linux-kernel - Re: [PATCH 2/9] ARC: timer: rtc: implement read loop in "C" vs. inline asm

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Date:   Thu, 3 Nov 2016 10:45:08 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
CC:     Noam Camus <noamca@...lanox.com>, <tglx@...utronix.de>,
        <linux-snps-arc@...ts.infradead.org>,
        <Alexey.Brodkin@...opsys.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] ARC: timer: rtc: implement read loop in "C" vs.
 inline asm

On 11/03/2016 10:02 AM, Daniel Lezcano wrote:
> On Mon, Oct 31, 2016 at 03:48:09PM -0700, Vineet Gupta wrote:
>> To allow for easy movement into drivers/clocksource
>>
>> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
>> ---
>>  arch/arc/kernel/time.c | 13 +++++--------
>>  1 file changed, 5 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/arc/kernel/time.c b/arch/arc/kernel/time.c
>> index a2db010cde18..2c51e3cafad0 100644
>> --- a/arch/arc/kernel/time.c
>> +++ b/arch/arc/kernel/time.c
>> @@ -153,14 +153,11 @@ static cycle_t arc_read_rtc(struct clocksource *cs)
>>  		cycle_t  full;
>>  	} stamp;
>>  
>> -
>> -	__asm__ __volatile(
>> -	"1:						\n"
>> -	"	lr		%0, [AUX_RTC_LOW]	\n"
>> -	"	lr		%1, [AUX_RTC_HIGH]	\n"
>> -	"	lr		%2, [AUX_RTC_CTRL]	\n"
>> -	"	bbit0.nt	%2, 31, 1b		\n"
>> -	: "=r" (stamp.low), "=r" (stamp.high), "=r" (status));
>> +	do {
>> +		stamp.low = read_aux_reg(AUX_RTC_LOW);
>> +		stamp.high = read_aux_reg(AUX_RTC_HIGH);
>> +		status = read_aux_reg(AUX_RTC_CTRL);
>> +	} while (!(status & 0x80000000UL));
> 
> Replace the literal 0x80000000UL by a macro.

OK !


> What is the 'status' for ?

Hardware keeps a internal state machine for atomic readout of low/high. So if an
interrupt is taken between reading low and high, or if high increments after low
is read, then the bit forces a loop to retry.

>   
>>  	return stamp.full;
>>  }
>> -- 
>> 2.7.4
>>