[PATCH v8 4/6] drm/ttm: Add a shrinker helper and export the LRU walker for driver use

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

Following the design direction communicated here:

https://lore.kernel.org/linux-mm/b7491378-defd-4f1c-31e2-29e4c77e2d67@amd.com/T/#ma918844aa8a6efe8768fdcda0c6590d5c93850c9

Export the LRU walker for driver shrinker use and add a bo
shrinker helper for initial use by the xe driver.

v8:
- Split out from another patch.
- Use a struct for bool arguments to increase readability (Matt Brost).
- Unmap user-space cpu-mappings before shrinking pages.
- Explain non-fatal error codes (Matt Brost)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 drivers/gpu/drm/ttm/ttm_bo_util.c | 65 +++++++++++++++++++++++++++++++
 include/drm/ttm/ttm_bo.h          | 17 ++++++++
 2 files changed, 82 insertions(+)

diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index 3c07f4712d5c..3490e3347de9 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -919,3 +919,68 @@ s64 ttm_lru_walk_for_evict(struct ttm_lru_walk *walk, struct ttm_device *bdev,
 
 	return progress;
 }
+EXPORT_SYMBOL(ttm_lru_walk_for_evict);
+
+/**
+ * ttm_bo_try_shrink - LRU walk helper to shrink a ttm buffer object.
+ * @walk: The struct xe_ttm_lru_walk that describes the walk.
+ * @bo: The buffer object.
+ * @flags: Flags governing the shrinking behaviour.
+ *
+ * The function uses the ttm_tt_back_up functionality to back up or
+ * purge a struct ttm_tt. If the bo is not in system, it's first
+ * moved there, unless @flags.allow_move is false.
+ *
+ * Return: The number of pages shrunken or purged, or
+ * negative error code on failure.
+ */
+long ttm_bo_try_shrink(struct ttm_lru_walk *walk, struct ttm_buffer_object *bo,
+		       const struct ttm_bo_shrink_flags flags)
+{
+	static const struct ttm_place sys_placement_flags = {
+		.fpfn = 0,
+		.lpfn = 0,
+		.mem_type = TTM_PL_SYSTEM,
+		.flags = 0,
+	};
+	static struct ttm_placement sys_placement = {
+		.num_placement = 1,
+		.placement = &sys_placement_flags,
+	};
+	struct ttm_operation_ctx *ctx = walk->ctx;
+	struct ttm_tt *tt = bo->ttm;
+	long lret;
+
+	dma_resv_assert_held(bo->base.resv);
+
+	if (!tt || !ttm_tt_is_populated(tt))
+		return 0;
+
+	if (flags.allow_move && bo->resource->mem_type != TTM_PL_SYSTEM) {
+		int ret = ttm_bo_validate(bo, &sys_placement, ctx);
+
+		/* Consider -ENOMEM and -ENOSPC non-fatal. */
+		if (ret) {
+			if (ret == -ENOMEM || ret == -ENOSPC)
+				ret = -EBUSY;
+			return ret;
+		}
+	}
+
+	ttm_bo_unmap_virtual(bo);
+	lret = ttm_bo_wait_ctx(bo, ctx);
+	if (lret < 0) {
+		if (lret == -ERESTARTSYS)
+			return lret;
+		return 0;
+	}
+
+	lret = ttm_tt_backup(bo->bdev, tt, (struct ttm_backup_flags)
+			     {.purge = flags.purge,
+			      .writeback = flags.writeback});
+	if (lret < 0 && lret != -EINTR)
+		return 0;
+
+	return lret;
+}
+EXPORT_SYMBOL(ttm_bo_try_shrink);
diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h
index d1a732d56259..479ada85cea1 100644
--- a/include/drm/ttm/ttm_bo.h
+++ b/include/drm/ttm/ttm_bo.h
@@ -229,6 +229,23 @@ struct ttm_lru_walk {
 s64 ttm_lru_walk_for_evict(struct ttm_lru_walk *walk, struct ttm_device *bdev,
 			   struct ttm_resource_manager *man, s64 target);
 
+/**
+ * struct ttm_bo_shrink_flags - flags to govern the bo shrinking behaviour
+ * @purge: Purge the content rather than backing it up.
+ * @writeback: Attempt to immediately write content to swap space.
+ * @allow_move: Allow moving to system before shrinking. This is typically
+ * not desired for zombie- or ghost objects (with zombie object meaning
+ * objects with a zero gem object refcount)
+ */
+struct ttm_bo_shrink_flags {
+	u32 purge : 1;
+	u32 writeback : 1;
+	u32 allow_move : 1;
+};
+
+long ttm_bo_try_shrink(struct ttm_lru_walk *walk, struct ttm_buffer_object *bo,
+		       const struct ttm_bo_shrink_flags flags);
+
 /**
  * ttm_bo_get - reference a struct ttm_buffer_object
  *
-- 
2.44.0