On Tue, Jul 30, 2013 at 08:19:28PM +0300, Ville Syrjälä wrote:
> On Tue, Jul 30, 2013 at 05:58:36PM +0100, Chris Wilson wrote:
> > Haswell GT3e has the unique feature of supporting Write-Through cacheing
> > of objects within the eLLC. The purpose of this is to enable the display
> > plane to remain coherent whilst objects lie resident in the eLLC - so
> > that we in theory get the best of both worlds, perfect display and fast
> > access.
>
> The description here talks about eLLC only, but you set the PTE for
> WT in LLC/eLLC both.
s/eLLC/LLC/
For some reason, I keep telling myself that it is a magic property of
the eLLC otherwise why wouldn't they do it for all LLC!
> > - ret = i915_gem_object_set_cache_level(obj, I915_CACHE_NONE);
> > + ret = i915_gem_object_set_cache_level(obj,
> > + HAS_WT(obj->base.dev) ?
> > I915_CACHE_WT : I915_CACHE_NONE);
>
> Don't we need to tweak the write domain like we do for UC to make sure
> already dirty lines get flushed from caches?
You need to explicitly do the flush, which gets ugly. I choose to ignore
the problem as unlike the LLC -> UC transition, I haven't spotted any
dirt. However, it doesn't look too bad if we replace it like so:
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ac1b9cd..0e089e2 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3362,27 +3362,32 @@ int i915_gem_object_set_cache_level(struct
drm_i915_gem_object *obj,
i915_gem_obj_ggtt_set_color(obj, cache_level);
}
- if (cache_level == I915_CACHE_NONE) {
- u32 old_read_domains, old_write_domain;
-
+ if (cache_level == I915_CACHE_NONE ||
+ cache_level == I915_CACHE_WT) {
/* 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.
+ * Do them now.
*/
- WARN_ON(obj->base.write_domain & ~I915_GEM_DOMAIN_CPU);
- WARN_ON(obj->base.read_domains & ~I915_GEM_DOMAIN_CPU);
+ if (obj->base.read_domains & I915_GEM_DOMAIN_CPU) {
+ u32 old_read_domains, old_write_domain;
- old_read_domains = obj->base.read_domains;
- old_write_domain = obj->base.write_domain;
+ BUG_ON(obj->pages == NULL);
- obj->base.read_domains = I915_GEM_DOMAIN_CPU;
- obj->base.write_domain = I915_GEM_DOMAIN_CPU;
+ trace_i915_gem_object_clflush(obj);
+ drm_clflush_sg(obj->pages);
- trace_i915_gem_object_change_domain(obj,
- old_read_domains,
- old_write_domain);
+ 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);
+ }
}
obj->cache_level = cache_level;
--
Chris Wilson, Intel Open Source Technology Centre
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