[PATCH v15 14/23] drm/shmem-helper: Add and use lockless drm_gem_shmem_get_pages()

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

Add lockless drm_gem_shmem_get_pages() helper that skips taking reservation
lock if pages_use_count is non-zero, leveraging from atomicity of the kref
counter. Make drm_gem_shmem_mmap() to utilize the new helper.

Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 5a2e37b3e51d..f386289c24fc 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -234,6 +234,20 @@ void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_put_pages_locked);
 
+static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
+{
+	int ret;
+
+	if (kref_get_unless_zero(&shmem->pages_use_count))
+		return 0;
+
+	dma_resv_lock(shmem->base.resv, NULL);
+	ret = drm_gem_shmem_get_pages_locked(shmem);
+	dma_resv_unlock(shmem->base.resv);
+
+	return ret;
+}
+
 static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
 {
 	int ret;
@@ -616,10 +630,7 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct
 		return ret;
 	}
 
-	dma_resv_lock(shmem->base.resv, NULL);
-	ret = drm_gem_shmem_get_pages_locked(shmem);
-	dma_resv_unlock(shmem->base.resv);
-
+	ret = drm_gem_shmem_get_pages(shmem);
 	if (ret)
 		return ret;
 
-- 
2.41.0