Re: [PATCH v3 6/8] drm/shmem-helper: Provide a vmap function for short-term mappings

On Thu, Jan 7, 2021 at 11:28 AM Thomas Zimmermann [off-list ref] wrote:

Hi Daniel

Am 11.12.20 um 10:50 schrieb Daniel Vetter:
[...]

quoted

quoted

+/**
+ * drm_gem_shmem_vmap_local - Create a virtual mapping for a shmem GEM object
+ * @shmem: shmem GEM object
+ * @map: Returns the kernel virtual address of the SHMEM GEM object's backing
+ *       store.
+ *
+ * This function makes sure that a contiguous kernel virtual address mapping
+ * exists for the buffer backing the shmem GEM object.
+ *
+ * The function is called with the BO's reservation object locked. Callers must
+ * hold the lock until after unmapping the buffer.
+ *
+ * This function can be used to implement &drm_gem_object_funcs.vmap_local. But
+ * it can also be called by drivers directly, in which case it will hide the
+ * differences between dma-buf imported and natively allocated objects.
+ *
+ * Acquired mappings should be cleaned up by calling drm_gem_shmem_vunmap_local().
+ *
+ * Returns:
+ * 0 on success or a negative error code on failure.
+ */
+int drm_gem_shmem_vmap_local(struct drm_gem_object *obj, struct dma_buf_map *map)
+{
+    struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
+    int ret;
+
+    dma_resv_assert_held(obj->resv);
+
+    ret = mutex_lock_interruptible(&shmem->vmap_lock);

This bites. You need to check for shmem->import_attach and call
dma_buf_vmap_local directly here before you take any shmem helper locks.
Real fix would be to replace both vmap_lock and pages_lock with dma_resv
lock, but that's more work. Same for vunmap_local

This comment confuses me. AFAICT vmap_lock protects vmap_use_count. Why
does this exact code work in drm_gem_shmem_vmap() but not in _local() ?

You'd hold the dma_resv lock both inside and outside of the
->vmap_lock mutex. Which deadlocks. For the _vmap version we more or
less mandate that the caller doens't hold the dma_resv lock already,
hence why we can get away with that.
-Daniel

Best regards
Thomas

quoted

With that fixed on the helper part of this patch:

Reviewed-by: Daniel Vetter <redacted>

quoted

+    if (ret)
+            return ret;
+    ret = drm_gem_shmem_vmap_locked(shmem, map);
+    mutex_unlock(&shmem->vmap_lock);
+
+    return ret;
+}
+EXPORT_SYMBOL(drm_gem_shmem_vmap_local);
+
  static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
                                     struct dma_buf_map *map)
  {

@@ -366,7 +406,7 @@ static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
     drm_gem_shmem_put_pages(shmem);
  }

-/*
+/**
   * drm_gem_shmem_vunmap - Unmap a virtual mapping fo a shmem GEM object
   * @shmem: shmem GEM object
   * @map: Kernel virtual address where the SHMEM GEM object was mapped

@@ -389,6 +429,33 @@ void drm_gem_shmem_vunmap(struct drm_gem_object *obj, struct dma_buf_map *map)
  }
  EXPORT_SYMBOL(drm_gem_shmem_vunmap);

+/**
+ * drm_gem_shmem_vunmap_local - Unmap a virtual mapping fo a shmem GEM object
+ * @shmem: shmem GEM object
+ * @map: Kernel virtual address where the SHMEM GEM object was mapped
+ *
+ * This function cleans up a kernel virtual address mapping acquired by
+ * drm_gem_shmem_vmap_local(). The mapping is only removed when the use count
+ * drops to zero.
+ *
+ * The function is called with the BO's reservation object locked.
+ *
+ * This function can be used to implement &drm_gem_object_funcs.vmap_local.
+ * But it can also be called by drivers directly, in which case it will hide
+ * the differences between dma-buf imported and natively allocated objects.
+ */
+void drm_gem_shmem_vunmap_local(struct drm_gem_object *obj, struct dma_buf_map *map)
+{
+    struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
+
+    dma_resv_assert_held(obj->resv);
+
+    mutex_lock(&shmem->vmap_lock);
+    drm_gem_shmem_vunmap_locked(shmem, map);
+    mutex_unlock(&shmem->vmap_lock);
+}
+EXPORT_SYMBOL(drm_gem_shmem_vunmap_local);
+
  struct drm_gem_shmem_object *
  drm_gem_shmem_create_with_handle(struct drm_file *file_priv,
                              struct drm_device *dev, size_t size,

diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c
index 1dfc42170059..a33e28d4c5e9 100644
--- a/drivers/gpu/drm/mgag200/mgag200_mode.c
+++ b/drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -1552,22 +1552,32 @@ mgag200_handle_damage(struct mga_device *mdev, struct drm_framebuffer *fb,
                   struct drm_rect *clip)
  {
     struct drm_device *dev = &mdev->base;
+    struct drm_gem_object *obj = fb->obj[0];
     struct dma_buf_map map;
     void *vmap;
     int ret;

