[PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[David Engraf]

On newer boards the TC can be read as single 32 bit value without locking.
Thus the clock can be used as reference for sched_clock which is much more
accurate than the jiffies implementation.

Tested on a Atmel SAMA5D2 board.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
---
 drivers/clocksource/tcb_clksrc.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/clocksource/tcb_clksrc.c b/drivers/clocksource/tcb_clksrc.c
index d4ca996..745844e 100644
--- a/drivers/clocksource/tcb_clksrc.c
+++ b/drivers/clocksource/tcb_clksrc.c
@@ -10,6 +10,7 @@
 #include <linux/io.h>
 #include <linux/platform_device.h>
 #include <linux/atmel_tc.h>
+#include <linux/sched_clock.h>
 
 
 /*
@@ -56,11 +57,16 @@ static u64 tc_get_cycles(struct clocksource *cs)
 	return (upper << 16) | lower;
 }
 
-static u64 tc_get_cycles32(struct clocksource *cs)
+static u32 tc_get_cv32(void)
 {
 	return __raw_readl(tcaddr + ATMEL_TC_REG(0, CV));
 }
 
+static u64 tc_get_cycles32(struct clocksource *cs)
+{
+	return tc_get_cv32();
+}
+
 static struct clocksource clksrc = {
 	.name           = "tcb_clksrc",
 	.rating         = 200,
@@ -69,6 +75,11 @@ static struct clocksource clksrc = {
 	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
 };
 
+static u64 notrace tc_read_sched_clock(void)
+{
+	return tc_get_cv32();
+}
+
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
 
 struct tc_clkevt_device {
@@ -339,6 +350,9 @@ static int __init tcb_clksrc_init(void)
 		clksrc.read = tc_get_cycles32;
 		/* setup ony channel 0 */
 		tcb_setup_single_chan(tc, best_divisor_idx);
+
+		/* register sched_clock on chips with single 32 bit counter */
+		sched_clock_register(tc_read_sched_clock, 32, divided_rate);
 	} else {
 		/* tclib will give us three clocks no matter what the
 		 * underlying platform supports.
-- 
2.9.3

Re: [PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[David Engraf]

Am 17.01.2017 um 10:13 schrieb Daniel Lezcano:
> On 11/01/2017 14:50, David Engraf wrote:
>> On newer boards the TC can be read as single 32 bit value without locking.
>> Thus the clock can be used as reference for sched_clock which is much more
>> accurate than the jiffies implementation.
>>
>> Tested on a Atmel SAMA5D2 board.
>
> Is this change backward compatible ?

The way how the driver reads the clock hasn't been changed. I've only 
introduced the call to sched_clock_register() which will use this clock 
for the scheduler as long as no better clock is already used. So I don't 
see a backward compatibility problem.

Best regards
- David

>
>> Signed-off-by: David Engraf <david.engraf@sysgo.com>
>> ---
>>  drivers/clocksource/tcb_clksrc.c | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/clocksource/tcb_clksrc.c b/drivers/clocksource/tcb_clksrc.c
>> index d4ca996..745844e 100644
>> --- a/drivers/clocksource/tcb_clksrc.c
>> +++ b/drivers/clocksource/tcb_clksrc.c
>> @@ -10,6 +10,7 @@
>>  #include <linux/io.h>
>>  #include <linux/platform_device.h>
>>  #include <linux/atmel_tc.h>
>> +#include <linux/sched_clock.h>
>>
>>
>>  /*
>> @@ -56,11 +57,16 @@ static u64 tc_get_cycles(struct clocksource *cs)
>>  	return (upper << 16) | lower;
>>  }
>>
>> -static u64 tc_get_cycles32(struct clocksource *cs)
>> +static u32 tc_get_cv32(void)
>>  {
>>  	return __raw_readl(tcaddr + ATMEL_TC_REG(0, CV));
>>  }
>>
>> +static u64 tc_get_cycles32(struct clocksource *cs)
>> +{
>> +	return tc_get_cv32();
>> +}
>> +
>>  static struct clocksource clksrc = {
>>  	.name           = "tcb_clksrc",
>>  	.rating         = 200,
>> @@ -69,6 +75,11 @@ static struct clocksource clksrc = {
>>  	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
>>  };
>>
>> +static u64 notrace tc_read_sched_clock(void)
>> +{
>> +	return tc_get_cv32();
>> +}
>> +
>>  #ifdef CONFIG_GENERIC_CLOCKEVENTS
>>
>>  struct tc_clkevt_device {
>> @@ -339,6 +350,9 @@ static int __init tcb_clksrc_init(void)
>>  		clksrc.read = tc_get_cycles32;
>>  		/* setup ony channel 0 */
>>  		tcb_setup_single_chan(tc, best_divisor_idx);
>> +
>> +		/* register sched_clock on chips with single 32 bit counter */
>> +		sched_clock_register(tc_read_sched_clock, 32, divided_rate);
>>  	} else {
>>  		/* tclib will give us three clocks no matter what the
>>  		 * underlying platform supports.
>>
>
>

Re: [PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[Mason]

On 11/01/2017 14:50, David Engraf wrote:

> Subject: [PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

I was puzzled by the TCB acronym :-)

Trusted Computing Base
Task Control Block
Transfer Control Block
Test Control Board

In fact, it (probably) stands for Timer Counter Block, which makes
sense for a clksrc patch.

Regards.

