linux-kernel - Re: [PATCH] arm64: kernel: compiling issue, need 'EXPORT_SYMBOL_GPL(read_current

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Date:	Tue, 21 May 2013 17:27:20 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	Will Deacon <will.deacon@....com>
CC:	Marc Zyngier <Marc.Zyngier@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Tony Lindgren <tony@...mide.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: kernel: compiling issue, need 'EXPORT_SYMBOL_GPL(read_current_timer)'

On 05/21/2013 04:53 PM, Will Deacon wrote:
> On Tue, May 21, 2013 at 05:06:52AM +0100, Chen Gang wrote:
>> On 05/20/2013 05:56 PM, Will Deacon wrote:
>>> Should be ok once the arch timer driver has moved exclusively to virtual
>>> time. I'm also not sure we even need to implement read_current_timer() --
>>> it's only used for delay-loop calibration, which we don't need for the
>>> arch timer.
>>>
>>
>> For whether we need implement read_current_timer():
>>
>>   many platforms have implemented it (openrisc, arm, sparc, hexagon, avr32, x86).
>>   it is called by init/calibrate.c when 'ARCH_HAS_READ_CURRENT_TIMER' is defined.
>>   since arm64 can implement it, better to provide it as an architect features to let outside use.
> 
> No, that code is not needed on arm64 because we calibrate the delay loop
> statically using a known timer frequency.
> 
>> For the implementation of read_current_timer():
>>
>>   it has to face various configurations
>>     (e.g. CONFIG_ARM_ARCH_TIMER, arch_timer_read_zero, arch_counter_get_cntvct, arch_counter_get_cntpct)
>>   so better still use variable instead of.
>>     (excuse me, I do not know what is 'CNTVCT_EL0', is it like a constant number ?)
> 
> cntvct_el0 is a system register, which provides the virtual counter value.
> 
>> For the implementation of get_cycles()
>>
>>   if read_current_timer() is provided,
>>   better to let get_cycles() to call it, instead of implement once again.
> 
> You can implement it as a macro if you like, I'm just suggesting that we
> might not need read_current_timer after all.
> 
> Will
> 
> 

Thanks, I should try patch v2.  :-)

-- 
Chen Gang

Asianux Corporation
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