Re: [Minios-devel] [UNIKRAFT PATCHv3 1/7] plat/common: Calculate shift factors for coversion between ns and tick

["Justin He (Arm Technology China)" <Justin.He@xxxxxxx>]

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 ?

>     +
>      /* 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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