Re: [PATCH v3 3/6] drm/i915 Implement LMEM backup and restore for suspend / resume

[Matthew Auld <matthew.auld@intel.com>]

On 14/09/2021 20:31, Thomas Hellström wrote:
> Just evict unpinned objects to system. For pinned LMEM objects,
> make a backup system object and blit the contents to that.
> 
> Backup is performed in three steps,
> 1: Opportunistically evict evictable objects using the gpu blitter.
> 2: After gt idle, evict evictable objects using the gpu blitter. This will
> be modified in an upcoming patch to backup pinned objects that are not used
> by the blitter itself.
> 3: Backup remaining pinned objects using memcpy.
> 
> Also move uC suspend to after 2) to make sure we have a functional GuC
> during 2) if using GuC submission.
> 
> v2:
> - Major refactor to make sure gem_exec_suspend@hang-SX subtests work, and
>    suspend / resume works with a slightly modified GuC submission enabling
>    patch series.
> 
> v3:
> - Fix a potential use-after-free (Matthew Auld)
> - Use i915_gem_object_create_shmem() instead of
>    i915_gem_object_create_region (Matthew Auld)
> - Minor simplifications (Matthew Auld)
> - Fix up kerneldoc for i195_ttm_restore_region().
> - Final lmem_suspend() call moved to i915_gem_backup_suspend from
>    i915_gem_suspend_late, since the latter gets called at driver unload
>    and we don't unnecessarily want to run it at that time.
> 
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

<snip>

> +
> +static int i915_ttm_restore(struct i915_gem_apply_to_region *apply,
> +			    struct drm_i915_gem_object *obj)
> +{
> +	struct i915_gem_ttm_pm_apply *pm_apply =
> +		container_of(apply, typeof(*pm_apply), base);
> +	struct drm_i915_gem_object *backup = obj->ttm.backup;
> +	struct ttm_buffer_object *backup_bo = i915_gem_to_ttm(backup);
> +	struct ttm_operation_ctx ctx = {};
> +	int err;
> +
> +	if (!backup)
> +		return 0;
> +
> +	if (!pm_apply->allow_gpu && (obj->flags & I915_BO_ALLOC_USER))
> +		return 0;

Hmm, do we ever hit this? I would presume anything that userspace 
directly allocated in lmem can be kicked out with ttm_bo_validate(sys) 
i.e backup == NULL?

> +
> +	err = i915_gem_object_lock(backup, apply->ww);
> +	if (err)
> +		return err;
> +
> +	/* Content may have been swapped. */
> +	err = ttm_tt_populate(backup_bo->bdev, backup_bo->ttm, &ctx);
> +	if (!err) {
> +		err = i915_gem_obj_copy_ttm(obj, backup, pm_apply->allow_gpu,
> +					    false);
> +		GEM_WARN_ON(err);
> +
> +		obj->ttm.backup = NULL;
> +		err = 0;
> +	}
> +
> +	i915_gem_ww_unlock_single(backup);
> +
> +	if (!err)
> +		i915_gem_object_put(backup);
> +
> +	return err;
> +}
> +