[PATCH v2 01/15] drm/shmem-helper: Make pages_use_count an atomic_t

[Boris Brezillon]

This way we can grab a pages ref without acquiring the resv lock when
pages_use_count > 0. Need to implement asynchronous map using the
drm_gpuva_mgr when the map/unmap operation triggers a mapping split,
requiring the new left/right regions to grab an additional page ref
to guarantee that the pages stay pinned when the middle section is
unmapped.

Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c  | 28 +++++++++++++------------
 drivers/gpu/drm/lima/lima_gem.c         |  2 +-
 drivers/gpu/drm/panfrost/panfrost_mmu.c |  2 +-
 include/drm/drm_gem_shmem_helper.h      |  2 +-
 4 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index a783d2245599..ca6938ea1b82 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -155,7 +155,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 		if (shmem->pages)
 			drm_gem_shmem_put_pages(shmem);
 
-		drm_WARN_ON(obj->dev, shmem->pages_use_count);
+		drm_WARN_ON(obj->dev, atomic_read(&shmem->pages_use_count));
 
 		dma_resv_unlock(shmem->base.resv);
 	}
@@ -172,14 +172,14 @@ static int drm_gem_shmem_get_pages(struct drm_gem_shmem_object *shmem)
 
 	dma_resv_assert_held(shmem->base.resv);
 
-	if (shmem->pages_use_count++ > 0)
+	if (atomic_inc_return(&shmem->pages_use_count) > 1)
 		return 0;
 
 	pages = drm_gem_get_pages(obj);
 	if (IS_ERR(pages)) {
 		drm_dbg_kms(obj->dev, "Failed to get pages (%ld)\n",
 			    PTR_ERR(pages));
-		shmem->pages_use_count = 0;
+		atomic_set(&shmem->pages_use_count, 0);
 		return PTR_ERR(pages);
 	}
 
@@ -210,10 +210,10 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
 
 	dma_resv_assert_held(shmem->base.resv);
 
-	if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
+	if (drm_WARN_ON_ONCE(obj->dev, !atomic_read(&shmem->pages_use_count)))
 		return;
 
-	if (--shmem->pages_use_count > 0)
+	if (atomic_dec_return(&shmem->pages_use_count) > 0)
 		return;
 
 #ifdef CONFIG_X86
@@ -263,6 +263,10 @@ int drm_gem_shmem_pin(struct drm_gem_shmem_object *shmem)
 
 	drm_WARN_ON(obj->dev, obj->import_attach);
 
+	/* If we are the first owner, we need to grab the lock. */
+	if (atomic_inc_not_zero(&shmem->pages_use_count))
+		return 0;
+
 	ret = dma_resv_lock_interruptible(shmem->base.resv, NULL);
 	if (ret)
 		return ret;
@@ -286,6 +290,10 @@ void drm_gem_shmem_unpin(struct drm_gem_shmem_object *shmem)
 
 	drm_WARN_ON(obj->dev, obj->import_attach);
 
+	/* If we are the last owner, we need to grab the lock. */
+	if (atomic_add_unless(&shmem->pages_use_count, -1, 1))
+		return;
+
 	dma_resv_lock(shmem->base.resv, NULL);
 	drm_gem_shmem_unpin_locked(shmem);
 	dma_resv_unlock(shmem->base.resv);
@@ -543,18 +551,12 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
 
 	drm_WARN_ON(obj->dev, obj->import_attach);
 
-	dma_resv_lock(shmem->base.resv, NULL);
-
 	/*
 	 * We should have already pinned the pages when the buffer was first
 	 * mmap'd, vm_open() just grabs an additional reference for the new
 	 * mm the vma is getting copied into (ie. on fork()).
 	 */
-	if (!drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
-		shmem->pages_use_count++;
-
-	dma_resv_unlock(shmem->base.resv);
-
+	drm_WARN_ON_ONCE(obj->dev, atomic_inc_return(&shmem->pages_use_count) == 1);
 	drm_gem_vm_open(vma);
 }
 
@@ -632,7 +634,7 @@ void drm_gem_shmem_print_info(const struct drm_gem_shmem_object *shmem,
 	if (shmem->base.import_attach)
 		return;
 
-	drm_printf_indent(p, indent, "pages_use_count=%u\n", shmem->pages_use_count);
+	drm_printf_indent(p, indent, "pages_use_count=%u\n", atomic_read(&shmem->pages_use_count));
 	drm_printf_indent(p, indent, "vmap_use_count=%u\n", shmem->vmap_use_count);
 	drm_printf_indent(p, indent, "vaddr=%p\n", shmem->vaddr);
 }
diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
index 4f9736e5f929..0116518b1601 100644
--- a/drivers/gpu/drm/lima/lima_gem.c
+++ b/drivers/gpu/drm/lima/lima_gem.c
@@ -47,7 +47,7 @@ int lima_heap_alloc(struct lima_bo *bo, struct lima_vm *vm)
 		}
 
 		bo->base.pages = pages;
-		bo->base.pages_use_count = 1;
+		atomic_set(&bo->base.pages_use_count, 1);
 
 		mapping_set_unevictable(mapping);
 	}
diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
index c0123d09f699..f66e63bf743e 100644
--- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
+++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
@@ -487,7 +487,7 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
 			goto err_unlock;
 		}
 		bo->base.pages = pages;
-		bo->base.pages_use_count = 1;
+		atomic_set(&bo->base.pages_use_count, 1);
 	} else {
 		pages = bo->base.pages;
 		if (pages[page_offset]) {
diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
index bf0c31aa8fbe..0661f87d3bda 100644
--- a/include/drm/drm_gem_shmem_helper.h
+++ b/include/drm/drm_gem_shmem_helper.h
@@ -37,7 +37,7 @@ struct drm_gem_shmem_object {
 	 * Reference count on the pages table.
 	 * The pages are put when the count reaches zero.
 	 */
-	unsigned int pages_use_count;
+	atomic_t pages_use_count;
 
 	/**
 	 * @madv: State for madvise
-- 
2.41.0