On 16 Jun 2014, at 19:36, Mark R V Murray <ma...@freebsd.org> wrote:
>
> On 16 Jun 2014, at 08:28, Michael Tuexen <tue...@freebsd.org> wrote:
>> your patch for accessing the value is correct. However, the initialisation
>> code also
>> needs to be adopted to the platform. So in addition to your patch, you also
>> need:
>
> Thanks!
>
>> Is there an easy test to see if the code actually works as expected and not
>> that it just
>> allows the system to boot?
>
> Yes. :-)
>
> #include <sys/types.h>
>
> #include <stdio.h>
>
> static __inline uint64_t
> get_cyclecount(void)
> {
> uint32_t ccnt;
> uint64_t tsc;
>
> /* Read CCNT. */
> __asm __volatile("mrc p15, 0, %0, c15, c12, 1": "=r" (ccnt));
>
> tsc = (uint64_t)ccnt;
>
> return (tsc);
> }
>
> #define N 100000
>
> int
> main(int argc, char *argv[])
> {
> int i;
> uint64_t ccnt[N];
>
> for (i = 0; i < N; i++)
> ccnt[i] = get_cyclecount();
>
> for (i = 1; i < N; i++)
> printf("%6d %016llX %16llu\n", i, ccnt[i], ccnt[i] - ccnt[i -
> 1]);
>
>
> return (0);
> }
>
> Should print a whole lotta numbers, incrementing, unless you hit the
> wraparound.
Don't I need to compile a kernel with _PMC_USER_READ_WRITE_ being defined, since
without it a user process can't access the register. When running it on a kernel
not defining _PMC_USER_READ_WRITE_, I get a core with Illegal instruction.
Let me build a kernel with the above define and retest.
>
>> Regarding the 32-bit limitation: Do we want to increment the register only
>> every
>> 64 clock cycle?
>
> Definitely not! The value is in the low bits; wrap is of little consequence.
OK.
Best regards
Michael
>
> M
> --
> Mark R V Murray
>
>
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