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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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | |||||||||
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