linux-kernel - Re: [PATCH] x86: Remove readq()/writeq() on 32-bit

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Message-ID: <4A0DFE43.7050705@garzik.org>
Date:	Fri, 15 May 2009 19:44:03 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Hitoshi Mitake <h.mitake@...il.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Roland Dreier <rdreier@...co.com>, Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	tglx@...utronix.de, rpjday@...shcourse.ca,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Remove readq()/writeq() on 32-bit

Hitoshi Mitake wrote:
> On Wed, 13 May 2009 20:49:26 -0400
> Jeff Garzik <jeff@...zik.org> wrote:
> 
>> H. Peter Anvin wrote:
>>> Jeff Garzik wrote:
>>>> Judging from this thread and past, I think people will continue to 
>>>> complain and get confused, even with the above.
>>>>
>>> Do you really think so?  Seems unfortunate, since an API rename would be
>>> way more invasive.  This is the entirety of the header patch
>>> (compile-tested using 32-bit allyesconfig).
>> The header patch does not lessen the confusion, because you cannot look 
>> at the code and immediately tell what is going on...
>>
>> Having a single function's behavior change based on #include selection 
>> is /not/ intuitive at all, particularly for driver writers.  That is 
>> unlike almost every other Linux API, where functions' behavior stays 
>> constant across platforms, regardless of magic "under the hood."
>>
>> That sort of trick is reserved for arch maintainers who know what they 
>> are doing :)
>>
>> 	Jeff
>>
>>
>>
> 
> I found another way:
> Making architecture with atomic readq/writeq provide HAVE_READQ_ATOMIC/HAVE_WRITEQ_ATOMIC
> and making architecture with non-atomic readq/writeq provide HAVE_READQ/HAVE_WRITEQ.
> (HAVE_READQ_ATOMIC/HAVE_WRITEQ_ATOMIC should double as HAVE_READQ/HAVE_WRITEQ.)
> 
> So driver programmers who need atomic readq/writeq can judge existence of API they really need.
> If platform doesn't provide atomic readq/writeq, drivers need these can be disabled by Kconfig.
> And bugs Roland and David talking about will be banished.
> How about this? > Roland and David
> I wrote a test patch. Request for comments.
> 
> Signed-off-by: Hitoshi Mitake <h.mitake@...il.com>
> 
> ---
>  arch/x86/Kconfig |   16 ++++++++++++++--
>  1 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index df9e885..c94fc48 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -19,8 +19,6 @@ config X86_64
>  config X86
>  	def_bool y
>  	select HAVE_AOUT if X86_32
> -	select HAVE_READQ
> -	select HAVE_WRITEQ
>  	select HAVE_UNSTABLE_SCHED_CLOCK
>  	select HAVE_IDE
>  	select HAVE_OPROFILE
> @@ -2022,6 +2020,20 @@ config HAVE_ATOMIC_IOMAP
>  	def_bool y
>  	depends on X86_32
>  
> +config HAVE_READQ
> +	def_bool y
> +
> +config HAVE_WRITEQ
> +	def_bool y
> +
> +config HAVE_READQ_ATOMIC
> +	def_bool y
> +	depends on X86_64
> +
> +config HAVE_WRITEQ_ATOMIC
> +	def_bool y
> +	depends on X86_64

If you create HAVE_{READQ,WRITEQ}_ATOMIC, then you don't really need 
HAVE_READQ -- the other relevant 32-bit platforms simply need a 
definition of readq and writeq.  Probably easy enough to have a common 
definition in asm-generic.

	Jeff




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