On Thu, 2002-12-05 at 01:47, David Gibson wrote:
> Do you have an example of where the second option is useful? Off hand
> the only places I can think of where you'd use a consistent_alloc()
> rather than map_single() and friends is in cases where the hardware's
> behaviour means you absolutely positively have to have consistent
> memory.
Looking at our implementation (ppc32 on non-coherent CPUs like 405) of
pci_map_single, which just flushes the cache, I still feel we need a
consistent_alloc, that is an implementation that _disables_ caching for
the area.
A typical example is an USB OHCI driver. You really don't want to play
cache tricks with the shared area here. That will happen each time you
have a shared area in memory in which both the CPU and the device may
read/write in the same cache line.
For things like ring descriptors of a net driver, I feel it's very much
simpler (and possibly more efficient too) to also allocate non-cacheable
space for consistent instead of continuously flushing/invalidating.
Actually, flush/invalidate here can also have nasty side effects if
several descriptors fit in the same cache line.
The data buffers, of course (skbuffs typically) would preferably use
pci_map_* like APIs (hrm... did we ever make sure skbuffs would _not_
mix the data buffer with control datas in the same cache line ? This
have been a problem with non-coherent CPUs in the past).
Ben.
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This archive was generated by hypermail 2b29 : Sat Dec 07 2002 - 22:00:22 EST