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

From: Thomas Huth <thuth@redhat.com>
To: Pierre Morel <pmorel@linux.ibm.com>, kvm@vger.kernel.org
Cc: linux-s390@vger.kernel.org, frankja@linux.ibm.com,
	david@redhat.com, cohuck@redhat.com
Subject: Re: [kvm-unit-tests PATCH v8 06/12] s390x: clock and delays caluculations
Date: Mon, 8 Jun 2020 17:55:53 +0200	[thread overview]
Message-ID: <2f449a7b-2701-abcc-42b7-5b7441c10761@redhat.com> (raw)
In-Reply-To: <1591603981-16879-7-git-send-email-pmorel@linux.ibm.com>

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