EDUP EP-AC1661 error: "Empty EFUSE with unknown REF type" - which REF type to use for BT coexistence?

[airdrummingfool]

Hello,
I'm creating this issue in case others run into this problem, but also I'm curious if anyone has an explanation/suggestion on how to proceed.

I have successfully compiled the driver (versions 5.4.1 and 5.8.1) on an RPi 3 (Buster 5.4.51). When I connect an EDUP EP-AC1661 there is an error shown in dmesg, and it does not work.

dmesg output:

[40760.025616] usb 1-1.4: new high-speed USB device number 5 using dwc_otg
[40760.156657] usb 1-1.4: New USB device found, idVendor=0bda, idProduct=c820, bcdDevice= 2.00
[40760.156673] usb 1-1.4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[40760.156683] usb 1-1.4: Product: 802.11ac NIC
[40760.156692] usb 1-1.4: Manufacturer: Realtek
[40760.156702] usb 1-1.4: SerialNumber: FF
[40760.158816] RTW: [HALMAC]11692M
               HALMAC_MAJOR_VER = 1
               HALMAC_PROTOTYPE_VER = 6
               HALMAC_MINOR_VER = 5
               HALMAC_PATCH_VER = 5
[40760.160609] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[40760.391042] RTW: HW EFUSE
[40760.391053] RTW: 0x000: 29 81 00 BC  09 00 21 00  6E 04 A4 34  10 00 30 0B
[40760.391099] RTW: 0x010: 1E 1F 20 20  20 20 26 27  28 28 28 FF  FF FF FF FF
[40760.391141] RTW: 0x020: FF FF 29 2A  2A 2A 2C 2C  2C 2C 2C 2C  2A 2B 2B 2B
[40760.391183] RTW: 0x030: 00 FF FF FF  FF FF FF FF  FF FF 1E 1F  20 20 20 20
[40760.391227] RTW: 0x040: 26 27 28 28  28 02 FF FF  FF FF FF FF  FF FF FF FF
[40760.391269] RTW: 0x050: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391311] RTW: 0x060: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391354] RTW: 0x070: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391396] RTW: 0x080: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391438] RTW: 0x090: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391481] RTW: 0x0A0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391523] RTW: 0x0B0: FF FF FF FF  FF FF FF FF  7F 04 20 00  FF FF FF FF
[40760.391565] RTW: 0x0C0: FF 21 00 00  00 00 00 00  00 26 FF FF  FF FF FF FF
[40760.391607] RTW: 0x0D0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391650] RTW: 0x0E0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391692] RTW: 0x0F0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391734] RTW: 0x100: DA 0B 20 C8  E3 46 FF E8  4E 06 86 BC  45 09 03 52
[40760.391776] RTW: 0x110: 65 61 6C 74  65 6B 0E 03  38 30 32 2E  31 31 61 63
[40760.391818] RTW: 0x120: 20 4E 49 43  04 03 46 46  FF FF FF FF  FF FF FF FF
[40760.391860] RTW: 0x130: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391901] RTW: 0x140: FF FF FF FF  00 00 31 0F  FF FF FF FF  FF FF FF FF
[40760.391944] RTW: 0x150: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.391986] RTW: 0x160: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392028] RTW: 0x170: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392070] RTW: 0x180: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392112] RTW: 0x190: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392154] RTW: 0x1A0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392196] RTW: 0x1B0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392238] RTW: 0x1C0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392280] RTW: 0x1D0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392322] RTW: 0x1E0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392364] RTW: 0x1F0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
[40760.392583] RTW: hal_com_config_channel_plan chplan:0x7F
[40760.392587] RTW: ERROR

               Empty EFUSE with unknown REF type!!

[40760.392590] RTW: ERROR please program efuse or specify correct RFE type.
[40760.392593] RTW: ERROR cmd: insmod rtl8821cx.ko rtw_RFE_type=<rfe_type>

The error appears to come from this line:

rtl8821CU/hal/rtl8821c/rtl8821c_ops.c

Line 410 in 45a8b43

RTW_ERR("\n\nEmpty EFUSE with unknown REF type!!\n\n");

The lines right after the error suggests manually providing a value for rtw_RFE_type when loading the module. Based on rtl8821c_ops.c there appear to be 8 options:

		Type 0 - (2-Ant, DPDT), (2G_WLG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)
		Type 1 - (1-Ant, SPDT@Ant1), (2G_WLG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)
		Type 2 -(1-Ant, SPDT@Ant1) , (2G_BTG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)
		Type 3 - (1-Ant, DPDT@Ant2), (2G_WLG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)
		Type 4 - (1-Ant, DPDT@Ant2), (2G_BTG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)
		Type 5 - (2-Ant), (2G_WLG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)
		Type 6 - (2-Ant), (2G_WLG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)
		Type 7 - (1-Ant), (2G_BTG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)

If I unload the module (sudo rmmod 8821cu.ko) and then re-load it specifying any one of the above types, it seems to work fine:
sudo insmod /lib/modules/$(uname -r)/kernel/drivers/net/wireless/realtek/rtl8821cu/8821cu.ko rtw_RFE_type=7

The wifi works just as well if I put rtw_RFE_type=0 or rtw_RFE_type=1, etc.

Digging a bit deeper, it appears that the RFE Type is used to determine antenna isolation and the BT coexistence mechanism. Is there a way to know which is correct for my wifi dongle, or which is optimal in any given situation?

[airdrummingfool]

Update: I bought a second EDUP EP-AC1661 just to see what would happen. Turns out that the EFUSE is a bit different in this one, and a valid rfe_type can be read (it's located at 0xCA in the EFUSE, per EEPROM_RFE_OPTION_8821C).

Full EFUSE of a valid chip:

RTW: HW EFUSE
RTW: 0x000: 29 81 00 BC  09 00 21 00  6E 04 A4 34  10 00 30 0B
RTW: 0x010: 1D 1D 1E 1F  1F 1F 25 25  26 27 27 FF  FF FF FF FF
RTW: 0x020: FF FF 28 28  28 28 2A 29  29 28 28 27  24 26 26 26
RTW: 0x030: 00 FF FF FF  FF FF FF FF  FF FF 1D 1D  1E 1F 1F 1F
RTW: 0x040: 25 25 26 27  27 02 FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x050: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x060: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x070: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x080: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x090: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x0A0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x0B0: FF FF FF FF  FF FF FF FF  7F 0A 1F 00  FF FF FF FF
RTW: 0x0C0: FF 21 00 00  00 00 00 00  00 FF 26 FF  FF FF FF FF
RTW: 0x0D0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x0E0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x0F0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x100: DA 0B 20 C8  E3 46 FF E8  4E 06 86 AA  8F 09 03 52
RTW: 0x110: 65 61 6C 74  65 6B 0E 03  38 30 32 2E  31 31 61 63
RTW: 0x120: 20 4E 49 43  04 03 46 46  FF FF FF FF  FF FF FF FF
RTW: 0x130: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x140: FF FF FF FF  00 00 31 0F  FF FF FF FF  FF FF FF FF
RTW: 0x150: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x160: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x170: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x180: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x190: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x1A0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x1B0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x1C0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x1D0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x1E0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF
RTW: 0x1F0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF

thus rfe_type: 0x26 (38 decimal).

Some other random debug output:

RTW: EEPROM rfe_type=0x26
RTW: EEPROM PAType_2G is 0x0, ExternalPA_2G = 0
RTW: EEPROM PAType_5G is 0x0, external_pa_5g = 0
RTW: EEPROM LNAType_2G is 0x0, ExternalLNA_2G = 0
RTW: EEPROM LNAType_5G is 0x0, external_lna_5g = 0
RTW: EEPROM TypeGPA = 0x0
RTW: EEPROM TypeAPA = 0x0
RTW: EEPROM TypeGLNA = 0x0
RTW: EEPROM TypeALNA = 0x0

[speculation]
A bit more digging and it appears that this value is assigned to rfe_type_expand in phydm.c, and then referenced in phydm+hal_api8821c.c, where a value of 0x26 indicates that the switch (that controls whether the antenna is being used for wifi or BT?) does not exist. Furthermore, rfe_type_expand is shifted right 3 to get rfe_type, and 0x26 shifted right 3 is 4. That corresponds to the Type 4 option listed above... so that's my guess as to what type this module is ¯_(ツ)_/¯
Type 4 - (1-Ant, DPDT@Ant2), (2G_BTG, iPA, iLNA, iSW), (5G, iPA, iLNA, iSW)
[/speculation]

If I wanted to continue using the errant dongle, I'd use the following code to load the driver (though I still don't think it makes much of a difference what rtw_RFE_type value is provided, as long as it's hex):

sudo insmod /lib/modules/$(uname -r)/kernel/drivers/net/wireless/realtek/rtl8821cu/8821cu.ko rtw_RFE_type=0x26

... but it's broken, so I'm sending it back.

Thanks for attending my Ted Talk.

[firepainting]

Hi, I just got the exact adapter today (EDUP EP-AC1661) and I can't get it to work. I'm just a regular user, not a coder of sorts, and I don't understand a thing you said above :)

So my question is: Will the adapter not work? Is that a conclusion after trying two of them?

I'm on Linux Mint 19.3. I made a post on the Mint forums about it if you want to check it out.

Because I would return this too if it won't work.

Thanks.

[airdrummingfool]

@firepainting the first adapter I bought did not work out of the box - and I decided to send it back - due to a problem with the contents of the EFUSE. I could have made it work by manually specifying the rtw_RFE_type when loading the driver.

The second adapter I purchased worked fine with this driver, without any changes or needing to manually load the driver.

[firepainting]

Thank you, I appreciate it.

If I understand it, two adapters of this exact model could be "different" that one might work and the other might not (out of the box), with this driver. I guess I should talk to the vendor, then.

[luckylittle]

@airdrummingfool @firepainting

I'm on a brand new Red Hat Enterprise Linux release 8.3 (Ootpa), kernel 4.18.0-240.el8.x86_64 and tried what you suggested:

echo "options 8821cu rtw_RFE_type=0x26" | sudo tee /etc/modprobe.d/8821cu.conf

To no avail. This is what happens:

[ 2805.561291] RTW: module init start
[ 2805.561293] RTW: rtl8821cu v5.4.1_28754.20180921_COEX20180712-3232
[ 2805.561293] RTW: build time: Dec  9 2020 21:44:46
[ 2805.561294] RTW: rtl8821cu BT-Coex version = COEX20180712-3232
[ 2805.561348] RTW: [HALMAC]11692M
               HALMAC_MAJOR_VER = 1
               HALMAC_PROTOTYPE_VER = 4
               HALMAC_MINOR_VER = 19
               HALMAC_PATCH_VER = 3
[ 2805.561829] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[ 2805.644256] RTW: HW EFUSE
[ 2805.644257] RTW: 0x000: 29 81 00 BC  09 00 21 00  6E 04 A4 34  10 00 30 0B  
[ 2805.644261] RTW: 0x010: 1C 1D 1E 20  20 20 24 25  26 28 28 FF  FF FF FF FF  
[ 2805.644263] RTW: 0x020: FF FF 26 28  28 28 2A 2A  2A 2A 2A 2A  25 26 26 26  
[ 2805.644266] RTW: 0x030: 00 FF FF FF  FF FF FF FF  FF FF 1C 1D  1E 20 20 20  
[ 2805.644268] RTW: 0x040: 24 25 26 28  28 02 FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644271] RTW: 0x050: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644273] RTW: 0x060: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644276] RTW: 0x070: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644278] RTW: 0x080: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644281] RTW: 0x090: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644283] RTW: 0x0A0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644286] RTW: 0x0B0: FF FF FF FF  FF FF FF FF  7F 0A 1F 00  FF FF FF FF  
[ 2805.644288] RTW: 0x0C0: FF 21 00 00  00 00 00 00  00 26 FF FF  FF FF FF FF  
[ 2805.644291] RTW: 0x0D0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644293] RTW: 0x0E0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644296] RTW: 0x0F0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644298] RTW: 0x100: DA 0B 20 C8  E3 46 FF E8  4E 06 88 EF  52 09 03 52  
[ 2805.644301] RTW: 0x110: 65 61 6C 74  65 6B 0E 03  38 30 32 2E  31 31 61 63  
[ 2805.644304] RTW: 0x120: 20 4E 49 43  04 03 46 46  FF FF FF FF  FF FF FF FF  
[ 2805.644306] RTW: 0x130: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644309] RTW: 0x140: FF FF FF FF  00 00 31 0F  FF FF FF FF  FF FF FF FF  
[ 2805.644311] RTW: 0x150: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644314] RTW: 0x160: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644316] RTW: 0x170: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644319] RTW: 0x180: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644321] RTW: 0x190: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644324] RTW: 0x1A0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644326] RTW: 0x1B0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644329] RTW: 0x1C0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644331] RTW: 0x1D0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644334] RTW: 0x1E0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644336] RTW: 0x1F0: FF FF FF FF  FF FF FF FF  FF FF FF FF  FF FF FF FF  
[ 2805.644416] RTW: hal_com_config_channel_plan chplan:0x7F
[ 2805.649616] RTW: [HALMAC][ALWAYS]shall R reg twice!!
[ 2805.651503] RTW: WARN [HALMAC][WARN]H2C/C2H ver is compatible!!
[ 2805.699239] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[ 2805.699318] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[ 2805.699398] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[ 2805.699478] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[ 2805.699558] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[ 2805.699638] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[ 2805.699718] RTW: ERROR [HALMAC][ERR]Dump efuse in suspend
[ 2805.699972] RTW: rtw_regsty_chk_target_tx_power_valid return _FALSE for band:0, path:0, rs:0, t:-1
[ 2805.700519] WARNING: CPU: 3 PID: 18695 at net/wireless/core.c:867 wiphy_register+0x68a/0x940 [cfg80211]
[ 2805.700520] Modules linked in: 8821cu(OE+) cfg80211 nls_utf8 isofs uas usb_storage hidp uinput rfcomm xt_CHECKSUM ipt_MASQUERADE xt_conntrack ipt_REJECT nf_nat_tftp nft_objref nf_conntrack_tftp nft_counter tun bridge stp llc nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nf_tables_set nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip6_tables nft_compat ip_set nf_tables nfnetlink cmac bnep sunrpc intel_rapl_msr intel_rapl_common x86_pkg_temp_thermal intel_powerclamp coretemp snd_hda_codec_realtek kvm_intel snd_hda_codec_generic btusb ledtrig_audio snd_hda_codec_hdmi iTCO_wdt btrtl mei_wdt wmi_bmof kvm iTCO_vendor_support btbcm btintel snd_hda_intel bluetooth snd_intel_dspcfg snd_hda_codec irqbypass intel_cstate snd_hda_core ecdh_generic snd_hwdep snd_seq intel_uncore rfkill snd_seq_device i2c_i801 pcspkr intel_rapl_perf snd_pcm lpc_ich snd_timer mei_me snd mei soundcore wmi ip_tables xfs libcrc32c dm_crypt
[ 2805.700542]  sd_mod sg i915 cec intel_gtt i2c_algo_bit drm_kms_helper crct10dif_pclmul crc32_pclmul crc32c_intel syscopyarea ahci sysfillrect sysimgblt fb_sys_fops libahci drm ghash_clmulni_intel libata e1000e video dm_mirror dm_region_hash dm_log dm_mod fuse [last unloaded: cfg80211]
[ 2805.700551] CPU: 3 PID: 18695 Comm: modprobe Tainted: G        W  OE    --------- -  - 4.18.0-240.el8.x86_64 #1
[ 2805.700552] Hardware name: LENOVO 10AACTO1WW/SHARKBAY, BIOS FBKT52AUS 09/23/2013
[ 2805.700561] RIP: 0010:wiphy_register+0x68a/0x940 [cfg80211]
[ 2805.700562] Code: 78 20 00 74 25 83 ea 01 48 8d 14 52 48 c1 e2 04 48 01 c2 48 83 78 10 00 74 1c 48 39 c2 74 5a 48 83 c0 30 48 83 78 20 00 75 e9 <0f> 0b b8 ea ff ff ff e9 8a fa ff ff 48 83 78 18 00 75 dd 0f 0b b8
[ 2805.700563] RSP: 0018:ffffb2e08200f9e0 EFLAGS: 00010246
[ 2805.700564] RAX: ffffffffc1197180 RBX: ffff9b46f6ca24a0 RCX: 0000000000000000
[ 2805.700564] RDX: 0000000000000002 RSI: ffff9b46bd5ca058 RDI: 0000000000000001
[ 2805.700565] RBP: ffffb2e08200fa70 R08: 0000000000000004 R09: 0000000000000003
[ 2805.700565] R10: ffff9b46f6ca7000 R11: ffff9b4706b2c22c R12: ffffb2e081ad64f0
[ 2805.700566] R13: 0000000000000000 R14: ffffb2e081ad644e R15: ffffffffc11a9438
[ 2805.700566] FS:  00007f7918b5f740(0000) GS:ffff9b470f8c0000(0000) knlGS:0000000000000000
[ 2805.700567] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2805.700568] CR2: 00007fd7a28d50e0 CR3: 00000003b4a5a002 CR4: 00000000001606e0
[ 2805.700568] Call Trace:
[ 2805.700580]  ? handle_channel_custom.isra.24.constprop.29+0x31/0x150 [cfg80211]
[ 2805.700616]  ? rtw_chbw_to_freq_range+0x40/0x96 [8821cu]
[ 2805.700645]  ? rtw_is_dfs_ch+0x34/0x70 [8821cu]
[ 2805.700681]  rtw_wiphy_register+0x2a/0x54 [8821cu]
[ 2805.700717]  rtw_cfg80211_ndev_res_register+0x15/0x56 [8821cu]
[ 2805.700750]  rtw_os_ndev_register+0x110/0x17b [8821cu]
[ 2805.700785]  rtw_os_ndevs_register+0xa7/0xf6 [8821cu]
[ 2805.700819]  rtw_os_ndevs_init+0x24/0x3a [8821cu]
[ 2805.700853]  rtw_drv_init+0x86/0xda [8821cu]
[ 2805.700856]  usb_probe_interface+0xe4/0x2f0
[ 2805.700860]  really_probe+0x212/0x440
[ 2805.700862]  driver_probe_device+0x49/0xc0
[ 2805.700863]  device_driver_attach+0x50/0x60
[ 2805.700865]  __driver_attach+0x61/0x130
[ 2805.700866]  ? device_driver_attach+0x60/0x60
[ 2805.700868]  bus_for_each_dev+0x77/0xc0
[ 2805.700871]  ? klist_add_tail+0x3b/0x70
[ 2805.700872]  bus_add_driver+0x14d/0x1e0
[ 2805.700874]  driver_register+0x6b/0xb0
[ 2805.700876]  usb_register_driver+0x7a/0x130
[ 2805.700877]  ? 0xffffffffc12c3000
[ 2805.700903]  rtw_drv_entry+0xc0/0x1000 [8821cu]
[ 2805.700906]  do_one_initcall+0x46/0x1c3
[ 2805.700909]  ? _cond_resched+0x15/0x30
[ 2805.700912]  ? kmem_cache_alloc_trace+0x140/0x1c0
[ 2805.700915]  do_init_module+0x5a/0x220
[ 2805.700916]  load_module+0x14b4/0x17e0
[ 2805.700919]  ? __do_sys_finit_module+0xa8/0x110
[ 2805.700920]  __do_sys_finit_module+0xa8/0x110
[ 2805.700923]  do_syscall_64+0x5b/0x1a0
[ 2805.700924]  entry_SYSCALL_64_after_hwframe+0x65/0xca
[ 2805.700925] RIP: 0033:0x7f7917b4878d
[ 2805.700927] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d cb 56 2c 00 f7 d8 64 89 01 48
[ 2805.700928] RSP: 002b:00007ffeaf4ee418 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[ 2805.700929] RAX: ffffffffffffffda RBX: 0000564b637d4cc0 RCX: 00007f7917b4878d
[ 2805.700929] RDX: 0000000000000000 RSI: 0000564b637d5310 RDI: 0000000000000004
[ 2805.700929] RBP: 0000564b637d5310 R08: 0000000000000000 R09: 0000000000000000
[ 2805.700930] R10: 0000000000000004 R11: 0000000000000246 R12: 0000000000000000
[ 2805.700930] R13: 0000564b637d4df0 R14: 0000000000040000 R15: 0000000000000000
[ 2805.700932] ---[ end trace ffc0ec4263e289df ]---
[ 2805.700971] WARNING: CPU: 3 PID: 18695 at /home/USER/Downloads/EP-AC1661/WiFi/rtl8821CU-5.4.1/os_dep/linux/os_intfs.c:1647 rtw_os_ndev_register+0x64/0x17b [8821cu]
[ 2805.700972] Modules linked in: 8821cu(OE+) cfg80211 nls_utf8 isofs uas usb_storage hidp uinput rfcomm xt_CHECKSUM ipt_MASQUERADE xt_conntrack ipt_REJECT nf_nat_tftp nft_objref nf_conntrack_tftp nft_counter tun bridge stp llc nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nf_tables_set nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip6_tables nft_compat ip_set nf_tables nfnetlink cmac bnep sunrpc intel_rapl_msr intel_rapl_common x86_pkg_temp_thermal intel_powerclamp coretemp snd_hda_codec_realtek kvm_intel snd_hda_codec_generic btusb ledtrig_audio snd_hda_codec_hdmi iTCO_wdt btrtl mei_wdt wmi_bmof kvm iTCO_vendor_support btbcm btintel snd_hda_intel bluetooth snd_intel_dspcfg snd_hda_codec irqbypass intel_cstate snd_hda_core ecdh_generic snd_hwdep snd_seq intel_uncore rfkill snd_seq_device i2c_i801 pcspkr intel_rapl_perf snd_pcm lpc_ich snd_timer mei_me snd mei soundcore wmi ip_tables xfs libcrc32c dm_crypt
[ 2805.700985]  sd_mod sg i915 cec intel_gtt i2c_algo_bit drm_kms_helper crct10dif_pclmul crc32_pclmul crc32c_intel syscopyarea ahci sysfillrect sysimgblt fb_sys_fops libahci drm ghash_clmulni_intel libata e1000e video dm_mirror dm_region_hash dm_log dm_mod fuse [last unloaded: cfg80211]
[ 2805.700990] CPU: 3 PID: 18695 Comm: modprobe Tainted: G        W  OE    --------- -  - 4.18.0-240.el8.x86_64 #1
[ 2805.700991] Hardware name: LENOVO 10AACTO1WW/SHARKBAY, BIOS FBKT52AUS 09/23/2013
[ 2805.701024] RIP: 0010:rtw_os_ndev_register+0x64/0x17b [8821cu]
[ 2805.701025] Code: 8d a7 f0 34 00 00 b9 20 00 00 00 48 c7 c2 95 9f 0d c1 4c 89 e6 48 89 ef e8 a9 13 87 c0 48 89 df e8 35 bf 01 00 83 f8 01 74 1a <0f> 0b 4c 89 e7 e8 72 03 87 c0 b8 00 00 00 00 5b 5d 41 5c 41 5d 41
[ 2805.701026] RSP: 0018:ffffb2e08200faa0 EFLAGS: 00010297
[ 2805.701027] RAX: 0000000000000000 RBX: ffffb2e081ad3000 RCX: 0000000000000000
[ 2805.701027] RDX: 0000000000000000 RSI: ffff9b46bd5ca058 RDI: 0000000000000001
[ 2805.701028] RBP: ffff9b46f6ca0000 R08: 0000000000000004 R09: 0000000000000003
[ 2805.701028] R10: ffff9b46f6ca7000 R11: ffff9b4706b2c22c R12: ffffb2e081ad64f0
[ 2805.701029] R13: 0000000000000000 R14: ffffb2e081ad644e R15: ffffffffc11a9438
[ 2805.701030] FS:  00007f7918b5f740(0000) GS:ffff9b470f8c0000(0000) knlGS:0000000000000000
[ 2805.701030] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2805.701031] CR2: 00007fd7a28d50e0 CR3: 00000003b4a5a002 CR4: 00000000001606e0
[ 2805.701031] Call Trace:
[ 2805.701066]  rtw_os_ndevs_register+0xa7/0xf6 [8821cu]
[ 2805.701099]  rtw_os_ndevs_init+0x24/0x3a [8821cu]
[ 2805.701160]  rtw_drv_init+0x86/0xda [8821cu]
[ 2805.701162]  usb_probe_interface+0xe4/0x2f0
[ 2805.701164]  really_probe+0x212/0x440
[ 2805.701166]  driver_probe_device+0x49/0xc0
[ 2805.701178]  device_driver_attach+0x50/0x60
[ 2805.701179]  __driver_attach+0x61/0x130
[ 2805.701180]  ? device_driver_attach+0x60/0x60
[ 2805.701182]  bus_for_each_dev+0x77/0xc0
[ 2805.701183]  ? klist_add_tail+0x3b/0x70
[ 2805.701184]  bus_add_driver+0x14d/0x1e0
[ 2805.701186]  driver_register+0x6b/0xb0
[ 2805.701187]  usb_register_driver+0x7a/0x130
[ 2805.701188]  ? 0xffffffffc12c3000
[ 2805.701223]  rtw_drv_entry+0xc0/0x1000 [8821cu]
[ 2805.701225]  do_one_initcall+0x46/0x1c3
[ 2805.701227]  ? _cond_resched+0x15/0x30
[ 2805.701228]  ? kmem_cache_alloc_trace+0x140/0x1c0
[ 2805.701230]  do_init_module+0x5a/0x220
[ 2805.701231]  load_module+0x14b4/0x17e0
[ 2805.701233]  ? __do_sys_finit_module+0xa8/0x110
[ 2805.701234]  __do_sys_finit_module+0xa8/0x110
[ 2805.701236]  do_syscall_64+0x5b/0x1a0
[ 2805.701237]  entry_SYSCALL_64_after_hwframe+0x65/0xca
[ 2805.701238] RIP: 0033:0x7f7917b4878d
[ 2805.701239] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d cb 56 2c 00 f7 d8 64 89 01 48
[ 2805.701240] RSP: 002b:00007ffeaf4ee418 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[ 2805.701241] RAX: ffffffffffffffda RBX: 0000564b637d4cc0 RCX: 00007f7917b4878d
[ 2805.701241] RDX: 0000000000000000 RSI: 0000564b637d5310 RDI: 0000000000000004
[ 2805.701242] RBP: 0000564b637d5310 R08: 0000000000000000 R09: 0000000000000000
[ 2805.701242] R10: 0000000000000004 R11: 0000000000000246 R12: 0000000000000000
[ 2805.701243] R13: 0000564b637d4df0 R14: 0000000000040000 R15: 0000000000000000
[ 2805.701244] ---[ end trace ffc0ec4263e289e0 ]---
[ 2805.710211] WARNING: CPU: 3 PID: 18695 at /home/USER/Downloads/EP-AC1661/WiFi/rtl8821CU-5.4.1/os_dep/linux/os_intfs.c:3105 rtw_os_ndevs_register+0xaf/0xf6 [8821cu]
[ 2805.710212] Modules linked in: 8821cu(OE+) cfg80211 nls_utf8 isofs uas usb_storage hidp uinput rfcomm xt_CHECKSUM ipt_MASQUERADE xt_conntrack ipt_REJECT nf_nat_tftp nft_objref nf_conntrack_tftp nft_counter tun bridge stp llc nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nf_tables_set nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip6_tables nft_compat ip_set nf_tables nfnetlink cmac bnep sunrpc intel_rapl_msr intel_rapl_common x86_pkg_temp_thermal intel_powerclamp coretemp snd_hda_codec_realtek kvm_intel snd_hda_codec_generic btusb ledtrig_audio snd_hda_codec_hdmi iTCO_wdt btrtl mei_wdt wmi_bmof kvm iTCO_vendor_support btbcm btintel snd_hda_intel bluetooth snd_intel_dspcfg snd_hda_codec irqbypass intel_cstate snd_hda_core ecdh_generic snd_hwdep snd_seq intel_uncore rfkill snd_seq_device i2c_i801 pcspkr intel_rapl_perf snd_pcm lpc_ich snd_timer mei_me snd mei soundcore wmi ip_tables xfs libcrc32c dm_crypt
