Re: [PATCH v5 1/3] drm: Add support for GEM buffers backed by non-coherent memory

[Tomi Valkeinen]

On 23/05/2021 20:04, Paul Cercueil wrote:
> 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.
>
> v4: Make sure map_noncoherent is always disabled when creating GEM
>      objects meant to be used with dma-buf.
>
> Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
> Acked-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> ---
>   drivers/gpu/drm/drm_gem_cma_helper.c | 38 +++++++++++++++++++++-------
>   include/drm/drm_gem_cma_helper.h     |  3 +++
>   2 files changed, 32 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
> index 7942cf05cd93..235c7a63da2b 100644
> --- a/drivers/gpu/drm/drm_gem_cma_helper.c
> +++ b/drivers/gpu/drm/drm_gem_cma_helper.c
> @@ -46,6 +46,7 @@ static const struct drm_gem_object_funcs drm_gem_cma_default_funcs = {
>    * __drm_gem_cma_create - Create a GEM CMA object without allocating memory
>    * @drm: DRM device
>    * @size: size of the object to allocate
> + * @private: true if used for internal purposes
>    *
>    * This function creates and initializes a GEM CMA object of the given size,
>    * but doesn't allocate any memory to back the object.
> @@ -55,11 +56,11 @@ static const struct drm_gem_object_funcs drm_gem_cma_default_funcs = {
>    * error code on failure.
>    */
>   static struct drm_gem_cma_object *
> -__drm_gem_cma_create(struct drm_device *drm, size_t size)
> +__drm_gem_cma_create(struct drm_device *drm, size_t size, bool private)
>   {
>   	struct drm_gem_cma_object *cma_obj;
>   	struct drm_gem_object *gem_obj;
> -	int ret;
> +	int ret = 0;
>   
>   	if (drm->driver->gem_create_object)
>   		gem_obj = drm->driver->gem_create_object(drm, size);
> @@ -73,7 +74,14 @@ __drm_gem_cma_create(struct drm_device *drm, size_t size)
>   
>   	cma_obj = container_of(gem_obj, struct drm_gem_cma_object, base);
>   
> -	ret = drm_gem_object_init(drm, gem_obj, size);
> +	if (private) {
> +		drm_gem_private_object_init(drm, gem_obj, size);
> +
> +		/* Always use writecombine for dma-buf mappings */
> +		cma_obj->map_noncoherent = false;
> +	} else {
> +		ret = drm_gem_object_init(drm, gem_obj, size);
> +	}
>   	if (ret)
>   		goto error;
>   
> @@ -111,12 +119,19 @@ struct drm_gem_cma_object *drm_gem_cma_create(struct drm_device *drm,
>   
>   	size = round_up(size, PAGE_SIZE);
>   
> -	cma_obj = __drm_gem_cma_create(drm, size);
> +	cma_obj = __drm_gem_cma_create(drm, size, false);
>   	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);
> @@ -432,7 +447,7 @@ drm_gem_cma_prime_import_sg_table(struct drm_device *dev,
>   		return ERR_PTR(-EINVAL);
>   
>   	/* Create a CMA GEM buffer. */
> -	cma_obj = __drm_gem_cma_create(dev, attach->dmabuf->size);
> +	cma_obj = __drm_gem_cma_create(dev, attach->dmabuf->size, true);
>   	if (IS_ERR(cma_obj))
>   		return ERR_CAST(cma_obj);
>   
> @@ -499,8 +514,13 @@ 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));

This breaks mmap on TI's J7 EVM (tidss driver). All DRM apps just die 
when doing mmap. Changing these lines back to dma_mmap_wc() makes it work.

Is dma_alloc_wc() even compatible with dma_mmap_pages()?

 Tomi