Re: [PATCH 1/4] drm/i915: Use a hybrid scheme for fast register waits

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 30/06/16 12:30, Chris Wilson wrote:
> Ville Syrjälä reported that in the majority of wait_for(I915_READ()) he
> inspect, most completed within the first couple of reads and that the
> delay between those wait_for() reads was the ratelimiting step for many
> code paths. For example, __gen6_update_ring_freq() was blamed for
> slowing down boot by many milliseconds, but under Ville's scrutiny the
> issue was just excessive delay waiting for sandybridge_pcode_write().
>
> We can eliminate the wait by initially using a busyspin upon the register
> read and only fallback to the sleeping loop in cases where the hardware
> is indeed too slow. A threshold of 2 microseconds is used as the initial
> ballpark.
>
> To avoid excessive code bloating from converting every wait_for() into a
> hybrid busy/sleep loop, we extend wait_for_register_fw() and export it
> for use by other callers.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_drv.h     | 11 +++++
>   drivers/gpu/drm/i915/intel_uncore.c | 83 ++++++++++++++++++++++++++++++++-----
>   2 files changed, 83 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 82b20e14b065..48d30676455e 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2968,6 +2968,17 @@ u64 intel_uncore_edram_size(struct drm_i915_private *dev_priv);
>
>   void assert_forcewakes_inactive(struct drm_i915_private *dev_priv);
>
> +int intel_wait_for_register(struct drm_i915_private *dev_priv,
> +			    i915_reg_t reg,
> +			    const u32 mask,
> +			    const u32 value,
> +			    const unsigned long timeout_ms);
> +int intel_wait_for_register_fw(struct drm_i915_private *dev_priv,
> +			       i915_reg_t reg,
> +			       const u32 mask,
> +			       const u32 value,
> +			       const unsigned long timeout_ms);
> +
>   static inline bool intel_gvt_active(struct drm_i915_private *dev_priv)
>   {
>   	return dev_priv->gvt.initialized;
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> index c1ca458d688e..4c166f6550be 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -1609,13 +1609,74 @@ static int gen6_reset_engines(struct drm_i915_private *dev_priv,
>   	return ret;
>   }
>
> -static int wait_for_register_fw(struct drm_i915_private *dev_priv,
> -				i915_reg_t reg,
> -				const u32 mask,
> -				const u32 value,
> -				const unsigned long timeout_ms)
> +/**
> + * intel_wait_for_register_fw - wait until register matches expected state
> + * @dev_priv: the i915 device
> + * @reg: the register to read
> + * @mask: mask to apply to register value
> + * @value: expected value
> + * @timeout_ms: timeout in millisecond
> + *
> + * This routine waits until the target register @reg contains the expected
> + * @value after applying the @mask, i.e. it waits until
> + *   (I915_READ_FW(@reg) & @mask) == @value
> + * Otherwise, the wait will timeout after @timeout_ms milliseconds.
> + *
> + * Note that this routine assumes the caller holds forcewake asserted, it is
> + * not suitable for very long waits. See intel_wait_for_register() if you
> + * wish to wait without holding forcewake for the duration (i.e. you expect
> + * the wait to be slow).
> + *
> + * Returns 0 if the register matches the desired condition, or -ETIMEOUT.
> + */
> +int intel_wait_for_register_fw(struct drm_i915_private *dev_priv,
> +			       i915_reg_t reg,
> +			       const u32 mask,
> +			       const u32 value,
> +			       const unsigned long timeout_ms)
> +{
> +#define done ((I915_READ_FW(reg) & mask) == value)
> +	int ret = wait_for_us(done, 2);
> +	if (ret)
> +		ret = wait_for(done, timeout_ms);
> +	return ret;
> +#undef done
> +}
> +
> +/**
> + * intel_wait_for_register - wait until register matches expected state
> + * @dev_priv: the i915 device
> + * @reg: the register to read
> + * @mask: mask to apply to register value
> + * @value: expected value
> + * @timeout_ms: timeout in millisecond
> + *
> + * This routine waits until the target register @reg contains the expected
> + * @value after applying the @mask, i.e. it waits until
> + *   (I915_READ(@reg) & @mask) == @value
> + * Otherwise, the wait will timeout after @timeout_ms milliseconds.
> + *
> + * Returns 0 if the register matches the desired condition, or -ETIMEOUT.
> + */
> +int intel_wait_for_register(struct drm_i915_private *dev_priv,
> +			    i915_reg_t reg,
> +			    const u32 mask,
> +			    const u32 value,
> +			    const unsigned long timeout_ms)
>   {
> -	return wait_for((I915_READ_FW(reg) & mask) == value, timeout_ms);
> +
> +	unsigned fw =
> +		intel_uncore_forcewake_for_reg(dev_priv, reg, FW_REG_READ);
> +	int ret;
> +
> +	intel_uncore_forcewake_get(dev_priv, fw);
> +	ret = wait_for_us((I915_READ_FW(reg) & mask) == value, 2);
> +	intel_uncore_forcewake_put(dev_priv, fw);
> +	if (ret)
> +		ret = wait_for((I915_READ_NOTRACE(reg) & mask) == value,
> +			       timeout_ms);
> +
> +	return ret;
>   }
>
>   static int gen8_request_engine_reset(struct intel_engine_cs *engine)
> @@ -1626,11 +1687,11 @@ static int gen8_request_engine_reset(struct intel_engine_cs *engine)
>   	I915_WRITE_FW(RING_RESET_CTL(engine->mmio_base),
>   		      _MASKED_BIT_ENABLE(RESET_CTL_REQUEST_RESET));
>
> -	ret = wait_for_register_fw(dev_priv,
> -				   RING_RESET_CTL(engine->mmio_base),
> -				   RESET_CTL_READY_TO_RESET,
> -				   RESET_CTL_READY_TO_RESET,
> -				   700);
> +	ret = intel_wait_for_register_fw(dev_priv,
> +					 RING_RESET_CTL(engine->mmio_base),
> +					 RESET_CTL_READY_TO_RESET,
> +					 RESET_CTL_READY_TO_RESET,
> +					 700);
>   	if (ret)
>   		DRM_ERROR("%s: reset request timeout\n", engine->name);
>
>

Looks good to me.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko


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