Re: [PATCH 01/12] drm/i915: Reduce CHV DDL multiplier to 16/8

[Arun R Murthy <arun.r.murthy@intel.com>]

>
> From: Ville Syrjälä<ville.syrjala@linux.intel.com>
>
> Apparently we must yet halve the DDL drain latency from what we're
> using currently. This little nugget is not in any spec, but came
> down through the grapevine.
>
> This makes the displays a bit more stable. Not quite fully stable but at
> least they don't fall over immediately on driver load.
>
> Signed-off-by: Ville Syrjälä<ville.syrjala@linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_reg.h | 1 +
>   drivers/gpu/drm/i915/intel_pm.c | 6 +++---
>   2 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 4ee1964..d8a0205 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -4166,6 +4166,7 @@ enum skl_disp_power_wells {
>   #define   DSPFW_PLANEA_WM1_HI_MASK	(1<<0)
>   
>   /* drain latency register values*/
> +#define DRAIN_LATENCY_PRECISION_8	8
>   #define DRAIN_LATENCY_PRECISION_16	16
>   #define DRAIN_LATENCY_PRECISION_32	32
>   #define DRAIN_LATENCY_PRECISION_64	64
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 3c64810..a70bce4 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -728,8 +728,8 @@ static bool vlv_compute_drain_latency(struct drm_crtc *crtc,
>   
>   	entries = DIV_ROUND_UP(clock, 1000) * pixel_size;
>   	if (IS_CHERRYVIEW(dev))
> -		*prec_mult = (entries > 128) ? DRAIN_LATENCY_PRECISION_32 :
> -					       DRAIN_LATENCY_PRECISION_16;
> +		*prec_mult = (entries > 32) ? DRAIN_LATENCY_PRECISION_16 :
> +					      DRAIN_LATENCY_PRECISION_8;
>   	else
>   		*prec_mult = (entries > 128) ? DRAIN_LATENCY_PRECISION_64 :
>   					       DRAIN_LATENCY_PRECISION_32;
> @@ -759,7 +759,7 @@ static void vlv_update_drain_latency(struct drm_crtc *crtc)
>   	enum pipe pipe = intel_crtc->pipe;
>   	int plane_prec, prec_mult, plane_dl;
>   	const int high_precision = IS_CHERRYVIEW(dev) ?
> -		DRAIN_LATENCY_PRECISION_32 : DRAIN_LATENCY_PRECISION_64;
The spec says that the drain latency precision are 32 and 16.
But just to be safe and have a stable environment, we further divide the
calculated DDL by 2 but precision remains the same 32 and 16.

Thanks and Regards,
Arun R Murthy
-------------------

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