On 2021-09-30 14:27, Matthew Auld wrote:
> On 30/09/2021 11:04, Michel Dänzer wrote:
>> On 2021-09-29 13:54, Thomas Hellström wrote:
>>> 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.a...@intel.com>
>>>> Cc: Thomas Hellström <thomas.hellst...@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.
>>
>> FWIW, the Mesa radeonsi driver avoids surprises due to this (e.g. some path
>> which involves CPU access suddenly goes faster if the BO was evicted from
>> VRAM) by asking for WC mapping of BOs intended to be in VRAM even while
>> they're evicted (via the AMDGPU_GEM_CREATE_CPU_GTT_USWC flag).
>>
>
> Ok, so amdgpu just defaults to cached system memory, even for evicted VRAM,
> unless userspace requests USWC, in which case it will use WC?
Right.
--
Earthling Michel Dänzer | https://redhat.com
Libre software enthusiast | Mesa and Xwayland developer
