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Issues compiling Realtek RTL8156 (r8152) driver for ASUS RT-AX86U

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I have a Plugable 2.5Gbe Ethernet USB adapter (Realtek RTL8156B) and an ASUS RT-AX86U router running the latest stable Merlin firmware (3004.388.6_2).

When I plug the adapter into the router, it is identified correctly as a USB device in the Network Map view, however I was unable to get it to work as a WAN connection (selecting WAN USB and all that), plus I see frequent log spam with dmesg. I've been following this well-known guide from a few years ago that mentions that this is due to a missing kernel module for this chipset and provides instructions how to compile one yourself (as well as optimize performance). The tutorial showed how to do this for the AX88U, so I adjusted a few things to be correct for the AX86U (i.e. to use release src-rt-5.02L.07p2axhnd). Everything seemed to work just fine compiling the firmware and tools (iperf3, ethtool) using the /gnuton/Asuswrt-Merlin-Toolchains-Docker docker container, except when building the r8152 driver, which gives me the following error:
$make ARCH=arm64 CROSS_COMPILE=aarch64-linux- -C /build/mnt/fastdata/asus/asuswrt-merlin.ng/release/src-rt-5.02L.07p2axhnd/kernel/linux-4.1 M=`pwd` modules
make: Entering directory '/build/mnt/fastdata/asus/asuswrt-merlin.ng/release/src-rt-5.02L.07p2axhnd/kernel/linux-4.1'
  CC [M]  /build/mnt/fastdata/asus/r8152-2.17.1/r8152.o
<command-line>:0:1: error: macro names must be identifiers
In file included from include/linux/kobject.h:21:0,
                 from include/linux/module.h:16,
                 from /build/mnt/fastdata/asus/r8152-2.17.1/r8152.c:15:
include/linux/sysfs.h: In function ‘sysfs_get_dirent’:
include/linux/sysfs.h:496:37: warning: pointer targets in passing argument 2 of ‘kernfs_find_and_get’ differ in signedness [-Wpointer-sign]
  return kernfs_find_and_get(parent, name);
make[1]: *** [scripts/Makefile.build:296: /build/mnt/fastdata/asus/r8152-2.17.1/r8152.o] Error 1
make: *** [Makefile:1748: _module_/build/mnt/fastdata/asus/r8152-2.17.1] Error 2
make: Leaving directory '/build/mnt/fastdata/asus/asuswrt-merlin.ng/release/src-rt-5.02L.07p2axhnd/kernel/linux-4.1'

