[Intel-gfx] [PATCH 2/4] drm/i915/gen9: Verify and enforce dc6 state writes

[Imre Deak]

On to, 2016-02-18 at 17:21 +0200, Mika Kuoppala wrote:
> It has been observed that sometimes disabling the dc6 fails
> and dc6 state pops back up, brief moment after disabling. This
> has to be dmc save/restore timing issue or other bug in the
> way dc states are handled.
> 
> Try to work around this issue as we don't have firmware fix
> yet available. Verify that the value we wrote for the dmc sticks,
> and also enforce it by rewriting it, if it didn't.
> 
> Testcase: kms_flip/basic-flip-vs-dpms
> References: https://bugs.freedesktop.org/show_bug.cgi?id=93768
> Cc: Patrik Jakobsson <patrik.jakobsson at linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> Cc: Imre Deak <imre.deak at intel.com>
> Signed-off-by: Mika Kuoppala <mika.kuoppala at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_runtime_pm.c | 40
> +++++++++++++++++++++++++++++++--
>  1 file changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_runtime_pm.c
> b/drivers/gpu/drm/i915/intel_runtime_pm.c
> index 8b9290fdb3b2..cb91540cfbad 100644
> --- a/drivers/gpu/drm/i915/intel_runtime_pm.c
> +++ b/drivers/gpu/drm/i915/intel_runtime_pm.c
> @@ -470,6 +470,42 @@ static void
> gen9_set_dc_state_debugmask_memory_up(
>  	}
>  }
>  
> +static void gen9_write_dc_state(struct drm_i915_private *dev_priv,
> +				u32 state)
> +{
> +	int rewrites = 0;
> +	int rereads = 0;
> +	u32 v;
> +
> +	I915_WRITE(DC_STATE_EN, state);
> +
> +	/* It has been observed that disabling the dc6 state
> sometimes
> +	 * doesn't stick and dmc keeps returning old value. Make
> sure
> +	 * the write really sticks enough times and also force
> rewrite until
> +	 * we are confident that state is exactly what we want.
> +	 */
> +	do  {
> +		v = I915_READ(DC_STATE_EN);
> +
> +		if (v != state) {
> +			I915_WRITE(DC_STATE_EN, state);
> +			rewrites++;

Could be rereads = 0; for extra paranoia. Either way:
Reviewed-by: Imre Deak <imre.deak at intel.com>

> +		} else if (rereads++ > 5) {
> +			break;
> +		}
> +
> +	} while (rewrites < 100);
> +
> +	if (v != state)
> +		DRM_ERROR("Writing dc state to 0x%x failed, now
> 0x%x\n",
> +			  state, v);
> +
> +	/* Most of the times we need one retry, avoid spam */
> +	if (rewrites > 1)
> +		DRM_DEBUG_KMS("Rewrote dc state to 0x%x %d times\n",
> +			      state, rewrites);
> +}
> +
>  static void gen9_set_dc_state(struct drm_i915_private *dev_priv,
> uint32_t state)
>  {
>  	uint32_t val;
> @@ -502,8 +538,8 @@ static void gen9_set_dc_state(struct
> drm_i915_private *dev_priv, uint32_t state)
>  
>  	val &= ~mask;
>  	val |= state;
> -	I915_WRITE(DC_STATE_EN, val);
> -	POSTING_READ(DC_STATE_EN);
> +
> +	gen9_write_dc_state(dev_priv, val);
>  
>  	dev_priv->csr.dc_state = val & mask;
>  }