v5.5-rc1 and beyond insta-kills some Comcast wifi routers

["Mancini, Jason" <Jason.Mancini@amd.com>]

[I can't seem to access the linux-net ml per kernel.org faq, apology
in advance.]

This change, which I think first appeared for v5.5-rc1, basically
within seconds, knocks out our [apparently buggy] Comcast wifi for
about 2-3 minutes.  Is there a boot option (or similar) where I can
achieve prior kernel behavior?  Otherwise I am stuck on kernel 5.4
(or Win10) it seems, or forever compiling custom kernels for my
choice of distribution [as I don't have physical access to the router
in question.]
Thanks!
Jason

================

127eef1d46f80056fe9f18406c6eab38778d8a06 is the first bad commit
commit 127eef1d46f80056fe9f18406c6eab38778d8a06
Author: Yan-Hsuan Chuang <yhchuang@realtek.com>
Date:   Wed Oct 2 14:35:23 2019 +0800

    rtw88: add TX-AMSDU support

    Based on the mac80211's TXQ implementation, TX-AMSDU can
    be used to get higher MAC efficiency. To make mac80211
    aggregate MSDUs, low level driver just need to leave skbs
    in the TXQ, and mac80211 will try to aggregate them if
    possible. As driver will schedule a tasklet when the TX
    queue is woke, until the tasklet being served, there will
    have some skbs in the queue if traffic is heavy.

    Driver can control the max AMSDU size depending on the
    current bit rate used by hardware/firmware. The higher
    rates are used, the larger AMSDU size can be.

    It is tested that can achieve higher T-Put at higher rates.
    If the environment is relatively clean, and the bit_rate
    is high enough, we can get about 80Mbps improvement.

    For lower bit rates, not much gain can we get, so leave
    the max_amsdu length low to prevent aggregation.

    Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
    Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

 drivers/net/wireless/realtek/rtw88/fw.c   | 24 ++++++++++++++++++++++++
 drivers/net/wireless/realtek/rtw88/main.c |  1 +
 2 files changed, 25 insertions(+)

------------------- drivers/net/wireless/realtek/rtw88/fw.c -------------------
index 4b41bf531998..51649df7cc98 100644
@@ -29,6 +29,28 @@ static void rtw_fw_c2h_cmd_handle_ext(struct rtw_dev *rtwdev,
        }
 }
 
+static u16 get_max_amsdu_len(u32 bit_rate)
+{
+       /* lower than ofdm, do not aggregate */
+       if (bit_rate < 550)
+               return 1;
+
+       /* lower than 20M 2ss mcs8, make it small */
+       if (bit_rate < 1800)
+               return 1200;
+
+       /* lower than 40M 2ss mcs9, make it medium */
+       if (bit_rate < 4000)
+               return 2600;
+
+       /* not yet 80M 2ss mcs8/9, make it twice regular packet size */
+       if (bit_rate < 7000)
+               return 3500;
+
+       /* unlimited */
+       return 0;
+}
+
 struct rtw_fw_iter_ra_data {
        struct rtw_dev *rtwdev;
        u8 *payload;
@@ -83,6 +105,8 @@ static void rtw_fw_ra_report_iter(void *data, struct ieee80211_sta *sta)
 
        si->ra_report.desc_rate = rate;
        si->ra_report.bit_rate = bit_rate;
+
+       sta->max_rc_amsdu_len = get_max_amsdu_len(bit_rate);
 }
 
 static void rtw_fw_ra_report_handle(struct rtw_dev *rtwdev, u8 *payload,

------------------ drivers/net/wireless/realtek/rtw88/main.c ------------------
index 690a5c4d64e7..f7044e8bcb5b 100644
@@ -1310,6 +1310,7 @@ int rtw_register_hw(struct rtw_dev *rtwdev, struct ieee80211_hw *hw)
        ieee80211_hw_set(hw, SUPPORT_FAST_XMIT);
        ieee80211_hw_set(hw, SUPPORTS_AMSDU_IN_AMPDU);
        ieee80211_hw_set(hw, HAS_RATE_CONTROL);
+       ieee80211_hw_set(hw, TX_AMSDU);
 
        hw->wiphy->interface_modes = BIT(NL80211_IFTYPE_STATION) |
                                     BIT(NL80211_IFTYPE_AP) |