I've searched all over the forum and I see some people mentioning that they got it working without compiling any driver (doesn't work for me?), while some others mentioning having similar issues when following the tutorial. If I look at the comments in the original tutorial, there are a few mentioning that they get the same issue for AX86U for some reason, but not for AX88U. When investigating the commands make is running, I can see that the issue is caused due to two missing/empty defines (" -D "):
$ make -n ARCH=arm64 CROSS_COMPILE=aarch64-linux- -C /build/mnt/fastdata/asus/asuswrt-merlin.ng/release/src-rt-5.02L.07p2axhnd/kernel/linux-4.1 M=`pwd` modules
make: Entering directory '/build/mnt/fastdata/asus/asuswrt-merlin.ng/release/src-rt-5.02L.07p2axhnd/kernel/linux-4.1'
mkdir -p /build/mnt/fastdata/asus/r8152-2.17.1/.tmp_versions ; rm -f /build/mnt/fastdata/asus/r8152-2.17.1/.tmp_versions/*
test -e ./Module.symvers || ( \
echo; \
echo "  WARNING: Symbol version dump ./Module.symvers"; \
echo "           is missing; modules will have no dependencies and modversions."; \
echo )
make -f ./scripts/Makefile.build obj=/build/mnt/fastdata/asus/r8152-2.17.1
set -e;            echo '  CC [M]  /build/mnt/fastdata/asus/r8152-2.17.1/r8152.o'; aarch64-linux-gcc -Wp,-MD,/build/mnt/fastdata/asus/r8152-2.17.1/.r8152.o.d  -nostdinc -isystem /home/docker/am-toolchains/brcm-arm-hnd/crosstools-aarch64-gcc-5.3-linux-4.1-glibc-2.22-binutils-2.25/usr/bin/../lib/gcc/aarch64-buildroot-linux-gnu/5.3.0/include -I./arch/arm64/include -Iarch/arm64/include/generated/uapi -Iarch/arm64/include/generated  -Iinclude -I./arch/arm64/include/uapi -Iarch/arm64/include/generated/uapi -I./include/uapi -Iinclude/generated/uapi -include ./include/linux/kconfig.h -D__KERNEL__ -mlittle-endian -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -std=gnu89 -D -mgeneral-regs-only -O2 -fno-omit-frame-pointer -fno-optimize-sibling-calls  -DMODULE -mcmodel=large  -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(r8152)"  -D"KBUILD_MODNAME=KBUILD_STR(r8152)" -c -o /build/mnt/fastdata/asus/r8152-2.17.1/r8152.o /build/mnt/fastdata/asus/r8152-2.17.1/r8152.c;    scripts/basic/fixdep /build/mnt/fastdata/asus/r8152-2.17.1/.r8152.o.d /build/mnt/fastdata/asus/r8152-2.17.1/r8152.o 'aarch64-linux-gcc -Wp,-MD,/build/mnt/fastdata/asus/r8152-2.17.1/.r8152.o.d  -nostdinc -isystem /home/docker/am-toolchains/brcm-arm-hnd/crosstools-aarch64-gcc-5.3-linux-4.1-glibc-2.22-binutils-2.25/usr/bin/../lib/gcc/aarch64-buildroot-linux-gnu/5.3.0/include -I./arch/arm64/include -Iarch/arm64/include/generated/uapi -Iarch/arm64/include/generated  -Iinclude -I./arch/arm64/include/uapi -Iarch/arm64/include/generated/uapi -I./include/uapi -Iinclude/generated/uapi -include ./include/linux/kconfig.h -D__KERNEL__ -mlittle-endian -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -std=gnu89 -D -mgeneral-regs-only -O2 -fno-omit-frame-pointer -fno-optimize-sibling-calls  -DMODULE -mcmodel=large  -D"KBUILD_STR(s)=\#s" -D"KBUILD_BASENAME=KBUILD_STR(r8152)"  -D"KBUILD_MODNAME=KBUILD_STR(r8152)" -c -o /build/mnt/fastdata/asus/r8152-2.17.1/r8152.o /build/mnt/fastdata/asus/r8152-2.17.1/r8152.c' > /build/mnt/fastdata/asus/r8152-2.17.1/.r8152.o.tmp; rm -f /build/mnt/fastdata/asus/r8152-2.17.1/.r8152.o.d; mv -f /build/mnt/fastdata/asus/r8152-2.17.1/.r8152.o.tmp /build/mnt/fastdata/asus/r8152-2.17.1/.r8152.o.cmd
{ echo /build/mnt/fastdata/asus/r8152-2.17.1/r8152.ko; echo /build/mnt/fastdata/asus/r8152-2.17.1/r8152.o; } > /build/mnt/fastdata/asus/r8152-2.17.1/.tmp_versions/r8152.mod
(cat /dev/null;   echo kernel//build/mnt/fastdata/asus/r8152-2.17.1/r8152.ko;) > /build/mnt/fastdata/asus/r8152-2.17.1/modules.order
echo '  Building modules, stage 2.';
make -f ./scripts/Makefile.modpost
echo '  MODPOST 0 modules'; find /build/mnt/fastdata/asus/r8152-2.17.1/.tmp_versions -name '*.mod' | xargs -r grep -h '\.ko$' | sort -u | sed 's/\.ko$/.o/' | scripts/mod/modpost   -i ./Module.symvers -I /build/mnt/fastdata/asus/r8152-2.17.1/Module.symvers  -o /build/mnt/fastdata/asus/r8152-2.17.1/Module.symvers -S -w  -s -T - vmlinux
make: Leaving directory '/build/mnt/fastdata/asus/asuswrt-merlin.ng/release/src-rt-5.02L.07p2axhnd/kernel/linux-4.1'

I'm not sure where these come from and if I get rid of them and run gcc manually, it does compile, but I see the mention in the comments of that tutorial that someone already tried that and the kernel module was not loadable as a result. I would like to understand where the issue comes from and hoping someone has successfully done this before for AX86U.

Thank you for any help!


  • make-output.txt
    5.8 KB · Views: 13
Last edited:
I managed to compile this manually without a makefile by executing a modified gcc command-line from the makefile above:
/opt/toolchains/crosstools-aarch64-gcc-5.5-linux-4.1-glibc-2.26-binutils-2.28.1/usr/bin/aarch64-buildroot-linux-gnu-gcc -Wp,-MD,/build/mnt/fastdata/asus/r8152-2.17.1/.r8152.o.d  -nostdinc -isystem /home/docker/am-toolchains/brcm-arm-hnd/crosstools-aarch64-gcc-5.3-linux-4.1-glibc-2.22-binutils-2.25/usr/bin/../lib/gcc/aarch64-buildroot-linux-gnu/5.3.0/include -I./arch/arm64/include -Iarch/arm64/include/generated/uapi -Iarch/arm64/include/generated  -Iinclude -I./arch/arm64/include/uapi -Iarch/arm64/include/generated/uapi -I./include/uapi -Iinclude/generated/uapi -include ./include/linux/kconfig.h -D__KERNEL__ -mlittle-endian -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -std=gnu89 -mgeneral-regs-only -O2 -fno-omit-frame-pointer -fno-optimize-sibling-calls -DMODULE -mcmodel=large  -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(r8152)"  -D"KBUILD_MODNAME=KBUILD_STR(r8152)" -c /build/mnt/fastdata/asus/r8152-2.17.1/r8152.c -o /build/mnt/fastdata/asus/r8152-2.17.1/r8152.ko

