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

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Date:   Mon, 23 May 2022 16:53:26 +0100
From:   Robert Beckett <bob.beckett@...labora.com>
To:     Thomas Hellström 
        <thomas.hellstrom@...ux.intel.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



On 11/05/2022 13:42, Thomas Hellström wrote:
> 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().

agreed. nice catch.
I'll fix for v2

looks like we could maybe do with some extra shrinker testing too? looks 
like nothing caught this during CI testing

> 
> /Thomas
> 
>