more about it.
>>
>> I don't know (and I mean it literally), but maybe these chips are special
>> as they typically end up with very small dividers and thus large frequency
>> steps? BTW, I do not consider myself a DRM guy, I have only tried to
>> fix that which did not work out for our needs...
>>
>>> Just as a notice, it may worth adding a print message saying what was
>>> frequency was requested and what frequency has been setup by driver.
>>
>> I have no problem with that.
> 
> Hi Peter,
> 
> I intend to prepare my v2 of this series. How would you like to proceed
> with the patch you provided? Are you OK if I add it to my v2 of this series
> or would you prefer to send it on your own?

It would be awesome if you shepherd it for me, thanks!

Cheers,
Peter