Re: [PATCH 1/2] drm/shmem: Use cached mappings by default

[Thomas Zimmermann <tzimmermann@suse.de>]

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Am 13.05.20 um 17:03 schrieb Thomas Zimmermann:
> SHMEM-buffer backing storage is allocated from system memory; which is
> typically cachable. Currently, only virtio uses cachable mappings; udl
> uses its own vmap/mmap implementation for cachable mappings. Other
> drivers default to writecombine mappings.
> 
> Use cached mappings by default. The exception is pages imported via
> dma-buf. DMA memory is usually not cached.
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c  | 6 ++++--
>  drivers/gpu/drm/virtio/virtgpu_object.c | 1 -
>  include/drm/drm_gem_shmem_helper.h      | 4 ++--
>  3 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index df31e5782eed1..1ce90325dfa31 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -259,7 +259,7 @@ static void *drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem)
>  	} else {
>  		pgprot_t prot = PAGE_KERNEL;
>  
> -		if (!shmem->map_cached)
> +		if (shmem->map_wc)
>  			prot = pgprot_writecombine(prot);
>  		shmem->vaddr = vmap(shmem->pages, obj->size >> PAGE_SHIFT,
>  				    VM_MAP, prot);
> @@ -546,7 +546,7 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>  
>  	vma->vm_flags |= VM_MIXEDMAP | VM_DONTEXPAND;
>  	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
> -	if (!shmem->map_cached)
> +	if (shmem->map_wc)
>  		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>  	vma->vm_ops = &drm_gem_shmem_vm_ops;
>  
> @@ -664,6 +664,8 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
>  	if (IS_ERR(shmem))
>  		return ERR_CAST(shmem);
>  
> +	shmem->map_wc = false; /* dma-buf mappings use writecombine */

Just when I posted the patch, I saw this bug. map_wc should be set to
true to enable writecombine mappings for dma-buf pages. Will be fixed in
the next iteration.

> +
>  	shmem->pages = kvmalloc_array(npages, sizeof(struct page *), GFP_KERNEL);
>  	if (!shmem->pages) {
>  		ret = -ENOMEM;
> diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c b/drivers/gpu/drm/virtio/virtgpu_object.c
> index 6ccbd01cd888c..80ba6b2b61668 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_object.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_object.c
> @@ -132,7 +132,6 @@ struct drm_gem_object *virtio_gpu_create_object(struct drm_device *dev,
>  
>  	dshmem = &shmem->base.base;
>  	dshmem->base.funcs = &virtio_gpu_shmem_funcs;
> -	dshmem->map_cached = true;
>  	return &dshmem->base;
>  }
>  
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index 294b2931c4cc0..a5bc082a77c48 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -98,9 +98,9 @@ struct drm_gem_shmem_object {
>  	unsigned int vmap_use_count;
>  
>  	/**
> -	 * @map_cached: map object cached (instead of using writecombine).
> +	 * @map_wc: map object using writecombine (instead of cached).
>  	 */
> -	bool map_cached;
> +	bool map_wc;
>  };
>  
>  #define to_drm_gem_shmem_obj(obj) \
> 

-- 
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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