Re: 2.6.18-rc5-mm1: drivers/infiniband/hw/amso1100/c2.c compile error

Andrew Morton
2006-09-01T20:11:16Z

On Fri, 01 Sep 2006 12:53:47 -0700
Roland Dreier <rdreier@cisco.com> wrote:

> 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.

No, driver-specific workarounds are not legitimate, sorry.

The driver should simply fail to compile on architectures which do not
implement __raw_writeq().

We can speed up the process by sending helpful emails to architecture
maintainers, but they'll notice either way.

Let's fix it once, and in the correct place.

> 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.

Well yeah. We should sort out the design issues before implementing
things ;)