Re: [RFC] generic device DMA implementation

["Adam J. Richter" <adam@yggdrasil.com>]

Benjamin Herrenschmidt wrote:
>On Wed, 2002-12-04 at 22:46, James Bottomley wrote:
>> 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 want
>dma_addr_t to 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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