Re: [Intel-gfx] [PATCH v5 12/13] drm/i915/ttm: use cached system pages when evicting lmem

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Mon, 2021-09-27 at 12:41 +0100, Matthew Auld wrote:
> This should let us do an accelerated copy directly to the shmem pages
> when temporarily moving lmem-only objects, where the i915-gem
> shrinker
> can later kick in to swap out the pages, if needed.
> 
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_ttm.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> index 194e5f1deda8..46d57541c0b2 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> @@ -134,11 +134,11 @@ static enum ttm_caching
>  i915_ttm_select_tt_caching(const struct drm_i915_gem_object *obj)
>  {
>         /*
> -        * Objects only allowed in system get cached cpu-mappings.
> -        * Other objects get WC mapping for now. Even if in system.
> +        * Objects only allowed in system get cached cpu-mappings, or
> when
> +        * evicting lmem-only buffers to system for swapping. Other
> objects get
> +        * WC mapping for now. Even if in system.
>          */
> -       if (obj->mm.region->type == INTEL_MEMORY_SYSTEM &&
> -           obj->mm.n_placements <= 1)
> +       if (obj->mm.n_placements <= 1)
>                 return ttm_cached;
>  
>         return ttm_write_combined;

We should be aware that with TTM, even evicted bos can be mapped by
user-space while evicted, and this will appear to user-space like the
WC-mapped object suddenly became WB-mapped. But it appears like mesa
doesn't care about this as long as the mappings are fully coherent.

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>