"Consolidate get_dma_ops" breaks Xen on ARM

[Catalin Marinas]

On Tue, Apr 11, 2017 at 01:43:28PM +0100, Julien Grall wrote:
> On 11/04/17 02:14, Bart Van Assche wrote:
> > On 04/10/17 17:31, Stefano Stabellini wrote:
> >> I think the reason is that, as you can see, if (dev && dev->dma_ops),
> >> dev->dma_ops is returned, while before this changes, xen_dma_ops was
> >> returned on Xen on ARM.
> >>
> >> Unfortunately DMA cannot work properly without using the appropriate
> >> xen_dma_ops. See drivers/xen/swiotlb-xen.c and arch/arm/xen/mm.c for
> >> more details. (The problem is easy to spot, but I wasn't CC'ed on the
> >> patch.)
> >>
> >> I don't know how to solve this problem without introducing some sort of
> >> if (xen()) in include/linux/dma-mapping.h.
> > 
> > Sorry but I don't have access to an ARM development system. Does your 
> > comment apply to dev == NULL only, dev != NULL only or perhaps to both? 
> > If your comment applies to dev != NULL only, can you check whether 
> > adding something like set_dma_ops(dev, get_arch_dma_ops(NULL)) to the 
> > appropriate ARM arch_setup_dma_ops() function is sufficient?
> 
> If I understand correctly, set_dma_ops will replace dev->dma_ops with
> Xen DMA ops.
> 
> However, Xen DMA ops will need in some places to call the device 
> specific DMA ops (see __generic_dma_ops(...)). So I think replacing
> dev->dma_ops is not a solution here.
> 
> The hackish patch below is fixing the problem for both ARM64 and ARM32.
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 0977317c6835..43a73ddeec7a 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -174,6 +174,8 @@ int dma_mmap_from_coherent(struct device *dev, struct vm_area_struct *vma,
>  #include <asm/dma-mapping.h>
>  static inline const struct dma_map_ops *get_dma_ops(struct device *dev)
>  {
> +       if (xen_initial_domain())
> +              return xen_dma_ops;
>         if (dev && dev->dma_ops)
>                 return dev->dma_ops;
>         return get_arch_dma_ops(dev ? dev->bus : NULL);

If we do this, I guess there is no need to check for
xen_initial_domain() in the get_arch_dma_ops() function. Anyway, this
hunk would break the other architectures since xen_dma_ops is only
defined for arm and arm64.

> It is not nice as this is common code, but I can't find a better solution
> so far. Any opinions?

A different hack would be to avoid the generic get_dma_ops
implementation on arm with some #ifdef hacks above.

Yet another way would be for dom0 to always set dev->dma_ops to
xen_dma_ops and preserve the real dma_ops somewhere under dev->archdata.
You could intercept the arch_setup_dma_ops() function for this or use
bus_register_notifier() (though I think the former is easier). The Xen
code making use of the real dma_ops would have to dig them out from
dev->archdata.

-- 
Catalin