Re: [PATCH v5 29/38] drm: rcar-du: fix common struct sg_table related issues

[Laurent Pinchart]

Hi Marek,

Thank you for the patch.

On Wed, May 13, 2020 at 03:32:36PM +0200, Marek Szyprowski wrote:
> The Documentation/DMA-API-HOWTO.txt states that the dma_map_sg() function
> returns the number of the created entries in the DMA address space.
> However the subsequent calls to the dma_sync_sg_for_{device,cpu}() and
> dma_unmap_sg must be called with the original number of the entries
> passed to the dma_map_sg().
> 
> struct sg_table is a common structure used for describing a non-contiguous
> memory buffer, used commonly in the DRM and graphics subsystems. It
> consists of a scatterlist with memory pages and DMA addresses (sgl entry),
> as well as the number of scatterlist entries: CPU pages (orig_nents entry)
> and DMA mapped pages (nents entry).
> 
> It turned out that it was a common mistake to misuse nents and orig_nents
> entries, calling DMA-mapping functions with a wrong number of entries or
> ignoring the number of mapped entries returned by the dma_map_sg()
> function.
> 
> To avoid such issues, lets use a common dma-mapping wrappers operating
> directly on the struct sg_table objects and use scatterlist page
> iterators where possible. This, almost always, hides references to the
> nents and orig_nents entries, making the code robust, easier to follow
> and copy/paste safe.
> 
> dma_map_sgtable() function returns zero or an error code, so adjust the
> return value check for the vsp1_du_map_sg() function.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

This is a very nice simplification, I've always foudn the dma_map_sg API
cumbersome to use.

Reviewed-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>

I assume you will get the whole series merged in one go. If I need to
pick the patch up at any point, please let me know. Otherwise I'll wait
until I see it upstream, no reply needed :-)

> ---
> For more information, see '[PATCH v5 00/38] DRM: fix struct sg_table nents
> vs. orig_nents misuse' thread:
> https://lore.kernel.org/linux-iommu/20200513132114.6046-1-m.szyprowski@xxxxxxxxxxx/T/
> ---
>  drivers/gpu/drm/rcar-du/rcar_du_vsp.c  | 3 +--
>  drivers/media/platform/vsp1/vsp1_drm.c | 8 ++++----
>  2 files changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_vsp.c b/drivers/gpu/drm/rcar-du/rcar_du_vsp.c
> index 5e4faf2..2fc1816 100644
> --- a/drivers/gpu/drm/rcar-du/rcar_du_vsp.c
> +++ b/drivers/gpu/drm/rcar-du/rcar_du_vsp.c
> @@ -197,9 +197,8 @@ int rcar_du_vsp_map_fb(struct rcar_du_vsp *vsp, struct drm_framebuffer *fb,
>  			goto fail;
>  
>  		ret = vsp1_du_map_sg(vsp->vsp, sgt);
> -		if (!ret) {
> +		if (ret) {
>  			sg_free_table(sgt);
> -			ret = -ENOMEM;
>  			goto fail;
>  		}
>  	}
> diff --git a/drivers/media/platform/vsp1/vsp1_drm.c b/drivers/media/platform/vsp1/vsp1_drm.c
> index a4a45d6..86d5e3f 100644
> --- a/drivers/media/platform/vsp1/vsp1_drm.c
> +++ b/drivers/media/platform/vsp1/vsp1_drm.c
> @@ -912,8 +912,8 @@ int vsp1_du_map_sg(struct device *dev, struct sg_table *sgt)
>  	 * skip cache sync. This will need to be revisited when support for
>  	 * non-coherent buffers will be added to the DU driver.
>  	 */
> -	return dma_map_sg_attrs(vsp1->bus_master, sgt->sgl, sgt->nents,
> -				DMA_TO_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
> +	return dma_map_sgtable(vsp1->bus_master, sgt, DMA_TO_DEVICE,
> +			       DMA_ATTR_SKIP_CPU_SYNC);
>  }
>  EXPORT_SYMBOL_GPL(vsp1_du_map_sg);
>  
> @@ -921,8 +921,8 @@ void vsp1_du_unmap_sg(struct device *dev, struct sg_table *sgt)
>  {
>  	struct vsp1_device *vsp1 = dev_get_drvdata(dev);
>  
> -	dma_unmap_sg_attrs(vsp1->bus_master, sgt->sgl, sgt->nents,
> -			   DMA_TO_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
> +	dma_unmap_sgtable(vsp1->bus_master, sgt, DMA_TO_DEVICE,
> +			  DMA_ATTR_SKIP_CPU_SYNC);
>  }
>  EXPORT_SYMBOL_GPL(vsp1_du_unmap_sg);
>  

-- 
Regards,

Laurent Pinchart