[PATCH 1/2] drm/i915: Allow evicting to use the requested placement

[Cavitt, Jonathan]

-----Original Message-----
From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of David Gow
Sent: Sunday, August 4, 2024 2:19 AM
To: Christian König <ckoenig.leichtzumerken at gmail.com>; Somalapuram Amaranath <Amaranath.Somalapuram at amd.com>; Thomas Hellström <thomas.hellstrom at linux.intel.com>; De Marchi, Lucas <lucas.demarchi at intel.com>; Andi Shyti <andi.shyti at linux.intel.com>; Thomas Zimmermann <tzimmermann at suse.de>
Cc: David Gow <david at davidgow.net>; Jani Nikula <jani.nikula at linux.intel.com>; Joonas Lahtinen <joonas.lahtinen at linux.intel.com>; Vivi, Rodrigo <rodrigo.vivi at intel.com>; Tvrtko Ursulin <tursulin at ursulin.net>; Ville Syrjälä <ville.syrjala at linux.intel.com>; intel-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org; linux-kernel at vger.kernel.org
Subject: [PATCH 1/2] drm/i915: Allow evicting to use the requested placement
> 
> In commit a78a8da51b36 ("drm/ttm: replace busy placement with flags v6"),
> the old system of having a separate placement list (for placements
> which should be used without eviction) and a 'busy' placement list (for
> placements which should be attempted if eviction is required) was
> replaced with a new one where placements could be marked 'FALLBACK' (to
> be attempted if eviction is required) or 'DESIRED' (to be attempted
> first, but not if eviction is required).
> 
> i915 had always included the requested placement in the list of
> 'busy' placements: i.e., the placement could be used either if eviction
> is required or not. But when the new system was put in place, the
> requested (first) placement was marked 'DESIRED', so would never be used
> if eviction became necessary. While a bug in the original commit
> prevented this flag from working, when this was fixed in
> 4a0e7b3c ("drm/i915: fix applying placement flag"), it caused long hangs
> on DG2 systems with small BAR.
> 
> Don't mark the requested placement DESIRED (or FALLBACK), allowing it to
> be used in both situations. This matches the old behaviour, and resolves
> the hangs.
> 
> Thanks to Justin Brewer for bisecting the issue.
> 
> Fixes: a78a8da51b36 ("drm/ttm: replace busy placement with flags v6")
> Fixes: 4a0e7b3c3753 ("drm/i915: fix applying placement flag")
> Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11255
> Signed-off-by: David Gow <david at davidgow.net>

Thank you for the thorough write-up of this issue.
Reviewed-by: Jonathan Cavitt <jonathan.cavitt at intel.com>
-Jonathan Cavitt
 
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_ttm.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> index e6f177183c0f..fb848fd8ba15 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> @@ -165,7 +165,6 @@ i915_ttm_placement_from_obj(const struct drm_i915_gem_object *obj,
>  	i915_ttm_place_from_region(num_allowed ? obj->mm.placements[0] :
>  				   obj->mm.region, &places[0], obj->bo_offset,
>  				   obj->base.size, flags);
> -	places[0].flags |= TTM_PL_FLAG_DESIRED;
>  
>  	/* Cache this on object? */
>  	for (i = 0; i < num_allowed; ++i) {
> -- 
> 2.46.0
> 
>