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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Minios-devel] [UNIKRAFT PATCHv3 1/7] plat/common: Calculate shift factors for coversion between ns and tick
Hi Justin,
Thanks for the reply. Please find a last comment on regards to one of the
points inline.
Thanks and best,
Santiago
On 16.09.19, 07:48, "Justin He (Arm Technology China)" <Justin.He@xxxxxxx>
wrote:
Hi Santiago, thanks for the review
Please see inline
> -----Original Message-----
> From: Santiago Pagani <Santiago.Pagani@xxxxxxxxx>
> Sent: 2019年9月10日 15:02
> To: Justin He (Arm Technology China) <Justin.He@xxxxxxx>; minios-
> devel@xxxxxxxxxxxxxxxxxxxx; Simon Kuenzer <simon.kuenzer@xxxxxxxxx>;
> Sharan Santhanam <Sharan.Santhanam@xxxxxxxxx>
> Cc: Julien Grall <Julien.Grall@xxxxxxx>; Kaly Xin (Arm Technology China)
> <Kaly.Xin@xxxxxxx>; Wei Chen (Arm Technology China)
> <Wei.Chen@xxxxxxx>; Jianyong Wu (Arm Technology China)
> <Jianyong.Wu@xxxxxxx>
> Subject: Re: [UNIKRAFT PATCHv3 1/7] plat/common: Calculate shift factors
> for coversion between ns and tick
>
> Hi all,
>
> Thanks for the patch. Please find my comments inline:
>
> On 30.07.19, 16:27, "Jia He" <justin.he@xxxxxxx> wrote:
>
> We had shift factor for coverting counter ticks to ns, but it's not
> enough. Sometime, we need to covert ns to ticks. For example, we'll
> transfer sleep(ns) to counter ticks. If we don't have the shift
factor,
> the conversion accuracy will be lower.
>
> Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
> Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx>
> Signed-off-by: Jia He <justin.he@xxxxxxx>
> ---
> plat/common/arm/time.c | 111
> +++++++++++++++++++++++++++++++----------
> 1 file changed, 86 insertions(+), 25 deletions(-)
>
> diff --git a/plat/common/arm/time.c b/plat/common/arm/time.c
> index 1b30903..4c66039 100644
> --- a/plat/common/arm/time.c
> +++ b/plat/common/arm/time.c
> @@ -47,21 +47,83 @@
> static uint64_t boot_ticks;
> static uint32_t counter_freq;
>
> -/*
> - * Shift factor for counter scaling multiplier; referred to as S in
the
> - * following comments.
> - */
> -static uint8_t counter_shift;
>
> -/* Multiplier for converting counter ticks to nsecs. (0.S) fixed
point. */
> +/* Shift factor for converting ticks to ns */
> +static uint8_t counter_shift_to_ns;
> +
> +/* Shift factor for converting ns to ticks */
> +static uint8_t counter_shift_to_tick;
> +
> +/* Multiplier for converting counter ticks to nsecs */
> static uint32_t ns_per_tick;
>
> +/* Multiplier for converting nsecs to counter ticks */
> +static uint32_t tick_per_ns;
> +
> +/*
> + * The maximum time range in seconds which can be converted by
> multiplier
> + * and shift factors. This will guarantee the converted value not to
> exceed
> + * 64-bit unsigned integer. Increase the time range will reduce the
> accuracy
> + * of conversion, because we will get smaller multiplier and shift
factors.
> + * In this case, we selected 3600s as the time range.
> + */
> +#define __MAX_CONVERT_SECS 3600UL
> COMMNET: As I understand it, this #define is also used when calculating
> the ns_per_tick, it not only imposes a limit to the time range, but also
to
> the tick range. That is, we do not support converting ns_to_ticks for more
> than 3600 seconds, and as well we do not support to convert ticks_to_ns
> for more than 3600000000 ticks, correct? Maybe change the name of the
> #define to something more generic.
Hmm, __MAX_CONVERT_SECS is for converting ticks_to_ns only, not the
converting of ns_to_ticks
So, how about __MAX_SECS_CONVERT_TO_NS ?
COMMENT: Well, the only place where __MAX_CONVERT_SECS is used is inside
function 'calculate_mult_shift' to compute 'sftacc'. Function
'calculate_mult_shift' is then used both to compute 'tick_per_ns and
counter_shift_to_tick', as well as 'ns_per_tick and counter_shift_to_ns'.
