Re: [kvm-unit-tests PATCH v8 06/12] s390x: clock and delays caluculations

Thomas Huth
2020-06-08T15:56:10Z

On 08/06/2020 10.12, Pierre Morel wrote:
> The hardware gives us a good definition of the microsecond,
> let's keep this information and let the routine accessing
> the hardware keep all the information and return microseconds.
>
> Calculate delays in microseconds and take care about wrapping
> around zero.
>
> Define values with macros and use inlines to keep the
> milliseconds interface.
>
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> ---
> lib/s390x/asm/time.h | 29 +++++++++++++++++++++++++++--
> 1 file changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/lib/s390x/asm/time.h b/lib/s390x/asm/time.h
> index 1791380..eb15941 100644
> --- a/lib/s390x/asm/time.h
> +++ b/lib/s390x/asm/time.h
> @@ -13,14 +13,39 @@
> #ifndef ASM_S390X_TIME_H
> #define ASM_S390X_TIME_H
>
> -static inline uint64_t get_clock_ms(void)
> +#define STCK_SHIFT (63 - 51)

Could you maybe rather call this STCK_SHIFT_US instead, so that it is
clear that we refer to the microsecond bit here?

> +#define STCK_MAX ((1 << (STCK_SHIFT + 1)) - 1)

Hmm, is that really right? Shouldn't that rather be

((1 << (51 + 1)) - 1)

instead?

(you want to have a max. value for the 52 bits that count the
microseconds, not a max value for the remainder bits, do you?)

> +static inline uint64_t get_clock_us(void)
> {
> uint64_t clk;
>
> asm volatile(" stck %0 " : : "Q"(clk) : "memory");
>
> /* Bit 51 is incrememented each microsecond */
> - return (clk >> (63 - 51)) / 1000;
> + return clk >> STCK_SHIFT;
> +}
> +
> +static inline void udelay(unsigned long us)
> +{
> + unsigned long startclk = get_clock_us();
> + unsigned long c;
> +
> + do {
> + c = get_clock_us();
> + if (c < startclk)
> + c += STCK_MAX;
> + } while (c < (startclk + us));

You could omit the parentheses around "startclk + us" here.

> +}
> +
> +static inline void mdelay(unsigned long ms)
> +{
> + udelay(ms * 1000);
> +}
> +
> +static inline uint64_t get_clock_ms(void)
> +{
> + return get_clock_us() / 1000;
> }
>
> #endif
>

Thomas