Re: [PATCH 1/3] drm/shmem-helper: Import dmabuf without mapping its sg_table

[Thomas Zimmermann <tzimmermann@suse.de>]

(cc'ing Christian)

Hi,

I don't feel qualified to fully review this patch.

It would be good to have the issue with sg-tables solved, but I dislike 
the dedicated initializer macros. So my question is if this has any 
drawbacks. Or could we make this available and the default for all 
shmem-based drivers?

Best regards
Thomas

Am 01.05.25 um 08:43 schrieb oushixiong1025@163.com:
> From: Shixiong Ou <oushixiong@kylinos.cn>
>
> [WHY]
> 1. Drivers using DRM_GEM_SHADOW_PLANE_HELPER_FUNCS and
>     DRM_GEM_SHMEM_DRIVER_OPS (e.g., udl, ast) do not require
>     sg_table import.
>     They only need dma_buf_vmap() to access the shared buffer's
>     kernel virtual address.
>
> 2. On certain Aspeed-based boards, a dma_mask of 0xffff_ffff may
>     trigger SWIOTLB during dmabuf import. However, IO_TLB_SEGSIZE
>     restricts the maximum DMA streaming mapping memory, resulting in
>     errors like:
>
>     ast 0000:07:00.0: swiotlb buffer is full (sz: 3145728 bytes), total 32768 (slots), used 0 (slots)
>
> [HOW]
> Provide a gem_prime_import implementation without sg_table mapping
> to avoid issues (e.g., "swiotlb buffer is full"). Drivers that do not
> require sg_table can adopt this.
>
> Signed-off-by: Shixiong Ou <oushixiong@kylinos.cn>
> ---
>   drivers/gpu/drm/drm_gem_shmem_helper.c | 95 ++++++++++++++++++++++++++
>   include/drm/drm_gem_shmem_helper.h     | 24 +++++++
>   2 files changed, 119 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index d99dee67353a..9e41e350ff6f 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -39,6 +39,7 @@ MODULE_IMPORT_NS("DMA_BUF");
>   static const struct drm_gem_object_funcs drm_gem_shmem_funcs = {
>   	.free = drm_gem_shmem_object_free,
>   	.print_info = drm_gem_shmem_object_print_info,
> +	.export = drm_gem_shmem_object_prime_export,
>   	.pin = drm_gem_shmem_object_pin,
>   	.unpin = drm_gem_shmem_object_unpin,
>   	.get_sg_table = drm_gem_shmem_object_get_sg_table,
> @@ -799,6 +800,100 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
>   }
>   EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import_sg_table);
>   
> +const struct dma_buf_ops drm_gem_shmem_prime_dmabuf_ops =  {
> +	.cache_sgt_mapping = true,
> +	.attach = drm_gem_map_attach,
> +	.detach = drm_gem_map_detach,
> +	.map_dma_buf = drm_gem_map_dma_buf,
> +	.unmap_dma_buf = drm_gem_unmap_dma_buf,
> +	.release = drm_gem_dmabuf_release,
> +	.mmap = drm_gem_dmabuf_mmap,
> +	.vmap = drm_gem_dmabuf_vmap,
> +	.vunmap = drm_gem_dmabuf_vunmap,
> +};
> +
> +/**
> + * drm_gem_shmem_prime_export - implementation of the export callback
> + * @shmem: shmem GEM object
> + */
> +struct dma_buf *drm_gem_shmem_prime_export(struct drm_gem_shmem_object *shmem,
> +					   int flags)
> +{
> +	struct drm_gem_object *obj = &shmem->base;
> +	struct drm_device *dev = obj->dev;
> +	struct dma_buf_export_info exp_info = {
> +		.exp_name = KBUILD_MODNAME, /* white lie for debug */
> +		.owner = dev->driver->fops->owner,
> +		.ops = &drm_gem_shmem_prime_dmabuf_ops,
> +		.size = obj->size,
> +		.flags = flags,
> +		.priv = obj,
> +		.resv = obj->resv,
> +	};
> +
> +	return drm_gem_dmabuf_export(dev, &exp_info);
> +}
> +
> +/**
> + * drm_gem_shmem_prime_import - Import dmabuf without mapping its sg_table
> + * @dev: Device to import into
> + * @dma_buf: dma-buf object to import
> + *
