Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE_WRITEQ vars

["H. Peter Anvin" <hpa@zytor.com>]

Ingo Molnar wrote:
> 
> Look at the drivers that define their own wrappers:
> 
> #ifndef readq
> static inline unsigned long long readq(void __iomem *addr)
> {
>         return readl(addr) | (((unsigned long long)readl(addr + 4)) << 32LL);
> }
> #endif
> 
> ... it's the obvious 32-bit semantics for reading a 64-bit value 
> from an mmio address. We made that available on 32-bit too.
> 
> It's being used ... and has been in use for some time. Where's the 
> problem? readl is serializing on all default-ioremap mmio targets on 
> x86 so there's no ambiguity in ordering.
> 

I think his point is that they're not atomic.  For what it's worth,
atomic readq()/writeq() *are* possible with any x86-32 CPU which
supports MMX, but it is very costly to do in the kernel since it
involves touching the FPU state.

For the vast number of users, a non-atomic primitive which is available
for both 32- and 64-bit x86 is a win.  For a small number of users,
it'll be confusing, and for a very small minority it's going to be
desirable to have the atomic primitive.

The reason the non-atomic is generally fine is because most device
drivers are inherently single-threaded on a per-hardware device basis.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.