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Jason Wessele3e2aaf2008-03-20 13:43:45 -05001<?xml version="1.0" encoding="UTF-8"?>
2<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
4
5<book id="kgdbOnLinux">
6 <bookinfo>
Jason Wessel84c08fd2010-05-20 21:04:24 -05007 <title>Using kgdb, kdb and the kernel debugger internals</title>
Jason Wessele3e2aaf2008-03-20 13:43:45 -05008
9 <authorgroup>
10 <author>
11 <firstname>Jason</firstname>
12 <surname>Wessel</surname>
13 <affiliation>
14 <address>
15 <email>jason.wessel@windriver.com</email>
16 </address>
17 </affiliation>
18 </author>
19 </authorgroup>
Jason Wessele3e2aaf2008-03-20 13:43:45 -050020 <copyright>
Jason Wessel84c08fd2010-05-20 21:04:24 -050021 <year>2008,2010</year>
Jason Wessele3e2aaf2008-03-20 13:43:45 -050022 <holder>Wind River Systems, Inc.</holder>
23 </copyright>
24 <copyright>
25 <year>2004-2005</year>
26 <holder>MontaVista Software, Inc.</holder>
27 </copyright>
28 <copyright>
29 <year>2004</year>
30 <holder>Amit S. Kale</holder>
31 </copyright>
32
33 <legalnotice>
34 <para>
35 This file is licensed under the terms of the GNU General Public License
36 version 2. This program is licensed "as is" without any warranty of any
37 kind, whether express or implied.
38 </para>
39
40 </legalnotice>
41 </bookinfo>
42
43<toc></toc>
44 <chapter id="Introduction">
45 <title>Introduction</title>
46 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -050047 The kernel has two different debugger front ends (kdb and kgdb)
48 which interface to the debug core. It is possible to use either
49 of the debugger front ends and dynamically transition between them
50 if you configure the kernel properly at compile and runtime.
Jason Wessele3e2aaf2008-03-20 13:43:45 -050051 </para>
52 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -050053 Kdb is simplistic shell-style interface which you can use on a
54 system console with a keyboard or serial console. You can use it
55 to inspect memory, registers, process lists, dmesg, and even set
56 breakpoints to stop in a certain location. Kdb is not a source
57 level debugger, although you can set breakpoints and execute some
58 basic kernel run control. Kdb is mainly aimed at doing some
59 analysis to aid in development or diagnosing kernel problems. You
60 can access some symbols by name in kernel built-ins or in kernel
61 modules if the code was built
62 with <symbol>CONFIG_KALLSYMS</symbol>.
63 </para>
64 <para>
65 Kgdb is intended to be used as a source level debugger for the
66 Linux kernel. It is used along with gdb to debug a Linux kernel.
67 The expectation is that gdb can be used to "break in" to the
68 kernel to inspect memory, variables and look through call stack
69 information similar to the way an application developer would use
70 gdb to debug an application. It is possible to place breakpoints
71 in kernel code and perform some limited execution stepping.
72 </para>
73 <para>
74 Two machines are required for using kgdb. One of these machines is
75 a development machine and the other is the target machine. The
76 kernel to be debugged runs on the target machine. The development
77 machine runs an instance of gdb against the vmlinux file which
Graham Whaleybb485c22015-02-03 16:38:02 +000078 contains the symbols (not a boot image such as bzImage, zImage,
Jason Wessel84c08fd2010-05-20 21:04:24 -050079 uImage...). In gdb the developer specifies the connection
80 parameters and connects to kgdb. The type of connection a
81 developer makes with gdb depends on the availability of kgdb I/O
82 modules compiled as built-ins or loadable kernel modules in the test
83 machine's kernel.
Jason Wessele3e2aaf2008-03-20 13:43:45 -050084 </para>
85 </chapter>
86 <chapter id="CompilingAKernel">
Jason Wessel84c08fd2010-05-20 21:04:24 -050087 <title>Compiling a kernel</title>
88 <para>
89 <itemizedlist>
90 <listitem><para>In order to enable compilation of kdb, you must first enable kgdb.</para></listitem>
91 <listitem><para>The kgdb test compile options are described in the kgdb test suite chapter.</para></listitem>
92 </itemizedlist>
93 </para>
94 <sect1 id="CompileKGDB">
95 <title>Kernel config options for kgdb</title>
Jason Wessele3e2aaf2008-03-20 13:43:45 -050096 <para>
Kees Cook0335cb42012-10-02 11:16:15 -070097 To enable <symbol>CONFIG_KGDB</symbol> you should look under
Graham Whaleybb485c22015-02-03 16:38:02 +000098 "Kernel hacking" / "Kernel debugging" and select "KGDB: kernel debugger".
Jason Wessel84c08fd2010-05-20 21:04:24 -050099 </para>
100 <para>
101 While it is not a hard requirement that you have symbols in your
102 vmlinux file, gdb tends not to be very useful without the symbolic
103 data, so you will want to turn
104 on <symbol>CONFIG_DEBUG_INFO</symbol> which is called "Compile the
105 kernel with debug info" in the config menu.
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500106 </para>
107 <para>
Graham Whaleybb485c22015-02-03 16:38:02 +0000108 It is advised, but not required, that you turn on the
Jason Wessel84c08fd2010-05-20 21:04:24 -0500109 <symbol>CONFIG_FRAME_POINTER</symbol> kernel option which is called "Compile the
110 kernel with frame pointers" in the config menu. This option
111 inserts code to into the compiled executable which saves the frame
112 information in registers or on the stack at different points which
113 allows a debugger such as gdb to more accurately construct
114 stack back traces while debugging the kernel.
