oprofile and ARM A9 hardware counter

[Shilimkar, Santosh]

On Tue, Feb 7, 2012 at 4:23 PM, Shilimkar, Santosh
<santosh.shilimkar at ti.com> wrote:
> ( Removing dead "linux-arm-kernel at lists.arm.linux.org.uk" and adding
> correct list
>
> On Tue, Feb 7, 2012 at 4:07 PM, stephane eranian <eranian at googlemail.com> wrote:
>> Hi,
>>
>> Ok, with Santosh's patch this is much better almost as expected, still
>> 10-15% off.
>>
[....]

>> So the fix does help. I am wondering why we're not getting closer to
>> 10k samples. But that
>> may be due to some overhead somewhere in there.
>>

There might be still a small corner case where few reads
might return the stale value. Counter need at least 1 32K
clock cycle for the sync. udelay is not accurate but
it will at least give min. 1/32768, so it should be
fine.

May be you can try out below patch and see if it helps.

Regards
Santosh

diff --git a/arch/arm/plat-omap/counter_32k.c b/arch/arm/plat-omap/counter_32k.c
index 5f0f229..014d8bd 100644
--- a/arch/arm/plat-omap/counter_32k.c
+++ b/arch/arm/plat-omap/counter_32k.c
@@ -18,6 +18,7 @@
 #include <linux/err.h>
 #include <linux/io.h>
 #include <linux/clocksource.h>
+#include <linux/delay.h>

 #include <asm/sched_clock.h>

@@ -38,6 +39,8 @@ static void __iomem *timer_32k_base;

 static u32 notrace omap_32k_read_sched_clock(void)
 {
+       /* Counter might take 1 clock cycle for OCP sync */
+       udelay(31);
        return timer_32k_base ? __raw_readl(timer_32k_base) : 0;
 }