lkml.org 
[lkml]   [2002]   [Aug]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH] (1/7) entropy, take 2 - log2
Take 2 of my entropy patches. Includes minor cleanups and should
address concerns about headless systems.

Patch 1:

This simplifies and properly names the log function.

Note that this is effectively a branchless generic_{fls,ffs,flz,ffz}.
I'll experiment with using this approach there later.

diff -ur a/drivers/char/random.c b/drivers/char/random.c
--- a/drivers/char/random.c 2002-07-20 14:11:07.000000000 -0500
+++ b/drivers/char/random.c 2002-08-23 12:43:20.000000000 -0500
@@ -253,6 +253,8 @@
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/tqueue.h>
+#include <linux/kernel_stat.h>
+#include <linux/bitops.h>

#include <asm/processor.h>
#include <asm/uaccess.h>
@@ -430,41 +432,19 @@
#endif

/*
- * More asm magic....
- *
* For entropy estimation, we need to do an integral base 2
* logarithm.
- *
- * Note the "12bits" suffix - this is used for numbers between
- * 0 and 4095 only. This allows a few shortcuts.
*/
-#if 0 /* Slow but clear version */
-static inline __u32 int_ln_12bits(__u32 word)
-{
- __u32 nbits = 0;
-
- while (word >>= 1)
- nbits++;
- return nbits;
-}
-#else /* Faster (more clever) version, courtesy Colin Plumb */
-static inline __u32 int_ln_12bits(__u32 word)
+static inline __u32 int_log2_16bits(__u32 word)
{
/* Smear msbit right to make an n-bit mask */
word |= word >> 8;
word |= word >> 4;
word |= word >> 2;
word |= word >> 1;
- /* Remove one bit to make this a logarithm */
- word >>= 1;
- /* Count the bits set in the word */
- word -= (word >> 1) & 0x555;
- word = (word & 0x333) + ((word >> 2) & 0x333);
- word += (word >> 4);
- word += (word >> 8);
- return word & 15;
+
+ return hweight16(word)-1;
}
-#endif

#if 0
#define DEBUG_ENT(fmt, arg...) printk(KERN_DEBUG "random: " fmt, ## arg)
@@ -782,7 +762,7 @@
delta >>= 1;
delta &= (1 << 12) - 1;

- entropy = int_ln_12bits(delta);
+ entropy = int_log2_16bits(delta);
}
batch_entropy_store(num, time, entropy);
}

--
"Love the dolphins," she advised him. "Write by W.A.S.T.E.."
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
\
 
 \ /
  Last update: 2005-03-22 13:28    [W:0.061 / U:12.712 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site