linux-kernel - Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE

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Date:	Sun, 19 Apr 2009 15:02:01 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Roland Dreier <rdreier@...co.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Robert P. J. Day" <rpjday@...shcourse.ca>,
	Hitoshi Mitake <h.mitake@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: arch/x86/Kconfig selects invalid HAVE_READQ, HAVE_WRITEQ vars

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.

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