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


  • To: Santiago Pagani <Santiago.Pagani@xxxxxxxxx>, "minios-devel@xxxxxxxxxxxxxxxxxxxx" <minios-devel@xxxxxxxxxxxxxxxxxxxx>, Simon Kuenzer <simon.kuenzer@xxxxxxxxx>, Sharan Santhanam <Sharan.Santhanam@xxxxxxxxx>
  • From: "Justin He (Arm Technology China)" <Justin.He@xxxxxxx>
  • Date: Mon, 16 Sep 2019 13:24:52 +0000
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  • Cc: "Kaly Xin \(Arm Technology China\)" <Kaly.Xin@xxxxxxx>, Julien Grall <Julien.Grall@xxxxxxx>, "Jianyong Wu \(Arm Technology China\)" <Jianyong.Wu@xxxxxxx>, "Wei Chen \(Arm Technology China\)" <Wei.Chen@xxxxxxx>
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  • Thread-topic: [UNIKRAFT PATCHv3 1/7] plat/common: Calculate shift factors for coversion between ns and tick


> -----Original Message-----
> From: Santiago Pagani <Santiago.Pagani@xxxxxxxxx>
> Sent: 2019年9月16日 14:48
> 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 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.
Sorry for previous mistakes.
__MAX_SECS_CONVERT_TO_NS is not correct

In your mention 1st case, e.g.  assume the freq is 25Mhz
calculate_mult_shift(&mult,&shift,1000000000L,25*1000000);
__MAX_CONVERT_SECS is to say the range ns is [0,3600*1000,000,000],
In your second case,
calculate_mult_shift(&mult,&shift ,25*1000000, 1000000000);
__MAX_CONVERT_SECS is to say the range ticks is [0, 3600*25*1000,000]

So, IMO, we *needn't change the name __MAX_CONVERT_SECS here.
What is your opinion?

--
Cheers,
Justin (Jia He)


>
>     >     +
>     >      /* 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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