Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit dacb16b1 authored by Mathieu Desnoyers's avatar Mathieu Desnoyers Committed by Linus Torvalds
Browse files

[PATCH] i386 and x86_64 TSC set_cyc2ns_scale imprecision 



I just found out that some precision is unnecessarily lost in the
arch/i386/kernel/timers/timer_tsc.c:set_cyc2ns_scale function.  It uses a
cpu_mhz parameter when it could use a cpu_khz.  In the specific case of an
Intel P4 running at 3001.171 Mhz, the truncation to 3001 Mhz leads to an
imprecision of 19 microseconds per second : this is very sad for a timer with
nearly nanosecond accuracy.

Fix the x86_64 architecture too.

Cc: george anzinger <george@mvista.com>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Andi Kleen <ak@muc.de>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent bfd51626

arch/i386/kernel/timers/timer_hpet.c

+11 −6
Original line number Original line Diff line number Diff line
@@ -30,23 +30,28 @@ static seqlock_t monotonic_lock = SEQLOCK_UNLOCKED; 
 *  basic equation:

 *  basic equation:

 *		ns = cycles / (freq / ns_per_sec)

 *		ns = cycles / (freq / ns_per_sec)

 *		ns = cycles * (ns_per_sec / freq)

 *		ns = cycles * (ns_per_sec / freq)

 *		ns = cycles * (10^9 / (cpu_mhz * 10^6))

 *		ns = cycles * (10^9 / (cpu_khz * 10^3))

 *		ns = cycles * (10^3 / cpu_mhz)

 *		ns = cycles * (10^6 / cpu_khz)

 *

 *

 *	Then we use scaling math (suggested by george@mvista.com) to get:

 *	Then we use scaling math (suggested by george@mvista.com) to get:

 *		ns = cycles * (10^3 * SC / cpu_mhz) / SC

 *		ns = cycles * (10^6 * SC / cpu_khz) / SC

 *		ns = cycles * cyc2ns_scale / SC

 *		ns = cycles * cyc2ns_scale / SC

 *

 *

 *	And since SC is a constant power of two, we can convert the div

 *	And since SC is a constant power of two, we can convert the div

 *  into a shift.

 *  into a shift.

 *

 *  We can use khz divisor instead of mhz to keep a better percision, since

 *  cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.

 *  (mathieu.desnoyers@polymtl.ca)

 *

 *			-johnstul@us.ibm.com "math is hard, lets go shopping!"

 *			-johnstul@us.ibm.com "math is hard, lets go shopping!"

 */

 */

static unsigned long cyc2ns_scale;

static unsigned long cyc2ns_scale;

#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */

#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */





static inline void set_cyc2ns_scale(unsigned long cpu_mhz)

static inline void set_cyc2ns_scale(unsigned long cpu_khz)

{

{

	cyc2ns_scale = (1000 << CYC2NS_SCALE_FACTOR)/cpu_mhz;

	cyc2ns_scale = (1000000 << CYC2NS_SCALE_FACTOR)/cpu_khz;

}

}





static inline unsigned long long cycles_2_ns(unsigned long long cyc)

static inline unsigned long long cycles_2_ns(unsigned long long cyc)
@@ -163,7 +168,7 @@ static int __init init_hpet(char* override) 
				printk("Detected %u.%03u MHz processor.\n",

				printk("Detected %u.%03u MHz processor.\n",

					cpu_khz / 1000, cpu_khz % 1000);

					cpu_khz / 1000, cpu_khz % 1000);

			}

			}

			set_cyc2ns_scale(cpu_khz/1000);

			set_cyc2ns_scale(cpu_khz);

		}

		}

		/* set this only when cpu_has_tsc */

		/* set this only when cpu_has_tsc */

		timer_hpet.read_timer = read_timer_tsc;

		timer_hpet.read_timer = read_timer_tsc;

arch/i386/kernel/timers/timer_tsc.c

+13 −8
Original line number Original line Diff line number Diff line
@@ -49,23 +49,28 @@ static seqlock_t monotonic_lock = SEQLOCK_UNLOCKED; 
 *  basic equation:

 *  basic equation:

 *		ns = cycles / (freq / ns_per_sec)

 *		ns = cycles / (freq / ns_per_sec)

 *		ns = cycles * (ns_per_sec / freq)

 *		ns = cycles * (ns_per_sec / freq)

 *		ns = cycles * (10^9 / (cpu_mhz * 10^6))

 *		ns = cycles * (10^9 / (cpu_khz * 10^3))

 *		ns = cycles * (10^3 / cpu_mhz)

 *		ns = cycles * (10^6 / cpu_khz)

 *

 *

 *	Then we use scaling math (suggested by george@mvista.com) to get:

 *	Then we use scaling math (suggested by george@mvista.com) to get:

 *		ns = cycles * (10^3 * SC / cpu_mhz) / SC

 *		ns = cycles * (10^6 * SC / cpu_khz) / SC

 *		ns = cycles * cyc2ns_scale / SC

 *		ns = cycles * cyc2ns_scale / SC

 *

 *

 *	And since SC is a constant power of two, we can convert the div

 *	And since SC is a constant power of two, we can convert the div

 *  into a shift.