Inside function 'calculate_mult_shift', the value of __MAX_CONVERT_SECS is
multiplied by 'from'. In one case when calling the function, 'from' is set to
NSEC_PER_SEC, however in the case, 'from' is set to ' counter_freq'. Therefore,
as we are multiplying __MAX_CONVERT_SECS with two variables with opposed
physical meanings, __MAX_CONVERT_SECS is in fact also been utilized in the
opposite way.
> +
> /* How many nanoseconds per second */
> #define NSEC_PER_SEC ukarch_time_sec_to_nsec(1)
>
> static inline uint64_t ticks_to_ns(uint64_t ticks)
> {
> - return (ns_per_tick * ticks) >> counter_shift;
> + return (ns_per_tick * ticks) >> counter_shift_to_ns;
> COMMENT: Maybe add an assertion for the maximum number of ticks as a
> sanity check?
Yes, reasonable to me
> +}
> +
> +static inline uint64_t ns_to_ticks(uint64_t ns)
> +{
> + return (tick_per_ns * ns) >> counter_shift_to_tick;
> COMMENT: Maybe add an assertion for the maximum number of seconds
> as a sanity check?
Yes
> +}
> +
> +/*
> + * Calculate multiplier/shift factors for scaled math.
> + */
> +static void calculate_mult_shift(uint32_t *mult, uint8_t *shift,
> + uint64_t from, uint64_t to)
> +{
> + uint64_t tmp;
> + uint32_t sft, sftacc = 32;
> +
> + /*
> + * Calculate the shift factor which is limiting the conversion
> + * range:
> + */
> + tmp = ((uint64_t)__MAX_CONVERT_SECS * from) >> 32;
> + while (tmp) {
> + tmp >>= 1;
> + sftacc--;
> + }
> +
> +
> + /*
> + * Calculate shift factor (S) and scaling multiplier (M).
> + *
> + * (S) needs to be the largest shift factor (<= max_shift) where
> + * the result of the M calculation below fits into uint32_t
> + * without truncation.
> + *
> + * multiplier = (target << shift) / source
> + */
> + for (sft = 32; sft > 0; sft--) {
> + tmp = (uint64_t) to << sft;
> +
> + /* Ensuring we round to nearest when calculating the
> + * multiplier
> + */
> + tmp += from / 2;
> + tmp /= from;
> + if ((tmp >> sftacc) == 0)
> + break;
> + }
> + *mult = tmp;
> + *shift = sft;
> }
>
> /*
> @@ -145,29 +207,28 @@ static __u64 generic_timer_epochoffset(void)
>
> static int generic_timer_init(void)
> {
> + counter_freq = get_counter_frequency();
> +
> /*
> - * Calculate counter shift factor and scaling multiplier.
> - *
> - * counter_shift (S) needs to be the largest (<=32) shift factor
where
> - * the result of the ns_per_tick calculation below fits into
uint32_t
> - * without truncation. Note that we disallow an S of zero to ensure
> - * the loop always terminates.
> - *
> - * (0.S) ns_per_tick = NSEC_PER_SEC (S.S) / counter_freq (S.0)
> + * Calculate the shift factor and scaling multiplier for
> + * cpnverting ticks to ns.
> */
> TYPO: Typo in comment: cpnverting -> converting
OK
> - uint64_t tmp;
> + calculate_mult_shift(&ns_per_tick, &counter_shift_to_ns,
> + counter_freq, NSEC_PER_SEC);
>
> - counter_freq = get_counter_frequency();
> - counter_shift = 32;
> - do {
> - tmp = (NSEC_PER_SEC << counter_shift) / counter_freq;
> - if ((tmp & 0xFFFFFFFF00000000L) == 0L)
> - ns_per_tick = (uint32_t)tmp;
> - else
> - counter_shift--;
> - } while (counter_shift > 0 && ns_per_tick == 0L);
> + /* We disallow zero ns_per_tick */
> UK_BUGON(!ns_per_tick);
>
> + /*
> + * Calculate the shift factor and scaling multiplier for
> + * cpnverting ns to ticks.
> + */
> TYPO: Typo in comment: cpnverting -> converting
OK
> + calculate_mult_shift(&tick_per_ns, &counter_shift_to_tick,
> + NSEC_PER_SEC, counter_freq);
> +
> + /* We disallow zero ns_per_tick */
> + UK_BUGON(!tick_per_ns);
> +
> /*
> * Monotonic time begins at boot_ticks (first read of counter
> * before calibration).
> --
> 2.17.1
>
>
>
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