Hi all,
Someone pointed out to me a change in the code with the reset rework,
due to my suggestion to use get_bitmask_order() which actually returns
highest bit set+1. Nick, can you comment whether the patch below does
the right thing and that the original code was correct?
[Forrest, is this idea via you, and may I attribute you as the patch
author like so?
From: Forrest Zhang <your preferred email>]
Here's a C program I used to confirm the problem (f1 is original code,
f2 is get_bitmask_order with generic fls(). Output is:
1: 0 1
0: 0 0
32767: 14 15
65535: 15 16
65536: 16 17
1048576: 20 21
--- start ---
#include <inttypes.h>
#include <stdio.h>
static int fls(int x)
{
int r = 32;
if (!x)
return 0;
if (!(x & 0xffff0000u)) {
x <<= 16;
r -= 16;
}
if (!(x & 0xff000000u)) {
x <<= 8;
r -= 8;
}
if (!(x & 0xf0000000u)) {
x <<= 4;
r -= 4;
}
if (!(x & 0xc0000000u)) {
x <<= 2;
r -= 2;
}
if (!(x & 0x80000000u)) {
x <<= 1;
r -= 1;
}
return r;
}
int f1(int c)
{
int bit;
for (bit = 31; bit > 0; bit--)
if ((c >> bit) & 1)
break;
return bit;
}
int f2(int c)
{
return fls(c);
}
int main()
{
int i;
int nums[] = { 1, 0, 32767, 65535, 65536, 1048576 };
for (i=0; i < sizeof(nums)/sizeof(int); i++)
printf("%d: %d %d\n", nums[i], f1(nums[i]), f2(nums[i]));
return 0;
}
--- end ---
>From d668403b04487a90378fc4ec7cd1ebe1e4b7bfe1 Mon Sep 17 00:00:00 2001
From: Bob Copeland <m...@bobcopeland.com>
Date: Tue, 12 May 2009 09:40:37 -0400
Subject: [PATCH] ath5k: fix off-by-one when computing OFDM delta slope
get_bitmask_order() actually returns the highest bit set plus one,
whereas the previous code wanted the highest bit set. Also change
type of coef_exp to int to deal with degenerate case of
coef_exp == 0.
---
drivers/net/wireless/ath/ath5k/reset.c | 8 ++++----
1 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/drivers/net/wireless/ath/ath5k/reset.c
b/drivers/net/wireless/ath/ath5k/reset.c
index 4a2d93e..cc75e77 100644
--- a/drivers/net/wireless/ath/ath5k/reset.c
+++ b/drivers/net/wireless/ath/ath5k/reset.c
@@ -51,8 +51,8 @@ static inline int ath5k_hw_write_ofdm_timings(struct
ath5k_hw *ah,
struct ieee80211_channel *channel)
{
/* Get exponent and mantissa and set it */
- u32 coef_scaled, coef_exp, coef_man,
- ds_coef_exp, ds_coef_man, clock;
+ u32 coef_scaled, coef_man, ds_coef_exp, ds_coef_man, clock;
+ int coef_exp;
if (!(ah->ah_version == AR5K_AR5212) ||
!(channel->hw_value & CHANNEL_OFDM))
@@ -69,10 +69,10 @@ static inline int ath5k_hw_write_ofdm_timings(struct
ath5k_hw *ah,
/* Get exponent
* ALGO: coef_exp = 14 - highest set bit position */
- coef_exp = get_bitmask_order(coef_scaled);
+ coef_exp = get_bitmask_order(coef_scaled) - 1;
/* Doesn't make sense if it's zero*/
- if (!coef_exp)
+ if (coef_exp <= 0)
return -EINVAL;
/* Note: we've shifted coef_scaled by 24 */
--
1.6.0.6
--
Bob Copeland %% www.bobcopeland.com
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