Re: [PATCH] drm/i915/glk: CDCLK calculation changes for glk

[Jani Nikula <jani.nikula@intel.com>]

On Tue, 07 Feb 2017, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> On Tue, Feb 07, 2017 at 05:48:46AM -0500, Madhav Chauhan wrote:
>> As per BSPEC, valid cdclk values for glk are 79.2, 158.4, 316.8 Mhz.
>> Practically we can achive only 99% of these cdclk values. So cdclk
>> should be calculated for the given pixclk as per that otherwise it may
>> lead to screen corruption for some scenarios.
>
> I this caused by the hw actually requiring some guardband or simply
> because of our sucky DPLL code not updating the dotclock with the actual
> value the hw is going to produce?
>
> If it's a genuine hw limitation is it documented? And if not has
> someone filed a spec issue for it? And what about other platforms
> (GLK isn't the only one without a guardband in our code after all)?

Indeed. What's the 99% based on? Where is it documented?

BR,
Jani.

>
>> 
>> Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
>> ---
>>  drivers/gpu/drm/i915/intel_display.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
>> index 45e5874..2e1bfe9 100644
>> --- a/drivers/gpu/drm/i915/intel_display.c
>> +++ b/drivers/gpu/drm/i915/intel_display.c
>> @@ -6544,9 +6544,9 @@ static int valleyview_calc_cdclk(struct drm_i915_private *dev_priv,
>>  
>>  static int glk_calc_cdclk(int max_pixclk)
>>  {
>> -	if (max_pixclk > 2 * 158400)
>> +	if (max_pixclk > DIV_ROUND_UP(2 * 158400 * 99, 100))
>>  		return 316800;
>> -	else if (max_pixclk > 2 * 79200)
>> +	else if (max_pixclk > DIV_ROUND_UP(2 * 79200 * 99, 100))
>>  		return 158400;
>>  	else
>>  		return 79200;
>> -- 
>> 1.9.1
>> 
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-- 
Jani Nikula, Intel Open Source Technology Center
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