Re: [RFC] generic device DMA implementation

[David Gibson]

On Thu, Dec 05, 2002 at 11:31:10AM +0900, Miles Bader wrote:
> James Bottomley <[e-mail]> writes:
> > > How is the driver supposed to tell whether a given dma_addr_t value
> > > represents consistent memory or not?  It seems like an
> > > (arch-specific) `dma_addr_is_consistent' function is necessary, but
> > > I couldn't see one in your patch.
> > 
> > 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).
> > 
> > The sync functions would just decode the type and either nop or perform the 
> > sync.
> 
> My thinking was that a driver might want to do things like --
> 
>   if (dma_addr_is_consistent (some_funky_addr)) {
>     do it quickly;
>   } else
>     do_it_the_slow_way (some_funky_addr);
> 
> in other words, something besides just calling the sync functions, in
> the case where the memory was consistent.

Yes, but using consistent memory is not necessarily the fast way - in
fact it probably won't be.  Machines which don't do DMA cache snooping
will need to disable caching to get consistent memory, so using
consistent memory is very slow - on such a machine explicit syncs are
preferable wherever possible.

On a machine which is nicely consistent, the cache "flushes" should
become NOPs, so we'd expect the two sides of that if to do the same
thing.

-- 
David Gibson			| For every complex problem there is a
[e-mail]	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson