rtlwifi: rtl8821ae: Fix connection lost problem correctly

There has been a coding error in rtl8821ae since it was first introduced,
namely that an 8-bit register was read using a 16-bit read in
_rtl8821ae_dbi_read(). This error was fixed with commit 40b368a
("rtlwifi: Fix alignment issues"); however, this change led to
instability in the connection. To restore stability, this change
was reverted in commit b8b8b16 ("rtlwifi: rtl8821ae: Fix connection
lost problem").

Unfortunately, the unaligned access causes machine checks in ARM
architecture, and we were finally forced to find the actual cause of the
problem on x86 platforms. Following a suggestion from Pkshih
<[email protected]>, it was found that increasing the ASPM L1
latency from 0 to 7 fixed the instability. This parameter was varied to
see if a smaller value would work; however, it appears that 7 is the
safest value. A new symbol is defined for this quantity, thus it can be
easily changed if necessary.

Fixes: b8b8b16 ("rtlwifi: rtl8821ae: Fix connection lost problem")
Cc: Stable <[email protected]> # 4.14+
Fix-suggested-by: Pkshih <[email protected]>
Signed-off-by: Larry Finger <[email protected]>
Tested-by: James Cameron <[email protected]>  # x86_64 OLPC NL3
Signed-off-by: Kalle Valo <[email protected]>

Author: lwfinger

drivers/net/wireless/realtek/rtlwifi/rtl8821ae/hw.c

@@ -1123,7 +1123,7 @@ static u8 _rtl8821ae_dbi_read(struct rtl_priv *rtlpriv, u16 addr)
 	}
 	if (0 == tmp) {
 		read_addr = REG_DBI_RDATA + addr % 4;
-		ret = rtl_read_word(rtlpriv, read_addr);
+		ret = rtl_read_byte(rtlpriv, read_addr);
 	}
 	return ret;
 }
@@ -1165,7 +1165,8 @@ static void _rtl8821ae_enable_aspm_back_door(struct ieee80211_hw *hw)
 	}
 
 	tmp = _rtl8821ae_dbi_read(rtlpriv, 0x70f);
-	_rtl8821ae_dbi_write(rtlpriv, 0x70f, tmp | BIT(7));
+	_rtl8821ae_dbi_write(rtlpriv, 0x70f, tmp | BIT(7) |
+			     ASPM_L1_LATENCY << 3);
 
 	tmp = _rtl8821ae_dbi_read(rtlpriv, 0x719);
 	_rtl8821ae_dbi_write(rtlpriv, 0x719, tmp | BIT(3) | BIT(4));

drivers/net/wireless/realtek/rtlwifi/wifi.h

@@ -99,6 +99,7 @@
 #define RTL_USB_MAX_RX_COUNT			100
 #define QBSS_LOAD_SIZE				5
 #define MAX_WMMELE_LENGTH			64
+#define ASPM_L1_LATENCY				7
 
 #define TOTAL_CAM_ENTRY				32