[Linux-Xtensa] [PATCH] xtensa: Fix mmap() support

Max Filippov jcmvbkbc at gmail.com
Wed Mar 29 22:48:24 UTC 2017

Hi Boris,

On Tue, Mar 28, 2017 at 1:20 AM, Boris Brezillon
<boris.brezillon at free-electrons.com> wrote:
> The xtensa architecture does not implement the dma_map_ops->mmap() hook,
> thus relying on the default dma_common_mmap() implementation.
> This implementation is only safe on DMA-coherent architecture (hence the
> !defined(CONFIG_ARCH_NO_COHERENT_DMA_MMAP) condition), but xtensa does not
> fall in this case.

Why not? AFAIU "DMA-coherent" may mean "having DMA memory accessible
w/o caching", is that right? We have all low memory mapped in the uncached
KSEG region, it may be mapped to the userspace w/o caching as well, that's
what dma_common_mmap does.

> This lead to bad pfn calculation when someone tries to mmap() one or
> several pages that are not part of the identity mapping because the
> dma_common_mmap() extract the pfn value from the virt address using
> virt_to_page() which is only applicable on DMA-coherent platforms (on
> other platforms, DMA coherent pages are mapped in a different region).

Oh, yes. Our __pa implementation is buggy in a sense that it doesn't work
correctly with addresses from the uncached KSEG.

> Implement xtensa_dma_mmap() (loosely based on __arm_dma_mmap()) in which
> pfn is extracted from the DMA address using PFN_DOWN().
> While we're at it, select ARCH_NO_COHERENT_DMA_MMAP from the XTENSA
> entry so that we never silently fallback to dma_common_mmap() if someone
> decides to drop the xtensa_dma_mmap() implementation.

I don't think this is right.

> Signed-off-by: Boris Brezillon <boris.brezillon at free-electrons.com>
> ---
> Hello,
> This bug has been detected while developping a DRM driver on an FPGA
> containing an Xtensa CPU. The DRM driver is using the generic CMA GEM
> implementation which is allocating DMA coherent buffers in kernel space
> and then allows userspace programs to mmap() these buffers.
> Whith the existing implementation, the userspace pointer was pointing
> to a completely different physical region, thus leading to bad display
> output and memory corruptions.
> I'm not sure the xtensa_dma_mmap() implementation is correct, but it
> seems to solve my problem.
> Don't hesitate to propose a different implementation.

Could you please instead check if the dma_common_mmap works for you
with the attached patch?


> diff --git a/arch/xtensa/kernel/pci-dma.c b/arch/xtensa/kernel/pci-dma.c
> index 70e362e6038e..8f3ef49ceba6 100644
> --- a/arch/xtensa/kernel/pci-dma.c
> +++ b/arch/xtensa/kernel/pci-dma.c
> @@ -249,9 +249,32 @@ int xtensa_dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
>         return 0;
>  }
> +static int xtensa_dma_mmap(struct device *dev, struct vm_area_struct *vma,
> +                          void *cpu_addr, dma_addr_t dma_addr, size_t size,
> +                          unsigned long attrs)
> +{
> +       int ret = -ENXIO;
> +#ifdef CONFIG_MMU
> +       unsigned long nr_vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
> +       unsigned long nr_pages = PAGE_ALIGN(size) >> PAGE_SHIFT;
> +       unsigned long pfn = PFN_DOWN(dma_addr);
> +       unsigned long off = vma->vm_pgoff;
> +
> +       if (dma_mmap_from_coherent(dev, vma, cpu_addr, size, &ret))
> +               return ret;
> +
> +       if (off < nr_pages && nr_vma_pages <= (nr_pages - off))
> +               ret = remap_pfn_range(vma, vma->vm_start, pfn + off,
> +                                     vma->vm_end - vma->vm_start,
> +                                     vma->vm_page_prot);

You use vma->vm_page_prot directly here, isn't it required to do

vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);

to make the mapping uncached? Because otherwise I guess you
get a mapping with cache which on Xtensa is not going to be

-- Max
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