Re: [PATCH] drm/i915: Release the atomic kmap relocation cache around snb GTT w/a

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

On 12/02/2018 21:11, Chris Wilson wrote:
> When we need to rebind the vma into the global GTT for snb, we need to
> drop the current reloc_cache as it will be holding a kmap_atomic() and
> we may need to sleep for i915_vma_bind(). In practice, this is not an
> issue as we already hold an rpm reference for the execbuffer, but with
> tighter error checking around rpm we need to be more careful.
> 
> References: 31a39207f04a ("drm/i915: Cache kmap between relocations")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_gem_execbuffer.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index b15305f2fb76..8c34b1b5a126 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1315,6 +1315,12 @@ eb_relocate_entry(struct i915_execbuffer *eb,
>   		 */
>   		if (reloc->write_domain == I915_GEM_DOMAIN_INSTRUCTION &&
>   		    IS_GEN6(eb->i915)) {
> +			/*
> +			 * Release the kmap_atomic cache in order to allow the
> +			 * i915_vma_bind() to sleep (if needs be).
> +			 */
> +			reloc_cache_reset(&eb->reloc_cache);
> +
>   			err = i915_vma_bind(target, target->obj->cache_level,
>   					    PIN_GLOBAL);
>   			if (WARN_ONCE(err,
> 

Hmm yes, very interesting. If you are happy with the performance hit then:

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

But in general could it also be solved by changing how 
intel_runtime_pm_get works - making our wakeref_count top level and only 
calling pm_runtime_get_sync on 0 to 1 transition? That would solve the 
issue with proposed might_sleep and code paths which know the condition 
is actually impossible.

Regards,

Tvrtko
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