-    ret = drm_gem_shmem_vmap(fb->obj[0], &map);
+    ret = dma_resv_lock(obj->resv, NULL);
     if (drm_WARN_ON(dev, ret))
-            return; /* BUG: SHMEM BO should always be vmapped */
+            return;
+    ret = drm_gem_shmem_vmap_local(obj, &map);
+    if (drm_WARN_ON(dev, ret))
+            goto err_dma_resv_unlock; /* BUG: SHMEM BO should always be vmapped */
     vmap = map.vaddr; /* TODO: Use mapping abstraction properly */

     drm_fb_memcpy_dstclip(mdev->vram, vmap, fb, clip);

-    drm_gem_shmem_vunmap(fb->obj[0], &map);
+    drm_gem_shmem_vunmap_local(obj, &map);
+    dma_resv_unlock(obj->resv);

     /* Always scanout image at VRAM offset 0 */
     mgag200_set_startadd(mdev, (u32)0);
     mgag200_set_offset(mdev, fb);
+
+    return;
+
+err_dma_resv_unlock:
+    dma_resv_unlock(obj->resv);
  }

  static void

diff --git a/drivers/gpu/drm/tiny/cirrus.c b/drivers/gpu/drm/tiny/cirrus.c
index 561c49d8657a..58c694964148 100644
--- a/drivers/gpu/drm/tiny/cirrus.c
+++ b/drivers/gpu/drm/tiny/cirrus.c

@@ -315,6 +315,7 @@ static int cirrus_fb_blit_rect(struct drm_framebuffer *fb,
                            struct drm_rect *rect)
  {
     struct cirrus_device *cirrus = to_cirrus(fb->dev);
+    struct drm_gem_object *obj = fb->obj[0];
     struct dma_buf_map map;
     void *vmap;
     int idx, ret;

@@ -323,9 +324,12 @@ static int cirrus_fb_blit_rect(struct drm_framebuffer *fb,
     if (!drm_dev_enter(&cirrus->dev, &idx))
             goto out;

-    ret = drm_gem_shmem_vmap(fb->obj[0], &map);
+    ret = dma_resv_lock(obj->resv, NULL);
     if (ret)
             goto out_dev_exit;
+    ret = drm_gem_shmem_vmap_local(fb->obj[0], &map);
+    if (ret)
+            goto out_dma_resv_unlock;
     vmap = map.vaddr; /* TODO: Use mapping abstraction properly */

     if (cirrus->cpp == fb->format->cpp[0])

@@ -345,9 +349,11 @@ static int cirrus_fb_blit_rect(struct drm_framebuffer *fb,
     else
             WARN_ON_ONCE("cpp mismatch");

-    drm_gem_shmem_vunmap(fb->obj[0], &map);
     ret = 0;

+    drm_gem_shmem_vunmap_local(obj, &map);
+out_dma_resv_unlock:
+    dma_resv_unlock(obj->resv);
  out_dev_exit:
     drm_dev_exit(idx);
  out:

diff --git a/drivers/gpu/drm/tiny/gm12u320.c b/drivers/gpu/drm/tiny/gm12u320.c
index 33f65f4626e5..b0c6e350f2b3 100644
--- a/drivers/gpu/drm/tiny/gm12u320.c
+++ b/drivers/gpu/drm/tiny/gm12u320.c

@@ -265,11 +265,16 @@ static void gm12u320_copy_fb_to_blocks(struct gm12u320_device *gm12u320)
     y1 = gm12u320->fb_update.rect.y1;
     y2 = gm12u320->fb_update.rect.y2;

