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

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

On Thu, 16 Feb 2017, Madhav Chauhan <madhav.chauhan@intel.com> 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(HW team
> checking on this). So cdclk should be calculated for the given pixclk as
> per that otherwise it may lead to screen corruption for some scenarios.
>
> v2: Rebased to new CDLCK code framework
>
> Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_cdclk.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_cdclk.c b/drivers/gpu/drm/i915/intel_cdclk.c
> index d643c0c..834df68 100644
> --- a/drivers/gpu/drm/i915/intel_cdclk.c
> +++ b/drivers/gpu/drm/i915/intel_cdclk.c
> @@ -1071,9 +1071,9 @@ static int bxt_calc_cdclk(int max_pixclk)
>  
>  static int glk_calc_cdclk(int max_pixclk)
>  {
> -	if (max_pixclk > 2 * 158400)
> +	if (max_pixclk > DIV_ROUND_UP(2 * 158400 * 99, 100))

Where do we ensure we don't use pixel clock 312841..316800? Clearly we
shouldn't use that because we can't guarantee it works, right?

Before we get the spec update to confirm what to do, I think we need a
comment here explaining what's going on.

BR,
Jani.

>  		return 316800;
> -	else if (max_pixclk > 2 * 79200)
> +	else if (max_pixclk > DIV_ROUND_UP(2 * 79200 * 99, 100))
>  		return 158400;
>  	else
>  		return 79200;

-- 
Jani Nikula, Intel Open Source Technology Center
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