> + * Drivers that use the shmem helpers but also wants to import dmabuf without
> + * mapping its sg_table can use this as their &drm_driver.gem_prime_import
> + * implementation.
> + */
> +struct drm_gem_object *drm_gem_shmem_prime_import(struct drm_device *dev,
> +						  struct dma_buf *dma_buf)
> +{
> +	struct dma_buf_attachment *attach;
> +	struct drm_gem_shmem_object *shmem;
> +	size_t size;
> +	int ret;
> +
> +	if (dma_buf->ops == &drm_gem_shmem_prime_dmabuf_ops) {
> +		struct drm_gem_object *obj;
> +
> +		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, dev->dev);
> +	if (IS_ERR(attach))
> +		return ERR_CAST(attach);
> +
> +	get_dma_buf(dma_buf);
> +
> +	size = PAGE_ALIGN(attach->dmabuf->size);
> +
> +	shmem = __drm_gem_shmem_create(dev, size, true, NULL);
> +	if (IS_ERR(shmem)) {
> +		ret = PTR_ERR(shmem);
> +		goto fail_detach;
> +	}
> +
> +	drm_dbg_prime(dev, "size = %zu\n", size);
> +
> +	shmem->base.import_attach = attach;
> +	shmem->base.resv = dma_buf->resv;
> +
> +	return &shmem->base;
> +
> +fail_detach:
> +	dma_buf_detach(dma_buf, attach);
> +	dma_buf_put(dma_buf);
> +
> +	return ERR_PTR(ret);
> +}
> +EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import);
> +
>   MODULE_DESCRIPTION("DRM SHMEM memory-management helpers");
>   MODULE_IMPORT_NS("DMA_BUF");
>   MODULE_LICENSE("GPL v2");
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index cef5a6b5a4d6..78ef91593a8e 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -110,6 +110,8 @@ int drm_gem_shmem_vmap(struct drm_gem_shmem_object *shmem,
>   void drm_gem_shmem_vunmap(struct drm_gem_shmem_object *shmem,
>   			  struct iosys_map *map);
>   int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct *vma);
> +struct dma_buf *drm_gem_shmem_prime_export(struct drm_gem_shmem_object *shmem,
> +						  int flags);
>   
>   int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem);
>   void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem);
> @@ -168,6 +170,18 @@ static inline void drm_gem_shmem_object_print_info(struct drm_printer *p, unsign
>   	drm_gem_shmem_print_info(shmem, p, indent);
>   }
>   
> +/**
> + * drm_gem_shmem_object_prime_export - GEM object function for export()
> + * @obj: GEM object
> + *
> + */
> +static inline struct dma_buf *drm_gem_shmem_object_prime_export(struct drm_gem_object *obj,
> +								int flags)
> +{
> +	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
> +
> +	return drm_gem_shmem_prime_export(shmem, flags);
> +}
>   /**
>    * drm_gem_shmem_object_pin - GEM object function for drm_gem_shmem_pin()
>    * @obj: GEM object
> @@ -276,6 +290,8 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
>   				    struct sg_table *sgt);
>   int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
>   			      struct drm_mode_create_dumb *args);
> +struct drm_gem_object *drm_gem_shmem_prime_import(struct drm_device *dev,
> +						  struct dma_buf *buf);
>   
>   /**
>    * DRM_GEM_SHMEM_DRIVER_OPS - Default shmem GEM operations
> @@ -287,4 +303,12 @@ 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
>   
> +/**
> + * This macro provides a shmem GEM operations that implementate a simple
> + * gem_prime_import.
> + */
> +#define DRM_GEM_SHMEM_SIMPLE_DRIVER_OPS \
> +	.gem_prime_import	= drm_gem_shmem_prime_import, \
> +	.dumb_create		= drm_gem_shmem_dumb_create
> +
>   #endif /* __DRM_GEM_SHMEM_HELPER_H__ */

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)