Re: [Xen-devel] [PATCH RFC 1/8] arm: make SWIOTLB available

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 1 Aug 2013, Russell King - ARM Linux wrote:
> On Wed, Jul 31, 2013 at 06:45:25PM +0100, Stefano Stabellini wrote:
> > +static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
> > +{
> > +   unsigned int offset = paddr & ~PAGE_MASK;
> > +   return pfn_to_dma(dev, paddr >> PAGE_SHIFT) + offset;
> > +}
> > +
> > +static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t 
> > dev_addr)
> > +{
> > +   unsigned int offset = dev_addr & ~PAGE_MASK;
> > +   return (dma_to_pfn(dev, dev_addr) << PAGE_SHIFT) + offset;
> > +}
> 
> These two helpers look fine on the face of it.
> 
> > +static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t 
> > size)
> > +{
> > +   if (!dev->dma_mask)
> > +           return 0;
> > +
> > +   return addr + size - 1 <= *dev->dma_mask;
> > +}
> 
> You may wish to have a closer look at the DMA bounce code, because this
> assumes that DMA masks are a set of zeros followed by a set of ones.
> That may not always be the case (and we have the odd platform where that
> isn't the case.)
> 
> It has always bugged me that we call this thing a dma _mask_ and then
> much kernel code treats it as a limit - it should've been called "dma
> limit" if that's how it was to be interpreted.  If it really is a _mask_
> then the right way to test whether a DMA address/size is possible is:
> 
>       u64 limit, mask = *dev->dma_mask;
> 
>       limit = (mask + 1) & ~mask;
>       if (limit && size > limit)
>               return 0;
> 
>       if ((addr | (addr + size - 1)) & ~mask)
>               return 0;
> 
>       return 1;
> 
> The first checks whether 'size' fits within the least significant
> contiguous set of '1' bits in the DMA mask, and the second checks
> whether the region itself contains any address bits which may not
> meet the DMA mask.
 
I'll make the change, thanks for the explanation.


> I guess if we aren't going to encounter any of these cases anymore,
> your test is entirely sufficient.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel