prime: Support importing DMA-BUF without sg_table

oushixiong Wed, 30 Apr 2025 07:22:07 -0700


在 2025/4/30 19:03, Christian König 写道:

On 4/30/25 10:56,oushixiong1...@163.com wrote:

From: Shixiong Ou<oushixi...@kylinos.cn>


[WHY]
On some boards, the dma_mask of Aspeed devices is 0xffff_ffff, this
quite possibly causes the SWIOTLB to be triggered when importing dmabuf.
However IO_TLB_SEGSIZE limits the maximum amount of available memory
for DMA Streaming Mapping, as dmesg following:

[   24.885303][ T1947] ast 0000:07:00.0: swiotlb buffer is full (sz: 3145728 
bytes), total 32768 (slots), used 0 (slots)

[HOW] Provide an interface so that attachment is not mapped when
importing dma-buf.

This is unecessary. The extra abstraction in DRM is only useful when you want to 
implement the obj->funcs->get_sg_table() callback.

When a driver doesn't want to expose an sg_table for a buffer or want some 
other special handling it can simply do so by implementing the DMA-buf 
interface directly.

See drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c for an example on how to do 
this.

Regards,
Christian.



Thanks for the reminder,

most drivers that use DRM_GEM_SHADOW_PLANE_HELPER_FUNCSandDRM_GEM_SHMEM_DRIVER_OPS

don't need to import the sg_table, such as the udl and the ast and so onat the moment.

They just need to call dma_buf_vmap() to get the kernel virtual addressof the shared buffer.

So I wondered if there was a simple generic PRIME implementation forthese drivers.

If you don't recommend this, Maybe try to implement it inDRM_GEM_SHMEM_DRIVER_OPS ?


Regards,

Shixiong Ou.

Signed-off-by: Shixiong Ou<oushixi...@kylinos.cn>
---
  drivers/gpu/drm/ast/ast_drv.c          |  2 +-
  drivers/gpu/drm/drm_gem_shmem_helper.c | 17 +++++++
  drivers/gpu/drm/drm_prime.c            | 67 ++++++++++++++++++++++++--
  drivers/gpu/drm/udl/udl_drv.c          |  2 +-
  include/drm/drm_drv.h                  |  3 ++
  include/drm/drm_gem_shmem_helper.h     |  6 +++
  6 files changed, 91 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/ast/ast_drv.c b/drivers/gpu/drm/ast/ast_drv.c
index 6fbf62a99c48..2dac6acf79e7 100644
--- a/drivers/gpu/drm/ast/ast_drv.c
+++ b/drivers/gpu/drm/ast/ast_drv.c
@@ -64,7 +64,7 @@ static const struct drm_driver ast_driver = {
        .minor = DRIVER_MINOR,
        .patchlevel = DRIVER_PATCHLEVEL,

- DRM_GEM_SHMEM_DRIVER_OPS,

+       DRM_GEM_SHMEM_SIMPLE_DRIVER_OPS,
        DRM_FBDEV_SHMEM_DRIVER_OPS,
  };

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c

index d99dee67353a..655d841df933 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -799,6 +799,23 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
  }
  EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import_sg_table);

+struct drm_gem_object *

+drm_gem_shmem_prime_import_attachment(struct drm_device *dev,
+                                     struct dma_buf_attachment *attach)
+{
+       size_t size = PAGE_ALIGN(attach->dmabuf->size);
+       struct drm_gem_shmem_object *shmem;
+
+       shmem = __drm_gem_shmem_create(dev, size, true, NULL);
+       if (IS_ERR(shmem))
+               return ERR_CAST(shmem);
+
+       drm_dbg_prime(dev, "size = %zu\n", size);
+
+       return &shmem->base;
+}
+EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import_attachment);
+
  MODULE_DESCRIPTION("DRM SHMEM memory-management helpers");
  MODULE_IMPORT_NS("DMA_BUF");
  MODULE_LICENSE("GPL v2");
diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 8e70abca33b9..522cf974e202 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -911,6 +911,62 @@ struct dma_buf *drm_gem_prime_export(struct drm_gem_object 
*obj,
  }
  EXPORT_SYMBOL(drm_gem_prime_export);

