Messages in this thread | | | Date | Fri, 2 Aug 2013 13:09:51 +0100 | From | Stefano Stabellini <> | Subject | Re: [PATCH RFC 1/8] arm: make SWIOTLB available |
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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.
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