linux-kernel - Re: [PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR

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Message-ID: <4F2B2F01.20601@codeaurora.org>
Date:	Thu, 02 Feb 2012 16:49:05 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Nicolas Pitre <nico@...xnic.net>,
	linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR

On 02/02/12 16:36, Russell King - ARM Linux wrote:
> On Thu, Feb 02, 2012 at 03:36:49PM -0800, Stephen Boyd wrote:
>> On 02/02/12 13:38, Nicolas Pitre wrote:
>>> On Thu, 2 Feb 2012, Russell King - ARM Linux wrote
>>>> On Thu, Feb 02, 2012 at 11:24:46AM -0800, Stephen Boyd wrote:
>>>>> Should we move get_thread_info into assembler.h? It seems odd
>>>>> to include entry-header.S but I saw that vfp was doing the same.
>>>> Probably yes, and probably also have preempt_disable and preempt_enable
>>>> assembler macros.  That's going to get rather icky if we have to
>>>> explicitly call the scheduler though (to solve (1)).
>>> What about a pair of helpers written in C instead?
>>>
>>> v7_flush_dcache_all() could be renamed, and a wrapper function called 
>>> v7_flush_dcache_all() would call the preemption disable helper, call the 
>>> former v7_flush_dcache_all code, then call the preemption enable helper.
>>>
>>> Then __v7_setup() could still call the core cache flush code without 
>>> issues.
>> I tried to put the preemption disable/enable right around the place
>> where it was needed. With this approach we would disable preemption
>> during the entire cache flush. I'm not sure if we want to make this
>> function worse for performance, do we? It certainly sounds easier than
>> writing all the preempt macros in assembly though.
> Err, why do you think it's a big task?
>
> preempt disable is a case of incrementing the thread preempt count, while
> preempt enable is a case of decrementing it, testing for zero, if zero,
> then checking whether TIF_NEED_RESCHED is set and calling a function.
>
> If that's too much, then the simple method in assembly to quickly disable
> preemption over a very few set of instructions is using mrs/msr and cpsid i.
> That'll be far cheaper than fiddling about with preempt counters or
> messing about with veneers in C code.

I'll try the macros. So far it isn't bad, just the __v7_setup to resolve.

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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