From: Sven Eckelmann <sven@narfation.org>
Date: Thu, 13 Oct 2022 16:40:21 +0200
Subject: openwrt: Add support for easily selectable kernel debugger support

When enabling this KERNEL_KGDB (after disabling KERNEL_DEBUG_INFO_REDUCED),
make sure to clean some packages to make sure that they are compiled with
the correct settings:

    make toolchain/gdb/clean
    make toolchain/gdb/compile -j$(nproc || echo 1)
    make target/linux/clean
    make -j$(nproc || echo 1)

The serial console will be shared between normal serial output and (k)gdb,
it is necessary to have an agent installed on your host system which
extracts the part relevant for (k)gdb.

    git clone https://git.kernel.org/pub/scm/utils/kernel/kgdb/agent-proxy.git/
    make -C agent-proxy
    ./agent-proxy/agent-proxy '127.0.0.1:5550^127.0.0.1:5551' 0 /dev/ttyUSB1,115200

It is then possible to see the full serial output in screen via:

    screen //telnet localhost 5550

On the target system, it is necessary to prepare the debugging session:

    echo ttyS0,115200 > /sys/module/kgdboc/parameters/kgdboc
    ubus call system watchdog '{"magicclose":true}'
    ubus call system watchdog '{"stop":true}'

Sometimes it might be necessary to force Linux to switch to kgdb:

    echo g > /proc/sysrq-trigger

The host can then connect to the kgdb using:

    cd "{LINUX_DIR}"
    cp ../vmlinux.debug vmlinux
    "${GDB}" -iex "set auto-load safe-path `pwd`/scripts/gdb/" -iex "target remote localhost:5551" vmlinux
    (gdb) lx-symbols ..
    (gdb) continue

Signed-off-by: Sven Eckelmann <sven@narfation.org>

diff --git a/config/Config-kernel.in b/config/Config-kernel.in
index ab827d6a71eb47ed0f6b71354b3372532d3656a5..817621ac9f37c365157470c7fc00c4f24e38f7f6 100644
--- a/config/Config-kernel.in
+++ b/config/Config-kernel.in
@@ -2,6 +2,50 @@
 #
 # Copyright (C) 2006-2014 OpenWrt.org
 
+config KERNEL_VT
+    bool
+
+config KERNEL_GDB_SCRIPTS
+    select GDB_PYTHON
+    bool
+
+config KERNEL_HW_CONSOLE
+    bool
+
+config KERNEL_CONSOLE_POLL
+    bool
+
+config KERNEL_MAGIC_SYSRQ
+    bool
+
+config KERNEL_MAGIC_SYSRQ_SERIAL
+    bool
+
+config KERNEL_KGDB_SERIAL_CONSOLE
+    bool
+
+config KERNEL_KGDB_HONOUR_BLOCKLIST
+    bool
+
+config KERNEL_MIPS_FP_SUPPORT
+    depends on (mips || mipsel || mips64 || mips64el)
+    bool
+
+config KERNEL_KGDB
+    select KERNEL_VT
+    select KERNEL_GDB_SCRIPTS
+    select KERNEL_HW_CONSOLE
+    select KERNEL_CONSOLE_POLL
+    select KERNEL_MAGIC_SYSRQ
+    select KERNEL_MAGIC_SYSRQ_SERIAL
+    select KERNEL_KGDB_SERIAL_CONSOLE
+    select KERNEL_KGDB_HONOUR_BLOCKLIST
+    select KERNEL_MIPS_FP_SUPPORT if (mips || mipsel || mips64 || mips64el)
+    
+    depends on KERNEL_DEBUG_INFO
+    bool "Enable kernel debugger over serial"
+
+
 config KERNEL_BUILD_USER
 	string "Custom Kernel Build User Name"
 	default "builder" if BUILDBOT
diff --git a/include/kernel-build.mk b/include/kernel-build.mk
index 5dd033cda6c89b4c302e678a3cd7dc1be6e661a1..37a859826c242d99c7451af3075bf3dda5fd7ac0 100644
--- a/include/kernel-build.mk
+++ b/include/kernel-build.mk
@@ -143,6 +143,7 @@ define BuildKernel
   $(LINUX_DIR)/.image: $(STAMP_CONFIGURED) $(if $(CONFIG_STRIP_KERNEL_EXPORTS),$(KERNEL_BUILD_DIR)/symtab.h) FORCE
 	$(Kernel/CompileImage)
 	$(Kernel/CollectDebug)
+	+[ -z "$(CONFIG_KERNEL_GDB_SCRIPTS)" ] || $(KERNEL_MAKE) scripts_gdb
 	touch $$@
 	
   mostlyclean: FORCE
diff --git a/target/linux/generic/config-5.10 b/target/linux/generic/config-5.10
index ffd29503ea78963d509b2a9eb63cdd2f03ff87d0..75fda4c9e0a29e79521f958620190416af18e4a9 100644
--- a/target/linux/generic/config-5.10
+++ b/target/linux/generic/config-5.10
@@ -1279,7 +1279,7 @@ CONFIG_DEBUG_FS_ALLOW_ALL=y
 # CONFIG_DEBUG_INFO_BTF is not set
 # CONFIG_DEBUG_INFO_COMPRESSED is not set
 # CONFIG_DEBUG_INFO_DWARF4 is not set
-CONFIG_DEBUG_INFO_REDUCED=y
+# CONFIG_DEBUG_INFO_REDUCED is not set
 # CONFIG_DEBUG_INFO_SPLIT is not set
 CONFIG_DEBUG_KERNEL=y
 # CONFIG_DEBUG_KMEMLEAK is not set
@@ -7181,3 +7181,15 @@ CONFIG_ZONE_DMA=y
 # CONFIG_ZRAM_MEMORY_TRACKING is not set
 # CONFIG_ZSMALLOC is not set
 # CONFIG_ZX_TDM is not set
+
+
+# KGDB specific "disabled" options
+# CONFIG_CONSOLE_TRANSLATIONS is not set
+# CONFIG_VT_CONSOLE is not set
+# CONFIG_VT_HW_CONSOLE_BINDING is not set
+# CONFIG_SERIAL_KGDB_NMI is not set
+# CONFIG_KGDB_TESTS is not set
+# CONFIG_KGDB_KDB is not set
+# CONFIG_KGDB_LOW_LEVEL_TRAP is not set
+# CONFIG_VGA_CONSOLE is not set
+CONFIG_MAGIC_SYSRQ_SERIAL_SEQUENCE="g"