[v8,4/9] clocksource/drivers/arm_arch_timer: use readq to get 64-bit CNTVCT

Message ID 1468952284-28942-5-git-send-email-fu.wei@linaro.org
State New
Headers show

Commit Message

Fu Wei July 19, 2016, 6:17 p.m.
From: Fu Wei <fu.wei@linaro.org>


This patch simplify arch_counter_get_cntvct_mem function by
using readq to get 64-bit CNTVCT value instead of readl_relaxed.

Signed-off-by: Fu Wei <fu.wei@linaro.org>

---
 drivers/clocksource/arm_arch_timer.c | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

-- 
2.5.5

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Comments

Will Deacon July 25, 2016, 9:02 a.m. | #1
On Wed, Jul 20, 2016 at 02:17:59AM +0800, fu.wei@linaro.org wrote:
> From: Fu Wei <fu.wei@linaro.org>

> 

> This patch simplify arch_counter_get_cntvct_mem function by

> using readq to get 64-bit CNTVCT value instead of readl_relaxed.

> 

> Signed-off-by: Fu Wei <fu.wei@linaro.org>

> ---

>  drivers/clocksource/arm_arch_timer.c | 10 +---------

>  1 file changed, 1 insertion(+), 9 deletions(-)

> 

> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c

> index e6fd42d..483d2f9 100644

> --- a/drivers/clocksource/arm_arch_timer.c

> +++ b/drivers/clocksource/arm_arch_timer.c

> @@ -418,15 +418,7 @@ u32 arch_timer_get_rate(void)

>  

>  static u64 arch_counter_get_cntvct_mem(void)

>  {

> -	u32 vct_lo, vct_hi, tmp_hi;

> -

> -	do {

> -		vct_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);

> -		vct_lo = readl_relaxed(arch_counter_base + CNTVCT_LO);

> -		tmp_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);

> -	} while (vct_hi != tmp_hi);

> -

> -	return ((u64) vct_hi << 32) | vct_lo;

> +	return readq(arch_counter_base + CNTVCT_LO);


What's the benefit of doing this? If you use readq here, how can we
guarantee that (a) the endpoint won't generate a SLVERR or similar and
(b) that we get an atomic read?

"If it ain't broke, don't fix it"

Will
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Fu Wei July 25, 2016, 3:50 p.m. | #2
Hi Will,

On 25 July 2016 at 17:02, Will Deacon <will.deacon@arm.com> wrote:
> On Wed, Jul 20, 2016 at 02:17:59AM +0800, fu.wei@linaro.org wrote:

>> From: Fu Wei <fu.wei@linaro.org>

>>

>> This patch simplify arch_counter_get_cntvct_mem function by

>> using readq to get 64-bit CNTVCT value instead of readl_relaxed.

>>

>> Signed-off-by: Fu Wei <fu.wei@linaro.org>

>> ---

>>  drivers/clocksource/arm_arch_timer.c | 10 +---------

>>  1 file changed, 1 insertion(+), 9 deletions(-)

>>

>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c

>> index e6fd42d..483d2f9 100644

>> --- a/drivers/clocksource/arm_arch_timer.c

>> +++ b/drivers/clocksource/arm_arch_timer.c

>> @@ -418,15 +418,7 @@ u32 arch_timer_get_rate(void)

>>

>>  static u64 arch_counter_get_cntvct_mem(void)

>>  {

>> -     u32 vct_lo, vct_hi, tmp_hi;

>> -

>> -     do {

>> -             vct_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);

>> -             vct_lo = readl_relaxed(arch_counter_base + CNTVCT_LO);

>> -             tmp_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);

>> -     } while (vct_hi != tmp_hi);

>> -

>> -     return ((u64) vct_hi << 32) | vct_lo;

>> +     return readq(arch_counter_base + CNTVCT_LO);

>


Sorry, right after posting v9, I got your comment,

> What's the benefit of doing this? If you use readq here, how can we


benefit:
1. simplify the code
2. from arch/arm64/include/asm/io.h, I guess readq is more efficient

> guarantee that (a) the endpoint won't generate a SLVERR or similar and

> (b) that we get ?


I think so, according to arch/arm64/include/asm/io.h.
readq Implement by "LDR" and "LDAR", So I think It is an atomic read.

Please correct me, If I misunderstand something, thanks

>

> "If it ain't broke, don't fix it"

>

> Will




-- 
Best regards,

Fu Wei
Software Engineer
Red Hat
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Patch hide | download patch | download mbox

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index e6fd42d..483d2f9 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -418,15 +418,7 @@  u32 arch_timer_get_rate(void)
 
 static u64 arch_counter_get_cntvct_mem(void)
 {
-	u32 vct_lo, vct_hi, tmp_hi;
-
-	do {
-		vct_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);
-		vct_lo = readl_relaxed(arch_counter_base + CNTVCT_LO);
-		tmp_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);
-	} while (vct_hi != tmp_hi);
-
-	return ((u64) vct_hi << 32) | vct_lo;
+	return readq(arch_counter_base + CNTVCT_LO);
 }
 
 /*