Re: [PATCH v2 1/2] drm/exynos: fix DMA_ATTR_NO_KERNEL_MAPPING usage

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Feb 04, 2015 at 11:20:19AM +0100, Marek Szyprowski wrote:
> >diff --git a/drivers/gpu/drm/exynos/exynos_drm_buf.c b/drivers/gpu/drm/exynos/exynos_drm_buf.c
> >index 9c80884..24994ba 100644
> >--- a/drivers/gpu/drm/exynos/exynos_drm_buf.c
> >+++ b/drivers/gpu/drm/exynos/exynos_drm_buf.c
> >@@ -63,11 +63,11 @@ static int lowlevel_buffer_allocate(struct drm_device *dev,
> >  			return -ENOMEM;
> >  		}
> >-		buf->kvaddr = (void __iomem *)dma_alloc_attrs(dev->dev,
> >+		buf->cookie = dma_alloc_attrs(dev->dev,
> >  					buf->size,
> >  					&buf->dma_addr, GFP_KERNEL,
> >  					&buf->dma_attrs);
> >-		if (!buf->kvaddr) {
> >+		if (!buf->cookie) {
> >  			DRM_ERROR("failed to allocate buffer.\n");
> >  			ret = -ENOMEM;
> >  			goto err_free;

I wonder whether anyone has looked at what exynos is doing with this.

                start_addr = buf->dma_addr;
                while (i < nr_pages) {
                        buf->pages[i] = phys_to_page(start_addr);
                        start_addr += PAGE_SIZE;
                        i++;
                }

There is no guarantee that DMA addresses are the same as physical addresses
in the kernel, so this is a layering violation.  If you want to do this,
then a better way to do this on ARM would be:

			buf->pages[i] = pfn_to_page(dma_to_pfn(dev, start_addr));

The difference here is that dma_to_pfn() knows how to convert a dma_addr_t
to a PFN which can then be converted to a struct page (provided it is
backed by kernel managed system memory.)

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