Re: PCI DMA to small buffers on cache-incoherent arch

Mark Zealey
2002-06-10T18:33:33Z

On Mon, Jun 10, 2002 at 11:07:40AM -0700, William Jhun wrote:

> On Sun, Jun 09, 2002 at 09:27:05PM -0700, David S. Miller wrote:
> > From: Paul Mackerras <paulus@samba.org>
> > Date: Sun, 9 Jun 2002 19:51:58 +1000 (EST)
> >
> > This is the problem scenario. Suppose we are doing DMA to a buffer B
> > and also independently accessing a variable X which is not part of B.
> > Suppose that X and the beginning of B are both in cache line C.
> >
> > I see what the problem is. Hmmm...
> >
> > I'm trying to specify this such that knowledge of cachelines and
> > whatnot don't escape the arch specific code, ho hum... Looks like
> > that isn't possible.
>
> Perhaps provide macros in asm/pci.h that will:
>
> - Take a buffer size and add an appropriate amount (one cache line for
> alignment and the remainder to fill out the last cache line) to be
> used for kmalloc(), etc, eg:
>
> #define DMA_SIZE_ROUNDUP(size) \
> ((size + 2*SMP_CACHE_BYTES - 1) & ~(SMP_CACHE_BYTES - 1))
>
> - Take a buffer address (as returned from kmalloc() with the modified
> size from above) and round it up to a cacheline boundary, eg:
>
> #define DMA_BUFFER_ALIGN(ptr) \
> (((unsigned long)ptr + SMP_CACHE_BYTES - 1) & ~(SMP_CACHE_BYTES - 1))
>
> These two, in conjunction, would provide a buffer that's aligned on a
> cacheline boundary and ends on a cacheline boundary. Kind of ugly, but
> would be sufficient and would hide the cacheline size specifics.
> Cache-coherent platforms would just returned the original argument.

Why not just make some dmalloc() macro in pci.h which will do the nessecory
magic resizing and alignments? seems a lot easier to do...

--

Mark Zealey
mark@zealos.org; mark@itsolve.co.uk