Commit 7f896126 authored by Bruno Randolf's avatar Bruno Randolf Committed by John W. Linville
Browse files

ath5k: Check and fix ATIM window

This patch adds sanity-checks for the beacon timers and especially the ATIM
window to ath5k. It is basically the same what i did for madwifi two years ago
and fixes a problem in IBSS mode which has been described as "ramping" pings.

See the code comments for a more detailed description and these links:

Signed-off-by: default avatarBruno Randolf <>
Signed-off-by: default avatarJohn W. Linville <>
parent 4a79f2c5
......@@ -1195,6 +1195,7 @@ u64 ath5k_hw_get_tsf64(struct ath5k_hw *ah);
void ath5k_hw_set_tsf64(struct ath5k_hw *ah, u64 tsf64);
void ath5k_hw_reset_tsf(struct ath5k_hw *ah);
void ath5k_hw_init_beacon(struct ath5k_hw *ah, u32 next_beacon, u32 interval);
bool ath5k_hw_check_beacon_timers(struct ath5k_hw *ah, int intval);
/* ACK bit rate */
void ath5k_hw_set_ack_bitrate_high(struct ath5k_hw *ah, bool high);
/* Clock rate related functions */
......@@ -1191,6 +1191,15 @@ ath5k_check_ibss_tsf(struct ath5k_softc *sc, struct sk_buff *skb,
if (hw_tu >= sc->nexttbtt)
ath5k_beacon_update_timers(sc, bc_tstamp);
/* Check if the beacon timers are still correct, because a TSF
* update might have created a window between them - for a
* longer description see the comment of this function: */
if (!ath5k_hw_check_beacon_timers(sc->ah, sc->bintval)) {
ath5k_beacon_update_timers(sc, bc_tstamp);
"fixed beacon timers after beacon receive\n");
......@@ -640,6 +640,97 @@ void ath5k_hw_init_beacon(struct ath5k_hw *ah, u32 next_beacon, u32 interval)
* ath5k_check_timer_win - Check if timer B is timer A + window
* @a: timer a (before b)
* @b: timer b (after a)
* @window: difference between a and b
* @intval: timers are increased by this interval
* This helper function checks if timer B is timer A + window and covers
* cases where timer A or B might have already been updated or wrapped
* around (Timers are 16 bit).
* Returns true if O.K.
static inline bool
ath5k_check_timer_win(int a, int b, int window, int intval)
* 1.) usually B should be A + window
* 2.) A already updated, B not updated yet
* 3.) A already updated and has wrapped around
* 4.) B has wrapped around
if ((b - a == window) || /* 1.) */
(a - b == intval - window) || /* 2.) */
((a | 0x10000) - b == intval - window) || /* 3.) */
((b | 0x10000) - a == window)) /* 4.) */
return true; /* O.K. */
return false;
* ath5k_hw_check_beacon_timers - Check if the beacon timers are correct
* @ah: The &struct ath5k_hw
* @intval: beacon interval
* This is a workaround for IBSS mode:
* The need for this function arises from the fact that we have 4 separate
* HW timer registers (TIMER0 - TIMER3), which are closely related to the
* next beacon target time (NBTT), and that the HW updates these timers
* seperately based on the current TSF value. The hardware increments each
* timer by the beacon interval, when the local TSF coverted to TU is equal
* to the value stored in the timer.
* The reception of a beacon with the same BSSID can update the local HW TSF
* at any time - this is something we can't avoid. If the TSF jumps to a
* time which is later than the time stored in a timer, this timer will not
* be updated until the TSF in TU wraps around at 16 bit (the size of the
* timers) and reaches the time which is stored in the timer.
* The problem is that these timers are closely related to TIMER0 (NBTT) and
* that they define a time "window". When the TSF jumps between two timers
* (e.g. ATIM and NBTT), the one in the past will be left behind (not
* updated), while the one in the future will be updated every beacon
* interval. This causes the window to get larger, until the TSF wraps
* around as described above and the timer which was left behind gets
* updated again. But - because the beacon interval is usually not an exact
* divisor of the size of the timers (16 bit), an unwanted "window" between
* these timers has developed!
* This is especially important with the ATIM window, because during
* the ATIM window only ATIM frames and no data frames are allowed to be
* sent, which creates transmission pauses after each beacon. This symptom
* has been described as "ramping ping" because ping times increase linearly
* for some time and then drop down again. A wrong window on the DMA beacon
* timer has the same effect, so we check for these two conditions.
* Returns true if O.K.
ath5k_hw_check_beacon_timers(struct ath5k_hw *ah, int intval)
unsigned int nbtt, atim, dma;
nbtt = ath5k_hw_reg_read(ah, AR5K_TIMER0);
atim = ath5k_hw_reg_read(ah, AR5K_TIMER3);
dma = ath5k_hw_reg_read(ah, AR5K_TIMER1) >> 3;
/* NOTE: SWBA is different. Having a wrong window there does not
* stop us from sending data and this condition is catched thru
* other means (SWBA interrupt) */
if (ath5k_check_timer_win(nbtt, atim, 1, intval) &&
ath5k_check_timer_win(dma, nbtt, AR5K_TUNE_DMA_BEACON_RESP,
return true; /* O.K. */
return false;
* ath5k_hw_set_coverage_class - Set IEEE 802.11 coverage class
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