Re: [PATCH v3 1/3] drm: Add support for GEM buffers backed by non-coherent memory

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 14.05.21 um 22:11 schrieb Paul Cercueil:
> Having GEM buffers backed by non-coherent memory is interesting in the
> particular case where it is faster to render to a non-coherent buffer
> then sync the data cache, than to render to a write-combine buffer, and
> (by extension) much faster than using a shadow buffer. This is true for
> instance on some Ingenic SoCs, where even simple blits (e.g. memcpy)
> are about three times faster using this method.
> 
> Add a 'map_noncoherent' flag to the drm_gem_cma_object structure, which
> can be set by the drivers when they create the dumb buffer.
> 
> Since this really only applies to software rendering, disable this flag
> as soon as the CMA objects are exported via PRIME.
> 
> v3: New patch. Now uses a simple 'map_noncoherent' flag to control how
>      the objects are mapped, and use the new dma_mmap_pages function.
> 
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> ---
>   drivers/gpu/drm/drm_gem_cma_helper.c | 41 +++++++++++++++++++++++++---
>   include/drm/drm_gem_cma_helper.h     |  7 ++++-
>   2 files changed, 43 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
> index 7942cf05cd93..81a31bcf7d68 100644
> --- a/drivers/gpu/drm/drm_gem_cma_helper.c
> +++ b/drivers/gpu/drm/drm_gem_cma_helper.c
> @@ -115,8 +115,15 @@ struct drm_gem_cma_object *drm_gem_cma_create(struct drm_device *drm,
>   	if (IS_ERR(cma_obj))
>   		return cma_obj;
>   
> -	cma_obj->vaddr = dma_alloc_wc(drm->dev, size, &cma_obj->paddr,
> -				      GFP_KERNEL | __GFP_NOWARN);
> +	if (cma_obj->map_noncoherent) {
> +		cma_obj->vaddr = dma_alloc_noncoherent(drm->dev, size,
> +						       &cma_obj->paddr,
> +						       DMA_TO_DEVICE,
> +						       GFP_KERNEL | __GFP_NOWARN);
> +	} else {
> +		cma_obj->vaddr = dma_alloc_wc(drm->dev, size, &cma_obj->paddr,
> +					      GFP_KERNEL | __GFP_NOWARN);
> +	}
>   	if (!cma_obj->vaddr) {
>   		drm_dbg(drm, "failed to allocate buffer with size %zu\n",
>   			 size);
> @@ -499,8 +506,13 @@ int drm_gem_cma_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>   
>   	cma_obj = to_drm_gem_cma_obj(obj);
>   
> -	ret = dma_mmap_wc(cma_obj->base.dev->dev, vma, cma_obj->vaddr,
> -			  cma_obj->paddr, vma->vm_end - vma->vm_start);
> +	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
> +	if (!cma_obj->map_noncoherent)
> +		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
> +
> +	ret = dma_mmap_pages(cma_obj->base.dev->dev,
> +			     vma, vma->vm_end - vma->vm_start,
> +			     virt_to_page(cma_obj->vaddr));
>   	if (ret)
>   		drm_gem_vm_close(vma);
>   
> @@ -556,3 +568,24 @@ drm_gem_cma_prime_import_sg_table_vmap(struct drm_device *dev,
>   	return obj;
>   }
>   EXPORT_SYMBOL(drm_gem_cma_prime_import_sg_table_vmap);
> +
> +/**
> + * drm_gem_cma_prime_mmap - PRIME mmap function for CMA GEM drivers
> + * @obj: GEM object
> + * @vma: Virtual address range
> + *
> + * Carbon copy of drm_gem_prime_mmap, but the 'map_noncoherent' flag is
> + * disabled to ensure that the exported buffers have the expected cache
> + * coherency.
> + */
> +int drm_gem_cma_prime_mmap(struct drm_gem_object *obj,
> +			   struct vm_area_struct *vma)
> +{
> +	struct drm_gem_cma_object *cma_obj = to_drm_gem_cma_obj(obj);
> +
> +	/* Use standard cache settings for PRIME-exported GEM buffers */
> +	cma_obj->map_noncoherent = false;
> +
> +	return drm_gem_prime_mmap(obj, vma);
> +}
> +EXPORT_SYMBOL(drm_gem_cma_prime_mmap);
> diff --git a/include/drm/drm_gem_cma_helper.h b/include/drm/drm_gem_cma_helper.h
> index 0a9711caa3e8..b597e00fd5f6 100644
> --- a/include/drm/drm_gem_cma_helper.h
> +++ b/include/drm/drm_gem_cma_helper.h
> @@ -16,6 +16,7 @@ struct drm_mode_create_dumb;
>    *       more than one entry but they are guaranteed to have contiguous
>    *       DMA addresses.
>    * @vaddr: kernel virtual address of the backing memory
> + * @map_noncoherent: if true, the GEM object is backed by non-coherent 
memory
>    */
>   struct drm_gem_cma_object {
>   	struct drm_gem_object base;
> @@ -24,6 +25,8 @@ struct drm_gem_cma_object {
>   
>   	/* For objects with DMA memory allocated by GEM CMA */
>   	void *vaddr;
> +
> +	bool map_noncoherent;
>   };
>   
>   #define to_drm_gem_cma_obj(gem_obj) \
> @@ -119,7 +122,7 @@ int drm_gem_cma_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma);
>   	.prime_handle_to_fd	= drm_gem_prime_handle_to_fd, \
>   	.prime_fd_to_handle	= drm_gem_prime_fd_to_handle, \
>   	.gem_prime_import_sg_table = drm_gem_cma_prime_import_sg_table, \
> -	.gem_prime_mmap		= drm_gem_prime_mmap
> +	.gem_prime_mmap		= drm_gem_cma_prime_mmap

gem_prime_mmap is deprecated and on the way out. Only mmap in 
drm_gem_object_funcs should be used. I have patches for other drivers 
that convert everything to drm_gem_prime_mmap. Afterwards the pointer 
can be removed.

Rather than writing a custom prime mmap function, update 
drm_gem_cma_prime_import_sg_table() so that it disables non-coherent 
mappings for imported buffers. For an example, see how SHMEM's internal 
create function uses the 'private' parameter. [1]

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_gem_shmem_helper.c#L39

>   
>   /**
>    * DRM_GEM_CMA_DRIVER_OPS - CMA GEM driver operations
> @@ -181,5 +184,7 @@ struct drm_gem_object *
>   drm_gem_cma_prime_import_sg_table_vmap(struct drm_device *drm,
>   				       struct dma_buf_attachment *attach,
>   				       struct sg_table *sgt);
> +int drm_gem_cma_prime_mmap(struct drm_gem_object *obj,
> +			   struct vm_area_struct *vma);
>   
>   #endif /* __DRM_GEM_CMA_HELPER_H__ */
> 

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


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