linux-kernel - Re: [PATCH] drm: Fix for GEM buffers with write-combine memory

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Date:   Fri, 28 May 2021 08:59:15 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To:     Paul Cercueil <paul@...pouillou.net>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>
Cc:     list@...ndingux.net, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: Fix for GEM buffers with write-combine memory

On 28/05/2021 02:03, Paul Cercueil wrote:
> The previous commit wrongly assumed that dma_mmap_wc() could be replaced
> by pgprot_writecombine() + dma_mmap_pages(). It did work on my setup,
> but did not work everywhere.
> 
> Use dma_mmap_wc() when the buffer has the write-combine cache attribute,
> and dma_mmap_pages() when it has the non-coherent cache attribute.
> 
> Signed-off-by: Paul Cercueil <paul@...pouillou.net>
> Reported-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> Fixes: cf8ccbc72d61 ("drm: Add support for GEM buffers backed by non-coherent memory")
> ---
>   drivers/gpu/drm/drm_gem_cma_helper.c | 16 ++++++++++------
>   1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
> index 235c7a63da2b..4c3772651954 100644
> --- a/drivers/gpu/drm/drm_gem_cma_helper.c
> +++ b/drivers/gpu/drm/drm_gem_cma_helper.c
> @@ -514,13 +514,17 @@ int drm_gem_cma_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
>   
>   	cma_obj = to_drm_gem_cma_obj(obj);
>   
> -	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);
> +	if (cma_obj->map_noncoherent) {
> +		vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
> +
> +		ret = dma_mmap_pages(cma_obj->base.dev->dev,
> +				     vma, vma->vm_end - vma->vm_start,
> +				     virt_to_page(cma_obj->vaddr));
> +	} else {
> +		ret = dma_mmap_wc(cma_obj->base.dev->dev, vma, cma_obj->vaddr,
> +				  cma_obj->paddr, vma->vm_end - vma->vm_start);
>   
> -	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);
>   
> 

Reviewed-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>

and

Tested-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>

Thanks!

Btw, the kernel-doc for drm_gem_cma_create doesn't quite match, as it 
says wc is always used.

  Tomi