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SubjectRe: [PATCH] preset loops_per_jiffy for faster booting
Here's an updated patch which incorporates suggestions from Todd Poynor
and Geert Uytterhoeven.

Signed-off-by: Adam Kropelin <akropel1@rochester.rr.com>

--- linux-2.6.7/init/main.c.orig Mon Jun 21 17:55:09 2004
+++ linux-2.6.7/init/main.c Sat Jul 10 11:02:49 2004
@@ -167,6 +167,15 @@
return 0;
}

+static unsigned long preset_lpj = CONFIG_PRESET_LPJ;
+static int __init lpj_setup(char *str)
+{
+ preset_lpj = simple_strtoul(str,NULL,0);
+ return 1;
+}
+
+__setup("lpj=", lpj_setup);
+
/* this should be approx 2 Bo*oMips to start (note initial shift), and will
still work even if initially too large, it will just take slightly longer */
unsigned long loops_per_jiffy = (1<<12);
@@ -183,40 +192,52 @@
unsigned long ticks, loopbit;
int lps_precision = LPS_PREC;

- loops_per_jiffy = (1<<12);
-
- printk("Calibrating delay loop... ");
- while ((loops_per_jiffy <<= 1) != 0) {
- /* wait for "start of" clock tick */
- ticks = jiffies;
- while (ticks == jiffies)
- /* nothing */;
- /* Go .. */
- ticks = jiffies;
- __delay(loops_per_jiffy);
- ticks = jiffies - ticks;
- if (ticks)
- break;
- }
-
-/* Do a binary approximation to get loops_per_jiffy set to equal one clock
- (up to lps_precision bits) */
- loops_per_jiffy >>= 1;
- loopbit = loops_per_jiffy;
- while ( lps_precision-- && (loopbit >>= 1) ) {
- loops_per_jiffy |= loopbit;
- ticks = jiffies;
- while (ticks == jiffies);
- ticks = jiffies;
- __delay(loops_per_jiffy);
- if (jiffies != ticks) /* longer than 1 tick */
- loops_per_jiffy &= ~loopbit;
+ if (preset_lpj) {
+ loops_per_jiffy = preset_lpj;
+ printk("Calibrating delay loop (skipped)... "
+ "%lu.%02lu BogoMIPS preset\n",
+ loops_per_jiffy/(500000/HZ),
+ (loops_per_jiffy/(5000/HZ)) % 100);
+ } else {
+ loops_per_jiffy = (1<<12);
+
+ printk("Calibrating delay loop... ");
+ while ((loops_per_jiffy <<= 1) != 0) {
+ /* wait for "start of" clock tick */
+ ticks = jiffies;
+ while (ticks == jiffies)
+ /* nothing */;
+ /* Go .. */
+ ticks = jiffies;
+ __delay(loops_per_jiffy);
+ ticks = jiffies - ticks;
+ if (ticks)
+ break;
+ }
+
+ /* Do a binary approximation to get loops_per_jiffy set to
+ equal one clock (up to lps_precision bits) */
+ loops_per_jiffy >>= 1;
+ loopbit = loops_per_jiffy;
+ while ( lps_precision-- && (loopbit >>= 1) ) {
+ loops_per_jiffy |= loopbit;
+ ticks = jiffies;
+ while (ticks == jiffies)
+ /* nothing */;
+ ticks = jiffies;
+ __delay(loops_per_jiffy);
+ if (jiffies != ticks) /* longer than 1 tick */
+ loops_per_jiffy &= ~loopbit;
+ }
+
+ /* Round the value and print it */
+ printk("%lu.%02lu BogoMIPS\n",
+ loops_per_jiffy/(500000/HZ),
+ (loops_per_jiffy/(5000/HZ)) % 100);
+ printk("Set 'Preset loops_per_jiffy'=%lu for preset lpj.\n",
+ loops_per_jiffy);
}

-/* Round the value and print it */
- printk("%lu.%02lu BogoMIPS\n",
- loops_per_jiffy/(500000/HZ),
- (loops_per_jiffy/(5000/HZ)) % 100);
}

static int __init debug_kernel(char *str)
--- linux-2.6.7/init/Kconfig.orig Mon Jun 21 17:55:09 2004
+++ linux-2.6.7/init/Kconfig Sat Jul 10 11:05:27 2004
@@ -218,6 +218,45 @@
This option enables access to kernel configuration file and build
information through /proc/config.gz.

+menuconfig FASTBOOT
+ bool "Fast boot options"
+ help
+ Say Y here to select among various options that can decrease
+ kernel boot time. These options commonly involve providing
+ hardcoded values for some parameters that the kernel usually
+ determines automatically.
+
+ This option is useful primarily on embedded systems.
+
+ If unsure, say N.
+
+config PRESET_LPJ
+ int "Preset loops_per_jiffy" if FASTBOOT
+ default 0
+ help
+ This is the number of loops used by delay() to achieve a single
+ jiffy of delay inside the kernel. It is normally calculated at
+ boot time, but that calculation can take up to 250 ms per CPU.
+ Specifying a constant value here will eliminate that delay.
+
+ A value of 0 results in the normal autodetect behavior.
+
+ loops_per_jiffy is roughly BogoMips * 5000. To determine the correct
+ value for your kernel, first set this option to 0, compile and boot
+ the kernel on your target hardware, then see what value is printed
+ during the kernel boot. Use that value here.
+
+ The kernel command line parameter "lpj=" can be used to override
+ the value configured here.
+
+ Note that on SMP systems the preset will be applied to all CPUs
+ which will cause problems if for some reason your CPUs need
+ significantly divergent settings.
+
+ If unsure, set this to 0. An incorrect value will cause delays in
+ the kernel to be wrong, leading to unpredictable I/O errors and
+ other breakage. Although unlikely, in the extreme case this might
+ damage your hardware.

menuconfig EMBEDDED
bool "Configure standard kernel features (for small systems)"
--- linux-2.6.7/Documentation/kernel-parameters.txt.orig Fri Jul 9 21:20:16 2004
+++ linux-2.6.7/Documentation/kernel-parameters.txt Fri Jul 9 21:17:11 2004
@@ -576,6 +576,12 @@
so, the driver will manage that printer.
See also header of drivers/char/lp.c.

+ lpj=n [KNL]
+ Sets loops_per_jiffy to given constant, thus avoiding
+ time-consuming boot-time autodetection.
+ 0 enables autodetection (default). See Kconfig help text
+ for PRESET_LPJ for details.
+
ltpc= [NET]
Format: <io>,<irq>,<dma>

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