Re: [Xen-devel] swiotlb-xen question

[Oleksandr Dmytryshyn <oleksandr.dmytryshyn@xxxxxxxxxxxxxxx>]

Hi, Stefano.

Thank You for advice!
I see that You just copied the content from the original function arm_dma_mmap()
to the swiotlb-xen driver.
I've tested the same solution (except that I've copied __get_dma_pgprot()
function to the swiotlb-xen.c file too because this function is static in the
original file). This solution is working (I've tested it). But there is
an easier solution:

diff --git a/arch/arm/xen/mm.c b/arch/arm/xen/mm.c
index b0e77de..f3e820f 100644
--- a/arch/arm/xen/mm.c
+++ b/arch/arm/xen/mm.c
@@ -48,6 +48,7 @@ static struct dma_map_ops xen_swiotlb_dma_ops = {
  .sync_sg_for_device = xen_swiotlb_sync_sg_for_device,
  .map_sg = xen_swiotlb_map_sg_attrs,
  .unmap_sg = xen_swiotlb_unmap_sg_attrs,
+ .mmap = arm_dma_mmap,
  .map_page = xen_swiotlb_map_page,
  .unmap_page = xen_swiotlb_unmap_page,
  .dma_supported = xen_swiotlb_dma_supported,

The original arm_dma_mmap() function is implemented in the file
arch/arm/mm/dma-mapping.c which always is compiled for arm arhitecture.
I've checked this solution too. It is working.

I have a question. Is my solution correct?

Oleksandr Dmytryshyn | Product Engineering and Development
GlobalLogic
M +38.067.382.2525
www.globallogic.com

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On Thu, Apr 3, 2014 at 1:51 PM, Stefano Stabellini
<stefano.stabellini@xxxxxxxxxxxxx> wrote:
> On Tue, 1 Apr 2014, Oleksandr Dmytryshyn wrote:
>> Hi to all. I've investigated my issue deeper.
>>
>> Currently we use mmap in 2 cases: video-in (camera) and video-out (display).
>>
>> In case video-out all is working because mmap callback in the video-out 
>> device
>> leads to the remap_pfn_range() function (this function doesn't use 
>> unimplemented
>> .mmap callback in the swiotlb-xen.c file).
>>
>> In case video-out mmap callback in the video-in device leads to the .mmap
>> callback from the dma ops (unimplemented callback in the swiotlb-xen.c file)
>>
>> I've investigated kernel code. We have patch from the Stefano Stabellini
>> (068a6e62 - arm/xen: get_dma_ops: return xen_dma_ops if we are running as
>> xen_initial_domain)
>>
>> Before this patch the function *get_dma_ops() always returned
>> __generic_dma_ops()
>> which returned &arm_dma_ops (if archdata.dma_ops is absent). In this case
>> for arm architecture .mmap callback is arm_dma_mmap() function.
>>
>> After this patch for domU all is the same as before this patch. But in case
>> dom0 kernel uses xen_dma_ops with unimplemented .mmap callback. If some
>> function tries to use .mmap callback in dom0 then kernel calls generic
>> function dma_common_mmap() which doesn't work properly.
>>
>> May be some mechanism for swiotlb-xen should be implemented for unimplemented
>> callbacks: if callback is not implemented then right architectural callback
>> should be used (.mmap - arm_dma_mmap() for ARM)?
>
> I understand now. Thanks for investingating!
> I think that we'll have to introduce our own version of .mmap,
> because dma_to_pfn doesn't work correctly on Xen on ARM.
> The implementation would be something like (untested):
>
> int xen_swiotlb_dma_mmap(struct device *dev, struct vm_area_struct *vma,
>                  void *cpu_addr, dma_addr_t dma_addr, size_t size,
>                  struct dma_attrs *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(xen_bus_to_phys(dev, dma_addr));
>         unsigned long off = vma->vm_pgoff;
>
>         vma->vm_page_prot = __get_dma_pgprot(attrs, vma->vm_page_prot);
>
>         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);
>         }
> #endif  /* CONFIG_MMU */
>
>         return ret;
> }
>
> then we can add the function to xen_swiotlb_dma_ops.
> Would you be up for coming up with a proper patch?

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