This page contains many helpful suggestions for debugging issue with the Linux kernel.

Common Tools

grep : The kernel has thousands of files. The grep tool is the most powerful way to find what you are looking for. Use the -R to search recursively . Remember that grep is case sensitive by default, so use -i if you need it to ignore case.
- And example to find a function name start_kernel: $ grep -r "start_kernel"
find : Sometimes someone will give you just a filename, and the find command can help you find where that file is. Also, it find is very helpful when combined with grep.
- For example, if you just want to search header files: $ find * -name "*.h" | grep purple

Error Messages

Many times an error message will be printed out. This happens a lot for a device driver that has trouble initializing during boot.
Your first step should be to find what source code file is print that message
Simply use the grep command with -R option and search for that error message.

Error Codes

If a error message prints out an error code, for example "-110", you should look up what the error code means.
You can find a list of the error code numbers mean in file include/uapi/asm-generic/errno-base.h
Since the code will use the #defrine name (not the actual number), you can then search driver file for when that error code/name is returned.

Finding the Device Driver File

When adding or configuring a driver to your system, you will probably be editing the Device Tree.
If there is an issue with that driver or peripheral, you will need to find the source code for the driver
You can use the "compatible" name listed in the device tree, and grep, in order to find the location of the device driver file.
Do you search starting in the "drivers" directory (to avoid all the matches that will be found in other device tree files)

Example: Find the Watchdog Timer Driver

RZ/G2L Device Tree: rz_linux-cip/arch/arm64/boot/dts/renesas/r9a07g044.dtsi

wdt0: watchdog@12800800 {
	compatible = "renesas,r9a07g044-wdt",
		     "renesas,rzg2l-wdt";
	reg = <0 0x12800800 0 0x400>;
	clocks = <&cpg CPG_MOD R9A07G044_WDT0_PCLK>,
		 <&cpg CPG_MOD R9A07G044_WDT0_CLK>;
	clock-names = "pclk", "oscclk";
	interrupts = <GIC_SPI 49 IRQ_TYPE_LEVEL_HIGH>,
		     <GIC_SPI 50 IRQ_TYPE_LEVEL_HIGH>;
	interrupt-names = "wdt", "perrout";
	resets = <&cpg R9A07G044_WDT0_PRESETN>;
	power-domains = <&cpg>;
	status = "disabled";
};

Use the grep command:

$ grep -R "renesas,rzg2l-wdt" *
arch/arm64/boot/dts/renesas/r9a07g044.dtsi:				     "renesas,rzg2l-wdt";
arch/arm64/boot/dts/renesas/r9a07g044.dtsi:				     "renesas,rzg2l-wdt";
arch/arm64/boot/dts/renesas/r9a07g044.dtsi:				     "renesas,rzg2l-wdt";
arch/arm64/boot/dts/renesas/r9a07g043.dtsi:				     "renesas,rzg2l-wdt";
arch/arm64/boot/dts/renesas/r9a07g043.dtsi:				     "renesas,rzg2l-wdt";
arch/arm64/boot/dts/renesas/r9a07g054.dtsi:				     "renesas,rzg2l-wdt";
arch/arm64/boot/dts/renesas/r9a07g054.dtsi:				     "renesas,rzg2l-wdt";
arch/arm64/boot/dts/renesas/r9a07g054.dtsi:				     "renesas,rzg2l-wdt";
Documentation/devicetree/bindings/watchdog/renesas,wdt.yaml:          - const: renesas,rzg2l-wdt     # RZ/G2L
Documentation/devicetree/bindings/watchdog/renesas,wdt.yaml:              - renesas,rzg2l-wdt
drivers/watchdog/rzg2l_wdt.c:	{ .compatible = "renesas,rzg2l-wdt", },
scripts/dtc/include-prefixes/arm64/renesas/r9a07g044.dtsi:				     "renesas,rzg2l-wdt";
scripts/dtc/include-prefixes/arm64/renesas/r9a07g044.dtsi:				     "renesas,rzg2l-wdt";
scripts/dtc/include-prefixes/arm64/renesas/r9a07g044.dtsi:				     "renesas,rzg2l-wdt";
scripts/dtc/include-prefixes/arm64/renesas/r9a07g043.dtsi:				     "renesas,rzg2l-wdt";
scripts/dtc/include-prefixes/arm64/renesas/r9a07g043.dtsi:				     "renesas,rzg2l-wdt";
scripts/dtc/include-prefixes/arm64/renesas/r9a07g054.dtsi:				     "renesas,rzg2l-wdt";
scripts/dtc/include-prefixes/arm64/renesas/r9a07g054.dtsi:				     "renesas,rzg2l-wdt";
scripts/dtc/include-prefixes/arm64/renesas/r9a07g054.dtsi:				     "renesas,rzg2l-wdt";

The very bottom of file drivers/watchdog/rzg2l_wdt.c

static const struct of_device_id rzg2l_wdt_ids[] = {
	{ .compatible = "renesas,rzg2l-wdt", },
	{ /* sentinel */ }
};
MODULE_DEVICE_TABLE(of, rzg2l_wdt_ids);

static struct platform_driver rzg2l_wdt_driver = {
	.driver = {
		.name = "rzg2l_wdt",
		.of_match_table = rzg2l_wdt_ids,
	},
	.probe = rzg2l_wdt_probe,
};
module_platform_driver(rzg2l_wdt_driver);

MODULE_DESCRIPTION("Renesas RZ/G2L WDT Watchdog Driver");
MODULE_AUTHOR("Biju Das ");
MODULE_LICENSE("GPL v2");