Re: [PATCH] drm/i915: Treat WC a separate cache domain

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

On ke, 2017-04-05 at 22:07 +0100, Chris Wilson wrote:
> When discussing a new WC mmap, we based the interface upon the
> assumption that GTT was fully coherent. How naive! Commits 3b5724d702ef
> ("drm/i915: Wait for writes through the GTT to land before reading
> back") and ed4596ea992d ("drm/i915/guc: WA to address the Ringbuffer
> coherency issue") demonstrate that writes through the GTT are indeed
> delayed and may be overtaken by direct WC access. To be safe, if
> userspace is mixing WC mmaps with other potential GTT access (pwrite,
> GTT mmaps) it should use set_domain(WC).
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96563
> Testcase: igt/gem_pwrite/small-gtt*
> Testcase: igt/drv_selftest/coherency
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

<SNIP>

> +int
> +i915_gem_object_set_to_wc_domain(struct drm_i915_gem_object *obj, bool write)
> +{
> +	int ret;
> +
> +	lockdep_assert_held(&obj->base.dev->struct_mutex);
> +
> > +	ret = i915_gem_object_wait(obj,
> > +				   I915_WAIT_INTERRUPTIBLE |
> > +				   I915_WAIT_LOCKED |
> +				   (write ? I915_WAIT_ALL : 0),

Could construct into flags variable.

> +	/* Flush and acquire obj->pages so that we are coherent through
> +	 * direct access in memory with previous cached writes through
> +	 * shmemfs and that our cache domain tracking remains valid.
> +	 * For example, if the obj->filp was moved to swap without us
> +	 * being notified and releasing the pages, we would mistakenly
> +	 * continue to assume that the obj remained out of the CPU cached
> +	 * domain.
> +	 */
> +	ret = i915_gem_object_pin_pages(obj);
> +	if (ret)
> +		return ret;
> +
> +	i915_gem_object_flush_cpu_write_domain(obj);
> +	i915_gem_object_flush_gtt_write_domain(obj);

I was thinking if WC should be a "substate" of CPU domain, because we
don't have i915_gem_object_flush_wc_write_domain.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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