This defines the __KERNEL__ and MODULE macros and specifies the isystem, so the result should be a valid module, but I guess I still did something wrong and the module refused to load:
admin@RT-AX86U-AE80:/jffs/drivers# insmod /jffs/drivers/r8152.ko
insmod: can't insert '/jffs/drivers/r8152.ko': invalid module format

While dmesg logs "No module found in object" and modinfo displays the appropriate information, but its probably metadata:
docker@422ad3617361:/build/mnt/fastdata/asus/r8152-2.17.1$ modinfo r8152.ko
filename:       /build/mnt/fastdata/asus/r8152-2.17.1/r8152.ko
version:        v2.17.1 (2023/06/13)
license:        GPL
description:    Realtek RTL8152/RTL8153 Based USB Ethernet Adapters
author:         Realtek nic sw <nic_swsd@realtek.com>

The binary seems to be correct (arm, aarch64) at fist glance:
docker@422ad3617361:/build/mnt/fastdata/asus/r8152-2.17.1$ file r8152.ko
r8152.ko: ELF 64-bit LSB relocatable, ARM aarch64, version 1 (SYSV), BuildID[sha1]=57002a32d66199cfd1d6ee4bd5d0a30007d6da6a, not stripped

I noticed that the r8152 driver is included in the merlin distribution (release/src-rt-5.02L.07p2axhnd/kernel/linux-4.1/drivers/net/usb/r8152.c), but this version is very old (v1.08.0 (2015/01/13)) and does not support the RTL8156B chipset that the new r8152 driver (v2.17.1 (2023/06/13)) from Realtek supports. I cannot see the old driver in the list of modules on the AX86U router though and I don't see a .ko file being generated when I compile the kernel (maybe something is skipping it?).

I can also see that cdc_ether module defines handling of Realtek 8152 and 8153 devices - not sure how that support is compiled in (maybe thats what is referencing the r8152.c code?). I have not done Linux kernel development since college (probably 20 years ago now) and I'm moving forward pretty slowly. I'm thinking that if I could get the included r8152 driver to build, maybe I can update it manually to the new version and get that one to build as well, but I'm not sure if I'm on the right path here...
Last edited:
Ok just an update - we have partial success!

I managed to compile everything - using the original Makefile method. I also created another Makefile to make things even easier:
KDIR  := /build/mnt/fastdata/asus/asuswrt-merlin.ng/release/src-rt-5.02L.07p2axhnd/kernel/linux-4.1
PWD   := $(shell pwd)
obj-m := r8152.o     # same name as the .c file of your module

    $(MAKE) -C $(KDIR) M=$(PWD) ARCH=arm64 CROSS_COMPILE=aarch64-linux- modules

The offending part that was causing all the issues was the missing BUILD_NAME variable in the main Makefile in the kernel. A simple "hack" to get past this was to just comment-out the KBUILD_CFLAGS line like so:
MODEL = $(subst -,,$(BUILD_NAME))
in asuswrt-merlin.ng/release/src-rt-5.02L.07p2axhndkernel/linux-4.1/Makefile

Module installs just fine, gets link, but I'm still having some issues:
1) I am unable to get the init-start and services-start scripts from the tutorial to execute on startup - or at least I don't see any evidence of them executing.
2) Even though the ETH8 device gets a link and an IP if I set it up manually, I couldn't get the ASUS router to use it as WAN. I tried setting WAN as USB and tried using both Manual Config and Legacy configurations for it. It does work fine as a LAN though, so thats how I'm using it for now (LAN -> 2.5Gbe switch -> 2.5Gbe NIC).

And the biggest issue of them all:
3) Even with the performance tweaks from the article, this router just isn't fast enough to use this beyond ~450Mbps - the CPU just gets pinned (mostly just Core 4) and I couldn't get it to do better. Not sure if I was doing something wrong or if the difference in CPU is that great between AX86U and AX88U.

If anyone wants the kernel module for the ASUS RT-AX86U and wants to experiment for themselves, you can use the one I compiled:
Last edited:

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