[PATCH] drm/i915/display: Add upper limit check for pixel clock

[Nautiyal, Ankit K]

On 7/3/2025 12:54 AM, Ville Syrjälä wrote:
> On Wed, Jul 02, 2025 at 09:42:04AM +0000, Borah, Chaitanya Kumar wrote:
>>
>>> -----Original Message-----
>>> From: Jani Nikula <jani.nikula at linux.intel.com>
>>> Sent: Wednesday, July 2, 2025 2:01 PM
>>> To: Borah, Chaitanya Kumar <chaitanya.kumar.borah at intel.com>; intel-
>>> xe at lists.freedesktop.org; intel-gfx at lists.freedesktop.org
>>> Cc: Deak, Imre <imre.deak at intel.com>; ville.syrjala at linux.intel.com; Shankar,
>>> Uma <uma.shankar at intel.com>; Borah, Chaitanya Kumar
>>> <chaitanya.kumar.borah at intel.com>
>>> Subject: Re: [PATCH] drm/i915/display: Add upper limit check for pixel clock
>>>
>>> On Wed, 02 Jul 2025, Chaitanya Kumar Borah
>>> <chaitanya.kumar.borah at intel.com> wrote:
>>>> Add upper limit check for pixel clock by platform. Limits don't apply
>>>> when DSC is enabled.
>>>>
>>>> For the currently supported versions of HDMI, pixel clock is already
>>>> limited to 600Mhz so nothing needs to be done there as of now.
>>>>
>>>> BSpec: 49199, 68912
>>>>
>>>> Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah at intel.com>
>>>> ---
>>>>   drivers/gpu/drm/i915/display/intel_display.c | 12 ++++++++++++
>>>> drivers/gpu/drm/i915/display/intel_display.h |  1 +
>>>>   drivers/gpu/drm/i915/display/intel_dp.c      |  3 +++
>>>>   3 files changed, 16 insertions(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/display/intel_display.c
>>>> b/drivers/gpu/drm/i915/display/intel_display.c
>>>> index 456fc4b04cda..bf0faff50c2e 100644
>>>> --- a/drivers/gpu/drm/i915/display/intel_display.c
>>>> +++ b/drivers/gpu/drm/i915/display/intel_display.c
>>>> @@ -7902,6 +7902,18 @@ void intel_setup_outputs(struct intel_display
>>> *display)
>>>>   	drm_helper_move_panel_connectors_to_head(display->drm);
>>>>   }
>>>>
>>>> +int intel_dotclock_limit(struct intel_display *display) {
>>>> +	if (DISPLAY_VERx100(display) == 3002)
>>>> +		return 937500;
>>>> +	else if (DISPLAY_VER(display) >= 30)
>>>> +		return 1350000;
>>>> +	else if (DISPLAY_VER(display) >= 13)
>>>> +		return 1200000;
>>>> +	else
>>>> +		return 1100000;
>>>> +}
>>> How does this relate to display->cdclk.max_dotclk_freq that's computed in
>>> intel_compute_max_dotclk(display)?
>>>
>> The Bspec defines this limit as "maximum validated frequency" and asks to limit the pixel clock accordingly when DSC is not enabled.
>> At least for PTL it is 97.65 % of the mathematically calculated maximum pixel clock (pixel per clock * max cdclk).
>>
>> I am not sure if intel_cdclk_guardband() used in intel_compute_max_dotclk() was intended for something similar. However, since the limit only applies
>> when dsc is not enabled it made sense to me that we apply this limit during intel_dp_mode_valid () instead of doing it in intel_compute_max_dotclk().
> I think what we need is something like:
> - implement the DSC bubble stuff and account for it in both
>    cdclk calculations and mode validation

I have some patch for accounting the DSC bubble stuff for cdclk 
calculations, but not for mode validation though.

https://patchwork.freedesktop.org/series/150083/

Still need to test properly.

Regards,

Ankit

> - implement this non-dsc cdclk limit checks also in a way that
>    covers both (also needs to cover all output types, not just DP).
>    I suspect the DSC bubble stuff might always end up more restrictive
>    than this, so perhaps this could just be covered by adjusting the
>    cdclk guardband
>