[ 2805.710225]  sd_mod sg i915 cec intel_gtt i2c_algo_bit drm_kms_helper crct10dif_pclmul crc32_pclmul crc32c_intel syscopyarea ahci sysfillrect sysimgblt fb_sys_fops libahci drm ghash_clmulni_intel libata e1000e video dm_mirror dm_region_hash dm_log dm_mod fuse [last unloaded: cfg80211]
[ 2805.710231] CPU: 3 PID: 18695 Comm: modprobe Tainted: G        W  OE    --------- -  - 4.18.0-240.el8.x86_64 #1
[ 2805.710232] Hardware name: LENOVO 10AACTO1WW/SHARKBAY, BIOS FBKT52AUS 09/23/2013
[ 2805.710269] RIP: 0010:rtw_os_ndevs_register+0xaf/0xf6 [8821cu]
[ 2805.710270] Code: 87 b4 48 00 00 49 8d b5 4e 34 00 00 84 c0 74 0d 49 8d b5 5e 34 00 00 3c 01 49 0f 45 f7 e8 eb f1 ff ff 41 89 c4 83 f8 01 74 b2 <0f> 0b 85 db 75 15 48 63 db 48 8b bc dd 50 01 00 00 48 85 ff 74 05
[ 2805.710271] RSP: 0018:ffffb2e08200fad8 EFLAGS: 00010297
[ 2805.710271] RAX: 0000000000000000 RBX: 0000000000000000 RCX: dead000000000200
[ 2805.710272] RDX: ffff9b46f6ca0060 RSI: ffffb2e081ad6588 RDI: ffffb2e081ad65b0
[ 2805.710273] RBP: ffff9b46f6ca7000 R08: 00000000000062ac R09: 00000000000062ac
[ 2805.710273] R10: 0000000000000000 R11: ffff9b470f8e8c64 R12: 0000000000000000
[ 2805.710274] R13: ffffb2e081ad3000 R14: 0000000000000001 R15: ffffffffc11a9438
[ 2805.710275] FS:  00007f7918b5f740(0000) GS:ffff9b470f8c0000(0000) knlGS:0000000000000000
[ 2805.710275] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2805.710276] CR2: 00007fd7a28d50e0 CR3: 00000003b4a5a002 CR4: 00000000001606e0
[ 2805.710276] Call Trace:
[ 2805.710315]  rtw_os_ndevs_init+0x24/0x3a [8821cu]
[ 2805.710352]  rtw_drv_init+0x86/0xda [8821cu]
[ 2805.710354]  usb_probe_interface+0xe4/0x2f0
[ 2805.710356]  really_probe+0x212/0x440
[ 2805.710357]  driver_probe_device+0x49/0xc0
[ 2805.710359]  device_driver_attach+0x50/0x60
[ 2805.710360]  __driver_attach+0x61/0x130
[ 2805.710361]  ? device_driver_attach+0x60/0x60
[ 2805.710363]  bus_for_each_dev+0x77/0xc0
[ 2805.710364]  ? klist_add_tail+0x3b/0x70
[ 2805.710365]  bus_add_driver+0x14d/0x1e0
[ 2805.710367]  driver_register+0x6b/0xb0
[ 2805.710368]  usb_register_driver+0x7a/0x130
[ 2805.710369]  ? 0xffffffffc12c3000
[ 2805.710396]  rtw_drv_entry+0xc0/0x1000 [8821cu]
[ 2805.710398]  do_one_initcall+0x46/0x1c3
[ 2805.710400]  ? _cond_resched+0x15/0x30
[ 2805.710401]  ? kmem_cache_alloc_trace+0x140/0x1c0
[ 2805.710403]  do_init_module+0x5a/0x220
[ 2805.710404]  load_module+0x14b4/0x17e0
[ 2805.710407]  ? __do_sys_finit_module+0xa8/0x110
[ 2805.710408]  __do_sys_finit_module+0xa8/0x110
[ 2805.710410]  do_syscall_64+0x5b/0x1a0
[ 2805.710411]  entry_SYSCALL_64_after_hwframe+0x65/0xca
[ 2805.710412] RIP: 0033:0x7f7917b4878d
[ 2805.710413] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d cb 56 2c 00 f7 d8 64 89 01 48
[ 2805.710414] RSP: 002b:00007ffeaf4ee418 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[ 2805.710415] RAX: ffffffffffffffda RBX: 0000564b637d4cc0 RCX: 00007f7917b4878d
[ 2805.710415] RDX: 0000000000000000 RSI: 0000564b637d5310 RDI: 0000000000000004
[ 2805.710416] RBP: 0000564b637d5310 R08: 0000000000000000 R09: 0000000000000000
[ 2805.710416] R10: 0000000000000004 R11: 0000000000000246 R12: 0000000000000000
[ 2805.710417] R13: 0000564b637d4df0 R14: 0000000000040000 R15: 0000000000000000
[ 2805.710418] ---[ end trace ffc0ec4263e289e1 ]---
[ 2805.710860] RTW: WARN free_recv_skb_queue not empty, 8
[ 2805.710889] usbcore: registered new interface driver rtl8821cu
[ 2805.710889] RTW: module init ret=0

UPDATE: With this little change, I was able to compile v5.8.1_34171.20190628_COEX20190509-4141, but the issue remains.