On Wed, Jul 15, 2009 at 08:41:30AM -0400, James Bottomley wrote: > On Wed, 2009-07-15 at 00:38 -0600, Grant Grundler wrote: > > IIRC, only pa880 and pa8900 pay attention to the U-bit. > > I thought all the previous CPUs ignored U-bit. > > Um, no, almost all pa chips use it ... it's the way we make coherent > memory on the very old 710 and similar systems: use an ordinary kmalloc > and turn off caching in the map. The 715 class systems with the oldest > pa chips don't respect the U bit ... these are the ones we have to pull > the driver cache flushing tricks on to get them working. Even the 715-class systems respect the U bit. The problem is that the memory controller in use on those platforms cannot cope with sub-cacheline-size accesses to memory. So if we were to set the U bit on memory, we would crash. > > Yes - but only ZX1 chipset (e.g. rp3440 and C8000) uses IO space that > > is outside of F-space. F-space is hardwired to be uncachable by the CPU. > > OK, so we don't really use the concept of F-Space in the linux kernel > virtual memory map on parisc linux. What we do is map the whole of > memory into the virtual address space, but none of the I/O space. the > readX/writeX macros actually go via absolute accesses. Because the Uhm, no. The gsc_read/writeX macros go via absolute accesses. __raw_read/writeX go via ioremapped addresses, as do the more common readX/writeX. > kernel virtual addresses are offset mapped from the absolute addresses, > we do get a hole in the virtual map corresponding to F-Space (simply > because there's no accessible memory there) but we're actually able to > fill that hole later with virtual mappings if we choose. The net result > is that I/O devices aren't mapped into the kernel at all until we call > ioremap. If a device uses readX/writeX only, it never actually gets an > appearance in our virtual space because we simply use absolute accesses > to get the data to and from the device. If it really needs a remapped > area, we use ioremap, then it does appear in our virtual map, and we set > the U bit on the mapping. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
