Roland> My understanding is that __raw_writeq() is like writeq()
Roland> except not strongly ordered and without the byte-swap on
Roland> big-endian architectures. The __raw_writeX() variants are
Roland> convenient to avoid having to write inefficient code like
Roland> writel(swab32(foo), ...) when talking to a PCI device that
Roland> wants big-endian data. Without the raw variant, you end
Roland> up with a double swap on big-endian architectures.
Oh, I left one other thing out: writeq() and __raw_writeq() shold be
atomic in the sense that no other transactions should be able to get
onto the IO bus in the middle -- so implementing writeq() as two
writel()s in a row is not allowed
Andrew> OK. Can we please stop hacking around this in drivers and
Andrew> a) work out what it's supposed to do
Andrew> b) document that (Documentation/DocBook/deviceiobook.tmpl
Andrew> or code comment or whatever)
Andrew> c) tell arch maintainers?
Yes, I agree that's a good plan, especially the documentation part.
However I would argue that what's in
drivers/infiniband/hw/mthca/mthca_doorbell.h
is legitimate: the driver uses __raw_writeq() when it exists and uses
two __raw_writel()s properly serialized with a device-specific lock to
get exactly the atomicity it needs on 32-bit archs.
It's an open question what drivers that don't actually need atomicity
but just want a convenient way to write 64 bits at time should do.
- R.
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