linux-kernel - Re: [RFC Patch v1 32/55] ARC: [optim] Cache "current" in Register r25

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Message-ID: <50A4C273.10102@synopsys.com>
Date:	Thu, 15 Nov 2012 15:52:43 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<tglx@...utronix.de>
Subject: Re: [RFC Patch v1 32/55] ARC: [optim] Cache "current" in Register
 r25

On Monday 12 November 2012 07:20 PM, Arnd Bergmann wrote:
> On Monday 12 November 2012, Vineet.Gupta1@...opsys.com wrote:
>>  endmenu # "Platform Board Configuration"
>>  
>> +config ARC_CURR_IN_REG
>> +       bool "Dedicate Register r25 for current_task pointer"
>> +       default y
>> +       help
>> +         This reserved Register R25 to point to Current Task in
>> +         kernel mode. This saves memory access for each such access
>> +
> This looks like a useful feature if you have 32 GPRs, but why make it
> optional? Are there cases where you would disable it?

Since it touched the low level guts of port, it made sense to start with
a config option in initial implementation. But I've kept it that way
since it annotates the code nicely and implicitly documents how to do
this for any other register + data structure.

> Another alternative to caching "current" would be to cache the per-cpu
> offset, and calculate current from that one.

Good tip - I'll add this to my TODO list -

> 	Arnd

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