On Mon, 2008-04-28 at 07:10 -0700, Arjan van de Ven wrote:
quoted text
> On Sun, 27 Apr 2008 18:39:24 -0400
> Jeff Garzik <jeff@garzik.org> wrote:
> 
> > James Bottomley wrote:
> > > Here's another piece of the x86 API that's designed to be cached.
> > > The dma_declare_coherent_memory() usually represents behind bridge
> > > memory that's fully participatory in the coherence model.
> > > 
> > > Making it uncached damages the utility of this memory because doing
> > > cacheline sized burst cycles when needed to it is far faster than
> > > individual byte/word/quad writes.
> > > 
> > > Signed-off-by: James Bottomley
> > > <James.Bottomley@HansenPartnership.com>
> > > 
> > > ---
> > > 
> > > diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> > > index 388b113..df83ffd 100644
> > > --- a/arch/x86/kernel/pci-dma.c
> > > +++ b/arch/x86/kernel/pci-dma.c
> > > @@ -214,7 +214,7 @@ int dma_declare_coherent_memory(struct device
> > > *dev, dma_addr_t bus_addr, 
> > >  	/* FIXME: this routine just ignores
> > > DMA_MEMORY_INCLUDES_CHILDREN */ 
> > > -	mem_base = ioremap(bus_addr, size);
> > > +	mem_base = ioremap_cache(bus_addr, size);
> > >  	if (!mem_base)
> > >  		goto out;
> 
> this patch patch is likely broken on x86; or rather, anyone who uses it is...
> thinking you can find cache coherent memory on a PCI or similar bus that is actually
> cachable... keep dreaming. (for now; there's talk about extending PCI)

No ... it works for me, and caching is a performance advantage for me
too.  The only current consumer of this API is the NCR_Q720 SCSI card
which keeps a bunch of cacheable memory remote across the MCA bus.

If you think about it logically, most busses are second citizens in the
caching hierarchy: they really only get to force a flush and invalidate
of the CPU cache line rather than being fully participatory in the
coherence protocol.  However, even being second class is enough of a
speed up on slow busses because it allows bursting of the cache line for
the bus transfers.

The other consumers are SoC embedded ... so yes, perhaps I should ask
about this on linux-arch.

James


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