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

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Date:	Tue, 24 Jan 2012 01:05:05 +0900
From:	Hitoshi Mitake <h.mitake@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Roland Dreier <roland@...estorage.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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

On Sun, Jan 22, 2012 at 01:58, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Hitoshi Mitake <h.mitake@...il.com> wrote:
>
>> On Sat, Jan 21, 2012 at 17:28, Ingo Molnar <mingo@...e.hu> wrote:
>> >
>> > * 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?
>>
>> It sounds nice. In the previous discussion, I suggested that
>> chaning the name of non-atomic readq/writeq to
>> readq_nonatomic/writeq_nonatomic. And James Bottomley
>> replied that it is fine but not really very useful:
>>
>> https://lkml.org/lkml/2011/5/19/13
>>
>> The idea of providing non-atomic readq/writeq in the new file
>> with the name which express non-atomicity clearly might be
>> able to satisfy both of safety and usefulness.
>>
>> It will reduce the duplication of the definition. In addition
>> readq/writeq users don't have to type the long symbols with
>> _nonatomic suffix and can know non-atomicity from the name
>> of header file.
>>
>> I'd like to hear opinions from James, Roland and folks who
>> dislike non-atomic readq/writeq.
>
> Drivers that want the definition add this line to their driver:
>
> #include <asm/io-nonatomic.h>
>
> and then readq()/writeq() does the obvious thing. No need for
> readq_nonatomic()/writeq_nonatomic() - that extra line declares
> things clearly enough and cannot be added accidentally.
>

I wrote the patch which adds the new file include/asm-generic/io-nonatomic.h.
io-nonatomic.h provides non-atomic version readq()/writeq().

The patch also removes some duplicated readq()/writeq() definitions
which are added in the commit: dbee8a0affd5e6eaa5d7c816c4bc233f6f110f50.
The commit was made by Roland Dreier and removed readq()/writeq()
from arch/x86/include/asm.h.

If I find no error after the building allyesconfig and allmodconfig test,
I'll send the patch for review. This may take a few hour.

BTW, I placed io-nonatomic.h under include/asm-generic/ because
non-atomic version readq()/writeq() is architecture dependent.
If you have comments on this point, I'd like to hear.

Thanks,
-- 
Hitoshi Mitake
h.mitake@...il.com
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