 *  into a shift.

 *

 *  We can use khz divisor instead of mhz to keep a better percision, since

 *  cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.

 *  (mathieu.desnoyers@polymtl.ca)

 *

 *			-johnstul@us.ibm.com "math is hard, lets go shopping!"

 *			-johnstul@us.ibm.com "math is hard, lets go shopping!"

 */

 */

static unsigned long cyc2ns_scale; 

static unsigned long cyc2ns_scale; 

#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */

#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */





static inline void set_cyc2ns_scale(unsigned long cpu_mhz)

static inline void set_cyc2ns_scale(unsigned long cpu_khz)

{

{

	cyc2ns_scale = (1000 << CYC2NS_SCALE_FACTOR)/cpu_mhz;

	cyc2ns_scale = (1000000 << CYC2NS_SCALE_FACTOR)/cpu_khz;

}

}





static inline unsigned long long cycles_2_ns(unsigned long long cyc)

static inline unsigned long long cycles_2_ns(unsigned long long cyc)
@@ -286,7 +291,7 @@ time_cpufreq_notifier(struct notifier_block *nb, unsigned long val, 
		if (use_tsc) {

		if (use_tsc) {

			if (!(freq->flags & CPUFREQ_CONST_LOOPS)) {

			if (!(freq->flags & CPUFREQ_CONST_LOOPS)) {

				fast_gettimeoffset_quotient = cpufreq_scale(fast_gettimeoffset_ref, freq->new, ref_freq);

				fast_gettimeoffset_quotient = cpufreq_scale(fast_gettimeoffset_ref, freq->new, ref_freq);

				set_cyc2ns_scale(cpu_khz/1000);

				set_cyc2ns_scale(cpu_khz);

			}

			}

		}

		}

#endif

#endif
@@ -536,7 +541,7 @@ static int __init init_tsc(char* override) 
				printk("Detected %u.%03u MHz processor.\n",

				printk("Detected %u.%03u MHz processor.\n",

					cpu_khz / 1000, cpu_khz % 1000);

					cpu_khz / 1000, cpu_khz % 1000);

			}

			}

			set_cyc2ns_scale(cpu_khz/1000);

			set_cyc2ns_scale(cpu_khz);

			return 0;

			return 0;

		}

		}

	}

	}

arch/x86_64/kernel/time.c

+4 −4
Original line number Original line Diff line number Diff line
@@ -481,9 +481,9 @@ static irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs) 
static unsigned int cyc2ns_scale;

static unsigned int cyc2ns_scale;

#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */

#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */





static inline void set_cyc2ns_scale(unsigned long cpu_mhz)

static inline void set_cyc2ns_scale(unsigned long cpu_khz)

{

{

	cyc2ns_scale = (1000 << CYC2NS_SCALE_FACTOR)/cpu_mhz;

	cyc2ns_scale = (1000000 << CYC2NS_SCALE_FACTOR)/cpu_khz;

}

}





static inline unsigned long long cycles_2_ns(unsigned long long cyc)

static inline unsigned long long cycles_2_ns(unsigned long long cyc)
@@ -655,7 +655,7 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val, 
			vxtime.tsc_quot = (1000L << 32) / cpu_khz;

			vxtime.tsc_quot = (1000L << 32) / cpu_khz;

	}

	}

	

	

	set_cyc2ns_scale(cpu_khz_ref / 1000);

	set_cyc2ns_scale(cpu_khz_ref);





	return 0;

	return 0;

}

}
@@ -939,7 +939,7 @@ void __init time_init(void) 
	rdtscll_sync(&vxtime.last_tsc);

	rdtscll_sync(&vxtime.last_tsc);

	setup_irq(0, &irq0);

	setup_irq(0, &irq0);





	set_cyc2ns_scale(cpu_khz / 1000);

	set_cyc2ns_scale(cpu_khz);





#ifndef CONFIG_SMP

#ifndef CONFIG_SMP

	time_init_gtod();

	time_init_gtod();