linux-kernel - Re: [PATCH 3/4] drm/i915: allow volatile buffers to use ttm pool allocator

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Date:   Wed, 11 May 2022 14:42:51 +0200
From:   Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>
To:     Robert Beckett <bob.beckett@...labora.com>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Matthew Auld <matthew.auld@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] drm/i915: allow volatile buffers to use ttm pool
 allocator

Hi, Bob,

On Tue, 2022-05-03 at 19:13 +0000, Robert Beckett wrote:
> internal buffers should be shmem backed.
> if a volatile buffer is requested, allow ttm to use the pool
> allocator
> to provide volatile pages as backing
> 
> Signed-off-by: Robert Beckett <bob.beckett@...labora.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_ttm.c | 3 ++-
>  1 file changed, 2 insertions(+), 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 4c25d9b2f138..fdb3a1c18cb6 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> @@ -309,7 +309,8 @@ static struct ttm_tt *i915_ttm_tt_create(struct
> ttm_buffer_object *bo,
>                 page_flags |= TTM_TT_FLAG_ZERO_ALLOC;
>  
>         caching = i915_ttm_select_tt_caching(obj);
> -       if (i915_gem_object_is_shrinkable(obj) && caching ==
> ttm_cached) {
> +       if (i915_gem_object_is_shrinkable(obj) && caching ==
> ttm_cached &&
> +           !i915_gem_object_is_volatile(obj)) {
>                 page_flags |= TTM_TT_FLAG_EXTERNAL |
>                               TTM_TT_FLAG_EXTERNAL_MAPPABLE;
>                 i915_tt->is_shmem = true;

While this is ok, I think it also needs adjustment in the i915_ttm
shrink callback. If someone creates a volatile smem object which then
hits the shrinker, I think we might hit asserts that it's a is_shem
ttm?

In this case, the shrink callback should just i915_ttm_purge().

/Thomas