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Subjectioremap() vs. ioremap_nocache()
Jes Sorensen was kind enough to point out a longstanding
mis-conception that I had about ioremap(): I thought that ioremap()
was being deprecated in favor of ioremap_nocache() because the former
does not clearly define what kind of memory attribute will be used to
access the mapped memory (cached, write-through cached,
write-coalescing, etc). But I seem to have been wrong about that.
Now, as far as I know, on x86, ioremap() will give write-through
cached mappings (in the absence of mtrr games). If this is true, how
can this work? There are many drivers out there that use ioremap() on
memory mapped I/O regions that do NOT have the "Prefetchable" bit set
in the PCI BAR. For example, the eepro100 driver does this and it has
a routine called wait_for_cmd_done(), which spins on an ioremapped
read. On an x86, what prevents these reads from being cached?


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