generic device DMA implementation
Quoted 8 lines · Benjamin Herrenschmidt
Quoted 2 lines · James Bottomley
If you have a machine that has both consistent and inconsistent blocks, you need to encode that in
dma_addr_t(which is a platform definable type).I don't agree here. Encoding things in
dma_addr_t, then special casing in consistent_{map,unmap,sync,....) looks really ugly to me ! You wantdma_addr_tto contain a bus address for the given bus you are working with and pass that to your device, period.
I don't think that James meant actually defining flag bits
inside of dma_addr_t, although I suppose you could do it for some unused high bits on some architectures. I think the implication was that you could have something like:
static inline int is_consistent(dma_addr_t addr)
{
return (addr >= CONSISTENT_AREA_START && addr < CONSISTENT_AREA_END);
} I also don't recall anyone proposing special casing
dma_{map,unmap,sync} based on the results of such a check. I think the only function that might use it would be maybe_wmb(addr,len), which, on some might machines might be:
static inline void maybe_wmb(dma_addr_t addr, size_t len)
{
if (!is_consistent(addr))
wmb();
} In practice, I think dma_malloc() would either always return
consistent memory or never return consistent memory on a given machine, so maybe_wmb would probably never do such range checking. Instead it would compile to nothing on machines where dma_alloc always returned consistent memory, would compile to wmb() on systems where dma_alloc would only succeed if it could return non-consistent memory, and would compile to a procedure pointer on parisc that would either set to point to a no-op or wmb, depending on which kind of machine the kernel was booted on.
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