[PATCH] preset loops_per_jiffy for faster booting

Adds a kernel boot parameter "lpj=NNN" which allows the operator to specify
the loops-per-jiffy value.  This shaves up to a quarter of a second off
boot times, which are critical for embedded appliances.

It's a bit thin, but the code is in __init.
Signed-off-by: Adam Kropelin <akropel1@rochester.rr.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/kernel-parameters.txt

@@ -579,6 +579,20 @@ running once the system is up.
 				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 (up to 250 ms per
+			CPU). 0 enables autodetection (default). To determine
+			the correct value for your kernel, boot with normal
+			autodetection and see what value is printed. Note that
+			on SMP systems the preset will be applied to all CPUs,
+			which is likely to cause problems if your CPUs need
+			significantly divergent settings. 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.
+
 	ltpc=		[NET]
 			Format: <io>,<irq>,<dma>
 

init/main.c

@@ -184,15 +184,28 @@ static int __init obsolete_checksetup(char *line)
 	return 0;
 }
 
-/* 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 */
+static unsigned long 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);
 
 EXPORT_SYMBOL(loops_per_jiffy);
 
-/* This is the number of bits of precision for the loops_per_jiffy.  Each
-   bit takes on average 1.5/HZ seconds.  This (like the original) is a little
-   better than 1% */
+/*
+ * This is the number of bits of precision for the loops_per_jiffy.  Each
+ * bit takes on average 1.5/HZ seconds.  This (like the original) is a little
+ * better than 1%
+ */
 #define LPS_PREC 8
 
 void __devinit calibrate_delay(void)
@@ -200,40 +213,53 @@ void __devinit calibrate_delay(void)
 	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;
-	}
+	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;
+		}
 
-/* 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;
+		/* Round the value and print it */
+		printk("%lu.%02lu BogoMIPS (lpj=%lu)\n",
+			loops_per_jiffy/(500000/HZ),
+			(loops_per_jiffy/(5000/HZ)) % 100,
+			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)
@@ -255,8 +281,10 @@ static int __init quiet_kernel(char *str)
 __setup("debug", debug_kernel);
 __setup("quiet", quiet_kernel);
 
-/* Unknown boot options get handed to init, unless they look like
-   failed parameters */
+/*
+ * Unknown boot options get handed to init, unless they look like
+ * failed parameters
+ */
 static int __init unknown_bootoption(char *param, char *val)
 {
 	/* Change NUL term back to "=", to make "param" the whole string. */
@@ -267,8 +295,10 @@ static int __init unknown_bootoption(char *param, char *val)
 	if (obsolete_checksetup(param))
 		return 0;
 
-	/* Preemptive maintenance for "why didn't my mispelled command
-           line work?" */
+	/*
+	 * Preemptive maintenance for "why didn't my mispelled command
+	 * line work?"
+	 */
 	if (strchr(param, '.') && (!val || strchr(param, '.') < val)) {
 		printk(KERN_ERR "Unknown boot option `%s': ignoring\n", param);
 		return 0;
@@ -308,7 +338,8 @@ static int __init init_setup(char *str)
 	unsigned int i;
 
 	execute_command = str;
-	/* In case LILO is going to boot us with default command line,
+	/*
+	 * In case LILO is going to boot us with default command line,
 	 * it prepends "auto" before the whole cmdline which makes
 	 * the shell think it should execute a script with such name.
 	 * So we ignore all arguments entered _before_ init=... [MJ]