linux-kernel - Re: [PATCH] NVMe: Fix compilation on architecturs without readq/writeq

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Date:	Sat, 21 Jan 2012 09:28:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hitoshi Mitake <h.mitake@...il.com>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Roland Dreier <roland@...estorage.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hitoshi Mitake <h.mitake@...il.com>,
	James Bottomley <James.Bottomley@...allels.com>,
	linux-kernel@...r.kernel.org, linux-nvme@...radead.org,
	hpa@...ux.intel.com
Subject: Re: [PATCH] NVMe: Fix compilation on architecturs without
 readq/writeq


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, Jan 19, 2012 at 5:01 PM, Matthew Wilcox
> <matthew.r.wilcox@...el.com> 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@cisco.com>).

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
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