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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

1				     ====================
2				     DEBUGGING FR-V LINUX
3				     ====================
4	
5	
6	The kernel contains a GDB stub that talks GDB remote protocol across a serial
7	port. This permits GDB to single step through the kernel, set breakpoints and
8	trap exceptions that happen in kernel space and interrupt execution. It also
9	permits the NMI interrupt button or serial port events to jump the kernel into
10	the debugger.
11	
12	On the CPUs that have on-chip UARTs (FR400, FR403, FR405, FR555), the
13	GDB stub hijacks a serial port for its own purposes, and makes it
14	generate level 15 interrupts (NMI). The kernel proper cannot see the serial
15	port in question under these conditions.
16	
17	On the MB93091-VDK CPU boards, the GDB stub uses UART1, which would otherwise
18	be /dev/ttyS1. On the MB93093-PDK, the GDB stub uses UART0. Therefore, on the
19	PDK there is no externally accessible serial port and the serial port to
20	which the touch screen is attached becomes /dev/ttyS0.
21	
22	Note that the GDB stub runs entirely within CPU debug mode, and so should not
23	incur any exceptions or interrupts whilst it is active. In particular, note
24	that the clock will lose time since it is implemented in software.
25	
26	
27	==================
28	KERNEL PREPARATION
29	==================
30	
31	Firstly, a debuggable kernel must be built. To do this, unpack the kernel tree
32	and copy the configuration that you wish to use to .config. Then reconfigure
33	the following things on the "Kernel Hacking" tab:
34	
35	  (*) "Include debugging information"
36	
37	      Set this to "Y". This causes all C and Assembly files to be compiled
38	      to include debugging information.
39	
40	  (*) "In-kernel GDB stub"
41	
42	      Set this to "Y". This causes the GDB stub to be compiled into the
43	      kernel.
44	
45	  (*) "Immediate activation"
46	
47	      Set this to "Y" if you want the GDB stub to activate as soon as possible
48	      and wait for GDB to connect. This allows you to start tracing right from
49	      the beginning of start_kernel() in init/main.c.
50	
51	  (*) "Console through GDB stub"
52	
53	      Set this to "Y" if you wish to be able to use "console=gdb0" on the
54	      command line. That tells the kernel to pass system console messages to
55	      GDB (which then prints them on its standard output). This is useful when
56	      debugging the serial drivers that'd otherwise be used to pass console
57	      messages to the outside world.
58	
59	Then build as usual, download to the board and execute. Note that if
60	"Immediate activation" was selected, then the kernel will wait for GDB to
61	attach. If not, then the kernel will boot immediately and GDB will have to
62	interrupt it or wait for an exception to occur before doing anything with
63	the kernel.
64	
65	
66	=========================
67	KERNEL DEBUGGING WITH GDB
68	=========================
69	
70	Set the serial port on the computer that's going to run GDB to the appropriate
71	baud rate. Assuming the board's debug port is connected to ttyS0/COM1 on the
72	computer doing the debugging:
73	
74		stty -F /dev/ttyS0 115200
75	
76	Then start GDB in the base of the kernel tree:
77	
78		frv-uclinux-gdb linux		[uClinux]
79	
80	Or:
81	
82		frv-uclinux-gdb vmlinux		[MMU linux]
83	
84	When the prompt appears:
85	
86		GNU gdb frv-031024
87		Copyright 2003 Free Software Foundation, Inc.
88		GDB is free software, covered by the GNU General Public License, and you are
89		welcome to change it and/or distribute copies of it under certain conditions.
90		Type "show copying" to see the conditions.
91		There is absolutely no warranty for GDB.  Type "show warranty" for details.
92		This GDB was configured as "--host=i686-pc-linux-gnu --target=frv-uclinux"...
93		(gdb)
94	
95	Attach to the board like this:
96	
97	        (gdb) target remote /dev/ttyS0
98		Remote debugging using /dev/ttyS0
99		start_kernel () at init/main.c:395
100		(gdb)
101	
102	This should show the appropriate lines from the source too. The kernel can
103	then be debugged almost as if it's any other program.
104	
105	
106	===============================
107	INTERRUPTING THE RUNNING KERNEL
108	===============================
109	
110	The kernel can be interrupted whilst it is running, causing a jump back to the
111	GDB stub and the debugger:
112	
113	  (*) Pressing Ctrl-C in GDB. This will cause GDB to try and interrupt the
114	      kernel by sending an RS232 BREAK over the serial line to the GDB
115	      stub. This will (mostly) immediately interrupt the kernel and return it
116	      to the debugger.
117	
118	  (*) Pressing the NMI button on the board will also cause a jump into the
119	      debugger.
120	
121	  (*) Setting a software breakpoint. This sets a break instruction at the
122	      desired location which the GDB stub then traps the exception for.
123	
124	  (*) Setting a hardware breakpoint. The GDB stub is capable of using the IBAR
125	      and DBAR registers to assist debugging.
126	
127	Furthermore, the GDB stub will intercept a number of exceptions automatically
128	if they are caused by kernel execution. It will also intercept BUG() macro
129	invocation.
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