Re: IOMMU setup vs DAC (PCI)

Grant Grundler writes:
 > My original quest was for an architecturally neutral way to pass
 > 64-bit physical memory addresses back to a 64-bit capable card.
 > 
 > pci_dma_supported() interface provides the right hook for the
 > driver to advertise device capabilities. dma_addr_t is defined
 > in most arches (read x86) to be 32-bit. But IA64 (u64) and mips*
 > (unsigned long) have broken ground here already. I'll explore
 > further to see if parisc*-linux can in fact use "unsigned long".
 > 
 > But I'm still interested in any comments or insights.
 > (ie am I out to lunch? ;^)

You are going into unchartered territory.  IA64 supports 64-bit
DMA as a HACK at best (see qla20xx driver) and uses a software
iommu implementation to handle all normal drivers using the
supported 32-bit pci_*() interfaces.

The 64-bit support API will appear in 2.5.x, no sooner.

And all that talk of IOMMU overhead assumes a shit implementation of
TLB flushing.  With a sane setup you only flush once per circle walk
of the page tables, see the tricks in sparc64/kernel/pci_iommu.c to
see what I'm talking about.  I can push 2 gigabytes to a disk over
SCSI and only take 18 PIOs to the IOMMU.  Also, many IOMMU based PCI
implementations do not offer the software managed
prefetching/write-behind facilities when 64-bit DAC is used.

I say stay at 32-bit IOMMU based stuff for now.

Later,
David S. Miller
davem@redhat.com
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