Linux Debug

Tips to do debugging under Linux.

Introduction

On this page, you will find usefull informations for debugging your Linux kernel/drivers.

Testing your custom Linux kernel before flashing it

You can test a linux kernel you've generated without having to reflash your board and destroy your currently working image. Indeed Linux kernel images can be loaded and started from SDRAM with U-Boot:

 BIOS> run download_kernel 
MAC: 00:1e:ac:00:00:02
operating at 100M full duplex mode
Using dm9000 device
TFTP from server 192.168.0.104; our IP address is 192.168.0.10
Filename 'apf9328-linux.bin'.
Load address: 0x8000000
Loading: #################################################################
	 #################################################################
	 #################################################################
	 #################################################################
	 #################################################################
	 ########
done
Bytes transferred = 1703940 (1a0004 hex)
BIOS> bootm
## Booting kernel from Legacy Image at 08000000 ...

Changing processor registers from Linux user space with imxregs

This tool allows you to access i.MX registers from Linux userspace/console. This way you can debug your driver or access i.MX hardware functionnalities directly from Linux console.

In recent releases (>= 3.0) you should find this tool in /usr/bin/ on your board.

Unlock registers access

Note: On APF51/APF28/OPOS6UL you don't have to explicitly unlock registers in U-Boot to access them under Linux, like explained just above

• To use it, you must clear i.MX PAR_1 & PAR_2 registers (registers access rights) before launching Linux kernel, so in U-Boot type (example here is for APF9328):

 BIOS> mw.l 0x00200008 0
 BIOS> mw.l 0x00210008 0

• on APF27:

 BIOS> mw 10000008 0
 BIOS> mw 10020008 0

If you use it frequently, a small script unlock_regs has been defined in U-Boot, and you can call it before booting your board:

 BIOS> run unlock_regs
 BIOS> boot

Usage

• Then in Linux console/terminal, launch imxregs like that:

 # imxregs REGISTER_NAME    (give it the register name as printed in i.MX Ref Manual or just the begining of the name)

or

 # imxregs    (to dump all supported registers)

Examples

• Show OCR1 registers of each GPIO Port:

 # imxregs OCR1

• Write 0x00000123 to OCR1_D register:

 # imxregs OCR1_D 123

Changing FPGA IP's registers from Linux user space

See fpgaregs tool.

Using DebugFS

DebugFS is a in-kernel filesystem, similar to procfs or sysfs, that allows Linux driver to easily communicate debug informations to user space. Full documentation: http://lxr.linux.no/linux+v2.6.32/Documentation/filesystems/debugfs.txt or http://www.linuxtopia.org/online_books/linux_kernel/kernel_configuration/ch09s07.html

Mounting it

# mount -t debugfs none /sys/kernel/debug

Showing already allocated GPIOs

# cat /sys/kernel/debug/gpio
GPIOs 0-31, gpio-0:          
 gpio-5   (LCD                 ) in  lo
 gpio-6   (LCD                 ) in  lo
 gpio-7   (LCD                 ) in  lo
 gpio-8   (LCD                 ) in  lo
....

Showing clock tree

Device Drivers  --->
    Common Clock Framework  --->
        [*]   DebugFS representation of clock tree

| Creates a directory hierarchy in debugfs for visualizing the clk
| tree structure.  Each directory contains read-only members
| that export information specific to that clk node: clk_rate,
| clk_flags, clk_prepare_count, clk_enable_count & clk_notifier_count. 

all clocks are availables under /sys/kernel/debug/clk:

# mount -t debugfs none /sys/kernel/debug/
# cat /sys/kernel/debug/clk/clk_summary

Tracers

• http://lxr.linux.no/#linux+v3.0.22/Documentation/trace/ftrace.txt#L1016

Function profiler

$ make linux-menuconfig

Kernel hacking  --->
    [*] Tracers  --->
        [*]   Kernel Function Tracer
        ...
        [*]   Kernel function profiler


 This option enables the kernel function profiler. A file is created
 in debugfs called function_profile_enabled which defaults to zero.
 When a 1 is echoed into this file profiling begins, and when a
 zero is entered, profiling stops. A "functions" file is created in
 the trace_stats directory; this file shows the list of functions that
 have been hit and their counters.

Observing system clocks

You can "export" and observe some clocks:

• On OPOS6UL/L: signal CLKO1 is available on pad JTAG_TMS (pin 7 of J36) and signal CLKO2 is available on pad JTAG_TDO (pin 13 of J36).

Example on OPOS6UL/L:

Mux CLKO1 and CLKO2 signals on JTAG_TMS and JTAG_DO:

# devmem 0x20E0048 32 3
# devmem 0x20E004C 32 3

Export ARM clock (divided by 8) on CLKO2:

# devmem 0x20C4060 32 0x1EA0001

See the reference manual of the OPOS6UL/L for the others clocks "exportable".

Links

• http://www-users.cs.umn.edu/~boutcher/kprobes/
• http://tree.celinuxforum.org/CelfPubWiki/PatchArchive