Hi,
On Sun, 2015-02-22 at 14:15 +0200, Valentin Manea wrote:
> 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.
>
Thank you for your report and analysis. You saved us a lot of time.
I'll send a fix.
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