[Intel-gfx] [PATCH] drm/i915: Performed deferred clflush inside set-cache-level

[Daniel Vetter]

On Tue, Jan 13, 2015 at 01:32:52PM +0000, Chris Wilson wrote:
> Currently we are hitting the WARN inside
> i915_gem_object_set_cache_level() as we can now have an unbound object
> in the GTT write domain (due to 43566dedde54f9 "drm/i915: Broaden
> application of set-domain(GTT)"). To avoid the warning, we need to track
> when we elided the clflush on a cacheable object and then evict the
> cache for the object when we move the object out of a cacheable domain.
>
> Reported-by: Jani Nikula <jani.nikula at linux.intel.com>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h |  1 +
>  drivers/gpu/drm/i915/i915_gem.c | 32 +++++++++-----------------------
>  2 files changed, 10 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index bf18a5238887..7070482000cd 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1998,6 +1998,7 @@ struct drm_i915_gem_object {
>   */
>   unsigned long gt_ro:1;
>   unsigned int cache_level:3;
> + unsigned int cache_dirty:1;
>
>   unsigned int has_dma_mapping:1;
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 9bafe50d3df7..aa089e7c31bf 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3639,11 +3639,14 @@ i915_gem_clflush_object(struct drm_i915_gem_object *obj,
>   * snooping behaviour occurs naturally as the result of our domain
>   * tracking.
>   */
> - if (!force && cpu_cache_is_coherent(obj->base.dev, obj->cache_level))
> + if (!force && cpu_cache_is_coherent(obj->base.dev, obj->cache_level)) {
> + obj->cache_dirty = true;
>   return false;
> + }
>
>   trace_i915_gem_object_clflush(obj);
>   drm_clflush_sg(obj->pages);
> + obj->cache_dirty = false;
>
>   return true;
>  }
> @@ -3826,28 +3829,11 @@ int i915_gem_object_set_cache_level(struct drm_i915_gem_object *obj,
>   vma->node.color = cache_level;
>   obj->cache_level = cache_level;
>
> - if (cpu_write_needs_clflush(obj)) {
> - u32 old_read_domains, old_write_domain;
> -
> - /* If we're coming from LLC cached, then we haven't
> - * actually been tracking whether the data is in the
> - * CPU cache or not, since we only allow one bit set
> - * in obj->write_domain and have been skipping the clflushes.
> - * Just set it to the CPU cache for now.
> - */
> - i915_gem_object_retire(obj);
> - WARN_ON(obj->base.write_domain & ~I915_GEM_DOMAIN_CPU);
> -
> - old_read_domains = obj->base.read_domains;
> - old_write_domain = obj->base.write_domain;
> -
> - obj->base.read_domains = I915_GEM_DOMAIN_CPU;
> - obj->base.write_domain = I915_GEM_DOMAIN_CPU;
> -
> - trace_i915_gem_object_change_domain(obj,
> -    old_read_domains,
> -    old_write_domain);
> - }
> + if (obj->cache_dirty &&
> +    obj->base.write_domain != I915_GEM_DOMAIN_CPU &&
> +    cpu_write_needs_clflush(obj) &&
> +    i915_gem_clflush_object(obj, true))

Imo hiding the actual action in the if condition like this is a bit too
evil. Also, can we please have a testcase to at lest exercise the
codepath? It sounds like a real functional tests using crc is a bit more
work, but just poking at the WARN_ON would be good already.
-Daniel

> + i915_gem_chipset_flush(obj->base.dev);
>
>   return 0;
>  }
> --
> 2.1.4
>
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch