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

Jesse Barnes
2015-02-27T17:44:06Z

On 02/10/2015 05:28 AM, ville.syrjala@linux.intel.com wrote:
> 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;
> + DRAIN_LATENCY_PRECISION_16 : DRAIN_LATENCY_PRECISION_64;
>
> plane_dl = I915_READ(VLV_DDL(pipe)) & ~(DDL_PLANE_PRECISION_HIGH |
> DRAIN_LATENCY_MASK | DDL_CURSOR_PRECISION_HIGH |
>

You're also changing the entries threshold; is that due to the reduced
drain latency precsion also?

Would be good to file a bspec bug on this so we don't lose it.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>