[Intel-gfx] [CI] drm/i915: Skip waking the device to service pwrite

[Tvrtko Ursulin]

On 19/10/2017 07:37, Chris Wilson wrote:
> If the device is in runtime suspend, resuming takes time and reduces our
> powersaving. If this was for a small write into an object, that resume
> will take longer than any savings in using the indirect GGTT access to
> avoid the cpu cache.

Commit talks about small writes but the patch takes no notice to size of 
requested writes. Is that intended?

Regards,

Tvrtko

> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> ---
>   drivers/gpu/drm/i915/i915_gem.c | 23 ++++++++++++++++++++---
>   1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index d699ea3ab80b..026cb52ece0b 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1240,7 +1240,23 @@ i915_gem_gtt_pwrite_fast(struct drm_i915_gem_object *obj,
>   	if (ret)
>   		return ret;
>   
> -	intel_runtime_pm_get(i915);
> +	if (i915_gem_object_has_struct_page(obj)) {
> +		/*
> +		 * Avoid waking the device up if we can fallback, as
> +		 * waking/resuming is very slow (worst-case 10-100 ms
> +		 * depending on PCI sleeps and our own resume time).
> +		 * This easily dwarfs any performance advantage from
> +		 * using the cache bypass of indirect GGTT access.
> +		 */
> +		if (!intel_runtime_pm_get_if_in_use(i915)) {
> +			ret = -EFAULT;
> +			goto out_unlock;
> +		}
> +	} else {
> +		/* No backing pages, no fallback, we must force GGTT access */
> +		intel_runtime_pm_get(i915);
> +	}
> +
>   	vma = i915_gem_object_ggtt_pin(obj, NULL, 0, 0,
>   				       PIN_MAPPABLE |
>   				       PIN_NONFAULT |
> @@ -1257,7 +1273,7 @@ i915_gem_gtt_pwrite_fast(struct drm_i915_gem_object *obj,
>   	if (IS_ERR(vma)) {
>   		ret = insert_mappable_node(ggtt, &node, PAGE_SIZE);
>   		if (ret)
> -			goto out_unlock;
> +			goto out_rpm;
>   		GEM_BUG_ON(!node.allocated);
>   	}
>   
> @@ -1320,8 +1336,9 @@ i915_gem_gtt_pwrite_fast(struct drm_i915_gem_object *obj,
>   	} else {
>   		i915_vma_unpin(vma);
>   	}
> -out_unlock:
> +out_rpm:
>   	intel_runtime_pm_put(i915);
> +out_unlock:
>   	mutex_unlock(&i915->drm.struct_mutex);
>   	return ret;
>   }
>