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

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Message-ID: <519AF2DC.2040200@asianux.com>
Date:	Tue, 21 May 2013 12:06:52 +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 05/20/2013 05:56 PM, Will Deacon wrote:
> On Mon, May 20, 2013 at 08:15:04AM +0100, Marc Zyngier wrote:
>> On Mon, 20 May 2013 14:48:05 +0800, Chen Gang <gang.chen@...anux.com>
>> wrote:
>>> Need 'EXPORT_SYMBOL_GPL(read_current_timer)' if build with allmodconfig.
>>>
>>> The related error:
>>>   ERROR: "read_current_timer" [lib/rbtree_test.ko] undefined!
>>>   ERROR: "read_current_timer" [lib/interval_tree_test.ko] undefined!
>>>   ERROR: "read_current_timer" [fs/ext4/ext4.ko] undefined!
>>>   ERROR: "read_current_timer" [crypto/tcrypt.ko] undefined!
>>>
>>>
>>> Signed-off-by: Chen Gang <gang.chen@...anux.com>
>>> ---
>>>  arch/arm64/kernel/time.c |    1 +
>>>  1 files changed, 1 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/arch/arm64/kernel/time.c b/arch/arm64/kernel/time.c
>>> index a551f88..7fcba80 100644
>>> --- a/arch/arm64/kernel/time.c
>>> +++ b/arch/arm64/kernel/time.c
>>> @@ -73,6 +73,7 @@ int read_current_timer(unsigned long *timer_value)
>>>  	*timer_value = arch_timer_read_counter();
>>>  	return 0;
>>>  }
>>> +EXPORT_SYMBOL_GPL(read_current_timer);
>>>  
>>>  void __init time_init(void)
>>>  {
>>
>> While this solves the problem, I'm not sure this is the best fix. The real
>> issue is with get_cycles, which is a macro around read_current_timer.
>>
>> AArch32 exports it because of the number of timer implementations. On
>> arm64, we should be able to just return CNTVCT_EL0.
>>
>> Catalin, Will, what do you think?
> 
> 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.

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 ?)

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.



Thanks.
-- 
Chen Gang

Asianux Corporation
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