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

From: Stephen Boyd
Date: Thu Feb 02 2012 - 19:49:15 EST

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.

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