[PATCH v19 24/30] drm/shmem-helper: Optimize unlocked get_pages_sgt()

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

SGT isn't refcounted. Once SGT pointer has been obtained, it remains the
same for both locked and unlocked get_pages_sgt(). Return cached SGT
directly without taking a potentially expensive lock.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 8fd7851c088b..e6e6e693ab95 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -962,6 +962,18 @@ struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_shmem_object *shmem)
 	    drm_WARN_ON(obj->dev, drm_gem_shmem_is_purgeable(shmem)))
 		return ERR_PTR(-EBUSY);
 
+	/*
+	 * Drivers that use shrinker should take into account that shrinker
+	 * may relocate BO, thus invalidating the returned SGT pointer.
+	 * Such drivers should pin GEM while they use SGT.
+	 *
+	 * Drivers that don't use shrinker should take into account that
+	 * SGT is released together with the GEM pages. Pages should be kept
+	 * alive while SGT is used.
+	 */
+	if (shmem->sgt)
+		return shmem->sgt;
+
 	ret = dma_resv_lock_interruptible(shmem->base.resv, NULL);
 	if (ret)
 		return ERR_PTR(ret);
-- 
2.43.0