Re: [PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[Romain Izard]

On 2017-01-11, David Engraf <david.engraf@sysgo.com> wrote:
> On newer boards the TC can be read as single 32 bit value without locking.
> Thus the clock can be used as reference for sched_clock which is much more
> accurate than the jiffies implementation.
>
> Tested on a Atmel SAMA5D2 board.
>
> Signed-off-by: David Engraf <david.engraf@sysgo.com>

Unfortunately, this leads to the current boot warning:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 1 at ../kernel/time/sched_clock.c:179 sched_clock_register+0x4c/0x21c
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 4.9.4-00041-ge780a8100f0d #1
Hardware name: Atmel SAMA5
[<c01103e4>] (unwind_backtrace) from [<c010d0e4>] (show_stack+0x20/0x24)
[<c010d0e4>] (show_stack) from [<c039c130>] (dump_stack+0x24/0x28)
[<c039c130>] (dump_stack) from [<c011f7b0>] (__warn+0xf4/0x10c)
[<c011f7b0>] (__warn) from [<c011f898>] (warn_slowpath_null+0x30/0x38)
[<c011f898>] (warn_slowpath_null) from [<c0c0d2c0>] (sched_clock_register+0x4c/0x21c)
[<c0c0d2c0>] (sched_clock_register) from [<c0c345fc>] (tcb_clksrc_init+0x1c8/0x424)
[<c0c345fc>] (tcb_clksrc_init) from [<c0101cdc>] (do_one_initcall+0x50/0x184)
[<c0101cdc>] (do_one_initcall) from [<c0c00e70>] (kernel_init_freeable+0x13c/0x1e0)
[<c0c00e70>] (kernel_init_freeable) from [<c08303ec>] (kernel_init+0x18/0x124)
[<c08303ec>] (kernel_init) from [<c0108c34>] (ret_from_fork+0x14/0x20)
---[ end trace 3d13186881cd5c91 ]---

"sched_clock_register" expects to be called with interrupts disabled, but
the tcb_clksrc initialization is called as an arch_initcall, which runs too
late in the boot sequence.

Best regards,
-- 
Romain Izard

Re: [PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[Daniel Lezcano]

On Wed, Jan 11, 2017 at 02:50:59PM +0100, David Engraf wrote:
> On newer boards the TC can be read as single 32 bit value without locking.
> Thus the clock can be used as reference for sched_clock which is much more
> accurate than the jiffies implementation.
> 
> Tested on a Atmel SAMA5D2 board.
> 
> Signed-off-by: David Engraf <david.engraf@sysgo.com>
> ---

Nicolas,

if you agree I will take this change.

  -- Daniel

Re: [PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[Nicolas Ferre]

Le 11/01/2017 à 14:50, David Engraf a écrit :
> On newer boards the TC can be read as single 32 bit value without locking.
> Thus the clock can be used as reference for sched_clock which is much more
> accurate than the jiffies implementation.
> 
> Tested on a Atmel SAMA5D2 board.
> 
> Signed-off-by: David Engraf <david.engraf@sysgo.com>

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>

Thanks,

> ---
>  drivers/clocksource/tcb_clksrc.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/clocksource/tcb_clksrc.c b/drivers/clocksource/tcb_clksrc.c
> index d4ca996..745844e 100644
> --- a/drivers/clocksource/tcb_clksrc.c
> +++ b/drivers/clocksource/tcb_clksrc.c
> @@ -10,6 +10,7 @@
>  #include <linux/io.h>
>  #include <linux/platform_device.h>
>  #include <linux/atmel_tc.h>
> +#include <linux/sched_clock.h>
>  
>  
>  /*
> @@ -56,11 +57,16 @@ static u64 tc_get_cycles(struct clocksource *cs)
>  	return (upper << 16) | lower;
>  }
>  
> -static u64 tc_get_cycles32(struct clocksource *cs)
> +static u32 tc_get_cv32(void)
>  {
>  	return __raw_readl(tcaddr + ATMEL_TC_REG(0, CV));
>  }
>  
> +static u64 tc_get_cycles32(struct clocksource *cs)
> +{
> +	return tc_get_cv32();
> +}
> +
>  static struct clocksource clksrc = {
>  	.name           = "tcb_clksrc",
>  	.rating         = 200,
> @@ -69,6 +75,11 @@ static struct clocksource clksrc = {
>  	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
>  };
>  
> +static u64 notrace tc_read_sched_clock(void)
> +{
> +	return tc_get_cv32();
> +}
> +
>  #ifdef CONFIG_GENERIC_CLOCKEVENTS
>  
>  struct tc_clkevt_device {
> @@ -339,6 +350,9 @@ static int __init tcb_clksrc_init(void)
>  		clksrc.read = tc_get_cycles32;
>  		/* setup ony channel 0 */
>  		tcb_setup_single_chan(tc, best_divisor_idx);
> +
> +		/* register sched_clock on chips with single 32 bit counter */
> +		sched_clock_register(tc_read_sched_clock, 32, divided_rate);
>  	} else {
>  		/* tclib will give us three clocks no matter what the
>  		 * underlying platform supports.
> 


-- 
Nicolas Ferre

Re: [PATCH] tcb_clksrc: Use 32 bit tcb as sched_clock

[Nicolas Ferre]

Le 23/01/2017 à 14:59, Daniel Lezcano a écrit :
> On Wed, Jan 11, 2017 at 02:50:59PM +0100, David Engraf wrote:
>> On newer boards the TC can be read as single 32 bit value without locking.
>> Thus the clock can be used as reference for sched_clock which is much more
>> accurate than the jiffies implementation.
>>
>> Tested on a Atmel SAMA5D2 board.
>>
>> Signed-off-by: David Engraf <david.engraf@sysgo.com>
>> ---
> 
> Nicolas,
> 
> if you agree I will take this change.

Absolutely. I've just added the Ack.

Thanks for the head-up. Bye,
-- 
Nicolas Ferre