+/**

+ * drm_gem_prime_import_dev_skip_map - core implementation of the import 
callback
+ * @dev: drm_device to import into
+ * @dma_buf: dma-buf object to import
+ * @attach_dev: struct device to dma_buf attach
+ *
+ * This function exports a dma-buf without get it's scatter/gather table.
+ *
+ * Drivers who need to get an scatter/gather table for objects need to call
+ * drm_gem_prime_import_dev() instead.
+ */
+struct drm_gem_object *drm_gem_prime_import_dev_skip_map(struct drm_device 
*dev,
+                                                        struct dma_buf 
*dma_buf,
+                                                        struct device 
*attach_dev)
+{
+       struct dma_buf_attachment *attach;
+       struct drm_gem_object *obj;
+       int ret;
+
+       if (dma_buf->ops == &drm_gem_prime_dmabuf_ops) {
+               obj = dma_buf->priv;
+               if (obj->dev == dev) {
+                       /*
+                        * Importing dmabuf exported from our own gem increases
+                        * refcount on gem itself instead of f_count of dmabuf.
+                        */
+                       drm_gem_object_get(obj);
+                       return obj;
+               }
+       }
+
+       attach = dma_buf_attach(dma_buf, attach_dev, true);
+       if (IS_ERR(attach))
+               return ERR_CAST(attach);
+
+       get_dma_buf(dma_buf);
+
+       obj = dev->driver->gem_prime_import_attachment(dev, attach);
+       if (IS_ERR(obj)) {
+               ret = PTR_ERR(obj);
+               goto fail_detach;
+       }
+
+       obj->import_attach = attach;
+       obj->resv = dma_buf->resv;
+
+       return obj;
+
+fail_detach:
+       dma_buf_detach(dma_buf, attach);
+       dma_buf_put(dma_buf);
+
+       return ERR_PTR(ret);
+}
+EXPORT_SYMBOL(drm_gem_prime_import_dev_skip_map);
+
  /**
   * drm_gem_prime_import_dev - core implementation of the import callback
   * @dev: drm_device to import into
@@ -946,9 +1002,6 @@ struct drm_gem_object *drm_gem_prime_import_dev(struct 
drm_device *dev,
                }
        }

- if (!dev->driver->gem_prime_import_sg_table)

-               return ERR_PTR(-EINVAL);
-
        attach = dma_buf_attach(dma_buf, attach_dev, false);
        if (IS_ERR(attach))
                return ERR_CAST(attach);
@@ -998,7 +1051,13 @@ EXPORT_SYMBOL(drm_gem_prime_import_dev);
  struct drm_gem_object *drm_gem_prime_import(struct drm_device *dev,
                                            struct dma_buf *dma_buf)
  {
-       return drm_gem_prime_import_dev(dev, dma_buf, dev->dev);
+       if (dev->driver->gem_prime_import_sg_table)
+               return drm_gem_prime_import_dev(dev, dma_buf, dev->dev);
+       else if (dev->driver->gem_prime_import_attachment)
+               return drm_gem_prime_import_dev_skip_map(dev, dma_buf, 
dev->dev);
+       else
+               return ERR_PTR(-EINVAL);
+
  }
  EXPORT_SYMBOL(drm_gem_prime_import);

diff --git a/drivers/gpu/drm/udl/udl_drv.c b/drivers/gpu/drm/udl/udl_drv.c

index 05b3a152cc33..c00d8b8834f2 100644
--- a/drivers/gpu/drm/udl/udl_drv.c
+++ b/drivers/gpu/drm/udl/udl_drv.c
@@ -72,7 +72,7 @@ static const struct drm_driver driver = {

/* GEM hooks */

        .fops = &udl_driver_fops,
-       DRM_GEM_SHMEM_DRIVER_OPS,
+       DRM_GEM_SHMEM_SIMPLE_DRIVER_OPS,
        .gem_prime_import = udl_driver_gem_prime_import,
        DRM_FBDEV_SHMEM_DRIVER_OPS,

diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h

index a43d707b5f36..aef8d9051fcd 100644
--- a/include/drm/drm_drv.h
+++ b/include/drm/drm_drv.h
@@ -326,6 +326,9 @@ struct drm_driver {
                                struct dma_buf_attachment *attach,
                                struct sg_table *sgt);

+ struct drm_gem_object *(*gem_prime_import_attachment)(

+                               struct drm_device *dev,
+                               struct dma_buf_attachment *attach);
        /**
         * @dumb_create:
         *
diff --git a/include/drm/drm_gem_shmem_helper.h 
b/include/drm/drm_gem_shmem_helper.h
index cef5a6b5a4d6..39a93c222aaa 100644
--- a/include/drm/drm_gem_shmem_helper.h
+++ b/include/drm/drm_gem_shmem_helper.h
@@ -274,6 +274,9 @@ struct drm_gem_object *
  drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
                                    struct dma_buf_attachment *attach,
                                    struct sg_table *sgt);
+struct drm_gem_object *
+drm_gem_shmem_prime_import_attachment(struct drm_device *dev,
+                                     struct dma_buf_attachment *attach);
  int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
                              struct drm_mode_create_dumb *args);

@@ -287,4 +290,7 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,

        .gem_prime_import_sg_table = drm_gem_shmem_prime_import_sg_table, \
        .dumb_create               = drm_gem_shmem_dumb_create

+#define DRM_GEM_SHMEM_SIMPLE_DRIVER_OPS \

+       .gem_prime_import_attachment = drm_gem_shmem_prime_import_attachment, \
+       .dumb_create                 = drm_gem_shmem_dumb_create
  #endif /* __DRM_GEM_SHMEM_HELPER_H__ */

Re: [PATCH 2/3] drm/prime: Support importing DMA-BUF without sg_table

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