Hi,
Recently I started using kernel 3.19 from Ubuntu ppa and I noticed a
big drop in Wifi throughput. After some investigation I found that my
router was reporting the AMPDU factor changed from 3(in 3.18) to 0(in 3.19):
cat
/sys/kernel/debug/ieee80211/phy1/netdev:wlan1/stations/c4:xx:xx:xx:xx:xx/ht_capa:
ht supported
cap: 0x1066
HT20/HT40
Dynamic SM Power Save
RX HT20 SGI
RX HT40 SGI
No RX STBC
Max AMSDU length: 3839 bytes
DSSS/CCK HT40
ampdu factor/density: 3/5
MCS mask: ff ff 00 00 00 00 00 00 00 00
MCS tx params: 1
ht supported
cap: 0x1066
HT20/HT40
Dynamic SM Power Save
RX HT20 SGI
RX HT40 SGI
No RX STBC
Max AMSDU length: 3839 bytes
DSSS/CCK HT40
ampdu factor/density: 0/5
MCS mask: ff ff 00 00 00 00 00 00 00 00
MCS tx params: 1
In my setup this means a 50% drop in downstream throughput(from
160Mbit/s to 80Mbit/s)
I tracked down the offending commit and it seems this is it:
iwlwifi: change max HT and VHT A-MPDU exponent
c064ddf318aa51647a30108f7cd151c208c62eef
A new mechanism was created to have per device ampd_factor:
- ht_info->ampdu_factor = IEEE80211_HT_MAX_AMPDU_64K;
+ ht_info->ampdu_factor = cfg->max_ht_ampdu_exponent;
however the max_ht_ampdu_exponent is not set for any of the iwl-6000
devices(including mine Intel Corporation Centrino Advanced-N 6235)
#define IWL_DEVICE_6035 \
.fw_name_pre = IWL6030_FW_PRE, \
.ucode_api_max = IWL6035_UCODE_API_MAX, \
.ucode_api_ok = IWL6035_UCODE_API_OK, \
.ucode_api_min = IWL6035_UCODE_API_MIN, \
.device_family = IWL_DEVICE_FAMILY_6030, \
.max_inst_size = IWL60_RTC_INST_SIZE, \
.max_data_size = IWL60_RTC_DATA_SIZE, \
.nvm_ver = EEPROM_6030_EEPROM_VERSION, \
.nvm_calib_ver = EEPROM_6030_TX_POWER_VERSION, \
.base_params = &iwl6000_g2_base_params, \
.eeprom_params = &iwl6000_eeprom_params, \
.led_mode = IWL_LED_RF_STATE
Since the original value of IEEE80211_HT_MAX_AMPDU_64K I would
suggest adding it to some iwl-6000 devices.
I did not submit any patches because I have no idea if there are
device which would not support this feature, however it worked before
with default value set to IEEE80211_HT_MAX_AMPDU_64K so I guess it must
be supported.
Regards,
Valentin
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