-    ret = drm_gem_shmem_vmap(fb->obj[0], &map);
+    ret = dma_resv_lock(fb->obj[0]->resv, NULL);
     if (ret) {
-            GM12U320_ERR("failed to vmap fb: %d\n", ret);
+            GM12U320_ERR("failed to reserve fb: %d\n", ret);
             goto put_fb;
     }
+    ret = drm_gem_shmem_vmap_local(fb->obj[0], &map);
+    if (ret) {
+            GM12U320_ERR("failed to vmap fb: %d\n", ret);
+            goto unlock_resv;
+    }
     vaddr = map.vaddr; /* TODO: Use mapping abstraction properly */

     if (fb->obj[0]->import_attach) {

@@ -321,8 +326,11 @@ static void gm12u320_copy_fb_to_blocks(struct gm12u320_device *gm12u320)
             if (ret)
                     GM12U320_ERR("dma_buf_end_cpu_access err: %d\n", ret);
     }
+
+unlock_resv:
+    dma_resv_unlock(fb->obj[0]->resv);
  vunmap:
-    drm_gem_shmem_vunmap(fb->obj[0], &map);
+    drm_gem_shmem_vunmap_local(fb->obj[0], &map);
  put_fb:
     drm_framebuffer_put(fb);
     gm12u320->fb_update.fb = NULL;

diff --git a/drivers/gpu/drm/udl/udl_modeset.c b/drivers/gpu/drm/udl/udl_modeset.c
index 9d34ec9d03f6..46b55b4d03c2 100644
--- a/drivers/gpu/drm/udl/udl_modeset.c
+++ b/drivers/gpu/drm/udl/udl_modeset.c

@@ -290,14 +290,18 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
     else if ((clip.x2 > fb->width) || (clip.y2 > fb->height))
             return -EINVAL;

+    ret = dma_resv_lock(fb->obj[0]->resv, NULL);
+    if (ret)
+            return ret;
+
     if (import_attach) {
             ret = dma_buf_begin_cpu_access(import_attach->dmabuf,
                                            DMA_FROM_DEVICE);
             if (ret)
-                    return ret;
+                    goto out_dma_resv_unlock;
     }

-    ret = drm_gem_shmem_vmap(fb->obj[0], &map);
+    ret = drm_gem_shmem_vmap_local(fb->obj[0], &map);
     if (ret) {
             DRM_ERROR("failed to vmap fb\n");
             goto out_dma_buf_end_cpu_access;

@@ -307,7 +311,7 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
     urb = udl_get_urb(dev);
     if (!urb) {
             ret = -ENOMEM;
-            goto out_drm_gem_shmem_vunmap;
+            goto out_drm_gem_shmem_vunmap_local;
     }
     cmd = urb->transfer_buffer;

@@ -320,7 +324,7 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
                                    &cmd, byte_offset, dev_byte_offset,
                                    byte_width);
             if (ret)
-                    goto out_drm_gem_shmem_vunmap;
+                    goto out_drm_gem_shmem_vunmap_local;
     }

     if (cmd > (char *)urb->transfer_buffer) {

@@ -336,8 +340,8 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,

     ret = 0;

-out_drm_gem_shmem_vunmap:
-    drm_gem_shmem_vunmap(fb->obj[0], &map);
+out_drm_gem_shmem_vunmap_local:
+    drm_gem_shmem_vunmap_local(fb->obj[0], &map);
  out_dma_buf_end_cpu_access:
     if (import_attach) {
             tmp_ret = dma_buf_end_cpu_access(import_attach->dmabuf,

@@ -345,6 +349,8 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
             if (tmp_ret && !ret)
                     ret = tmp_ret; /* only update ret if not set yet */
     }
+out_dma_resv_unlock:
+    dma_resv_unlock(fb->obj[0]->resv);

     return ret;
  }

diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
index 434328d8a0d9..3f59bdf749aa 100644
--- a/include/drm/drm_gem_shmem_helper.h
+++ b/include/drm/drm_gem_shmem_helper.h

@@ -114,7 +114,9 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem);
  int drm_gem_shmem_pin(struct drm_gem_object *obj);
  void drm_gem_shmem_unpin(struct drm_gem_object *obj);
  int drm_gem_shmem_vmap(struct drm_gem_object *obj, struct dma_buf_map *map);
+int drm_gem_shmem_vmap_local(struct drm_gem_object *obj, struct dma_buf_map *map);
  void drm_gem_shmem_vunmap(struct drm_gem_object *obj, struct dma_buf_map *map);
+void drm_gem_shmem_vunmap_local(struct drm_gem_object *obj, struct dma_buf_map *map);

  int drm_gem_shmem_madvise(struct drm_gem_object *obj, int madv);

--

2.29.2

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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