Based on kernel version 4.2. Page generated on 2015-09-09 12:13 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.