Re: [PATCH] NVMe: Fix compilation on architecturs withoutreadq/writeq
From: Ingo Molnar
Date: Sat Jan 21 2012 - 03:29:45 EST
* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Jan 19, 2012 at 5:01 PM, Matthew Wilcox
> <matthew.r.wilcox@xxxxxxxxx> wrote:
> > The only places that uses readq/writeq are in the initialisation
> > path. Since they're not performance critical, always use readl/writel.
>
> The arch rules are that i fthe architecture has readq/writeq, they
> will be #define'd (they may be inline functions, but there will be a
>
> #define readq readq
>
> to make it visible to the preprocessor as well).
>
> So if you don't need the atomicity guarantees of a "real" readq, you
> can do this instead:
>
> #ifndef readq
> static inline u64 readq(void __iomem *addr)
> {
> return readl(addr) | (((u64) readl(addr + 4)) << 32LL);
> }
> #endif
>
> and then use readq() as if it existed.
>
> And I do think we should expose this in some generic manner. Because
> we currently don't, we already have that pattern copied in quite a few
> drivers.
>
> Maybe <asm-generic/io-nonatomic.h> or something? Making it
> clear that its not atomic, but avoiding the silly duplication
> we do now..
>
> This whole mess was introduced in commit dbee8a0affd5 ("x86:
> remove 32-bit versions of readq()/writeq()"), and it already
> talked about the problems but didn't help with the drivers
> that simply don't care.
>
> All the people in those threads were doing their
> self-satisfied "writeq is broken", without much acknowledging
> that 99% of users simply don't seem to care.
>
> "Occupy Writeq - We are the 99%"
Agreed, and offering a generic facility for silly duplication
was the motivation of the original commit by Hitoshi Mitake.
This:
| The presense of a writeq() implementation on 32-bit x86 that
| splits the 64-bit write into two 32-bit writes turns out to
| break the mpt2sas driver (and in general is risky for drivers
| as was discussed in
| <http://lkml.kernel.org/r/adaab6c1h7c.fsf@xxxxxxxxx>).
is actually a mostly bogus statement and creates more problems
than it solves.
Hitoshi-san, would you be interested in re-adding the generic
readq/writeq definitions in a slight variation to 2c5643b1c5, to
a separate io-nonatomic.h file, so that drivers that want it can
#include that file and be happy?
Thanks,
Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/