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

On 4/30/25 10:56, oushixiong1025@163.com wrote:

From: Shixiong Ou oushixiong@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.

Signed-off-by: Shixiong Ou oushixiong@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__ */