Jason Wessel5f5ddfb2008-08-01 08:39:34 -0500115 </para>
116 <para>
Jason Wessela9b60bf2008-08-01 08:39:34 -0500117 If the architecture that you are using supports the kernel option
118 CONFIG_DEBUG_RODATA, you should consider turning it off. This
119 option will prevent the use of software breakpoints because it
120 marks certain regions of the kernel's memory space as read-only.
121 If kgdb supports it for the architecture you are using, you can
122 use hardware breakpoints if you desire to run with the
123 CONFIG_DEBUG_RODATA option turned on, else you need to turn off
124 this option.
125 </para>
126 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500127 Next you should choose one of more I/O drivers to interconnect
128 debugging host and debugged target. Early boot debugging requires
129 a KGDB I/O driver that supports early debugging and the driver
130 must be built into the kernel directly. Kgdb I/O driver
131 configuration takes place via kernel or module parameters which
132 you can learn more about in the in the section that describes the
133 parameter "kgdboc".
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500134 </para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500135 <para>Here is an example set of .config symbols to enable or
136 disable for kgdb:
137 <itemizedlist>
138 <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
139 <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
140 <listitem><para>CONFIG_KGDB=y</para></listitem>
141 <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
142 </itemizedlist>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500143 </para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500144 </sect1>
145 <sect1 id="CompileKDB">
146 <title>Kernel config options for kdb</title>
147 <para>Kdb is quite a bit more complex than the simple gdbstub
148 sitting on top of the kernel's debug core. Kdb must implement a
149 shell, and also adds some helper functions in other parts of the
150 kernel, responsible for printing out interesting data such as what
151 you would see if you ran "lsmod", or "ps". In order to build kdb
152 into the kernel you follow the same steps as you would for kgdb.
153 </para>
154 <para>The main config option for kdb
155 is <symbol>CONFIG_KGDB_KDB</symbol> which is called "KGDB_KDB:
156 include kdb frontend for kgdb" in the config menu. In theory you
157 would have already also selected an I/O driver such as the
158 CONFIG_KGDB_SERIAL_CONSOLE interface if you plan on using kdb on a
159 serial port, when you were configuring kgdb.
160 </para>
161 <para>If you want to use a PS/2-style keyboard with kdb, you would
162 select CONFIG_KDB_KEYBOARD which is called "KGDB_KDB: keyboard as
163 input device" in the config menu. The CONFIG_KDB_KEYBOARD option
164 is not used for anything in the gdb interface to kgdb. The
165 CONFIG_KDB_KEYBOARD option only works with kdb.
166 </para>
167 <para>Here is an example set of .config symbols to enable/disable kdb:
168 <itemizedlist>
169 <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
170 <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
171 <listitem><para>CONFIG_KGDB=y</para></listitem>
172 <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
173 <listitem><para>CONFIG_KGDB_KDB=y</para></listitem>
174 <listitem><para>CONFIG_KDB_KEYBOARD=y</para></listitem>
175 </itemizedlist>
176 </para>
177 </sect1>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500178 </chapter>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500179 <chapter id="kgdbKernelArgs">
180 <title>Kernel Debugger Boot Arguments</title>
181 <para>This section describes the various runtime kernel
182 parameters that affect the configuration of the kernel debugger.
183 The following chapter covers using kdb and kgdb as well as
Graham Whaleybb485c22015-02-03 16:38:02 +0000184 providing some examples of the configuration parameters.</para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500185 <sect1 id="kgdboc">
186 <title>Kernel parameter: kgdboc</title>
187 <para>The kgdboc driver was originally an abbreviation meant to
188 stand for "kgdb over console". Today it is the primary mechanism
189 to configure how to communicate from gdb to kgdb as well as the
190 devices you want to use to interact with the kdb shell.
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500191 </para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500192 <para>For kgdb/gdb, kgdboc is designed to work with a single serial
193 port. It is intended to cover the circumstance where you want to
194 use a serial console as your primary console as well as using it to
195 perform kernel debugging. It is also possible to use kgdb on a
196 serial port which is not designated as a system console. Kgdboc
197 may be configured as a kernel built-in or a kernel loadable module.
198 You can only make use of <constant>kgdbwait</constant> and early
199 debugging if you build kgdboc into the kernel as a built-in.
Rajaneesh Acharyadd8f30c2014-12-31 17:43:19 +0000200 </para>
Jason Wessel65b5ac12010-08-05 09:22:33 -0500201 <para>Optionally you can elect to activate kms (Kernel Mode
202 Setting) integration. When you use kms with kgdboc and you have a
203 video driver that has atomic mode setting hooks, it is possible to
204 enter the debugger on the graphics console. When the kernel
205 execution is resumed, the previous graphics mode will be restored.
206 This integration can serve as a useful tool to aid in diagnosing
207 crashes or doing analysis of memory with kdb while allowing the
208 full graphics console applications to run.
209 </para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500210 <sect2 id="kgdbocArgs">
211 <title>kgdboc arguments</title>
Jason Wessel65b5ac12010-08-05 09:22:33 -0500212 <para>Usage: <constant>kgdboc=[kms][[,]kbd][[,]serial_device][,baud]</constant></para>
213 <para>The order listed above must be observed if you use any of the
214 optional configurations together.
215 </para>
216 <para>Abbreviations:
217 <itemizedlist>
218 <listitem><para>kms = Kernel Mode Setting</para></listitem>
219 <listitem><para>kbd = Keyboard</para></listitem>
220 </itemizedlist>
221 </para>
Graham Whaleybb485c22015-02-03 16:38:02 +0000222 <para>You can configure kgdboc to use the keyboard, and/or a serial
223 device depending on if you are using kdb and/or kgdb, in one of the
Jason Wessel65b5ac12010-08-05 09:22:33 -0500224 following scenarios. The order listed above must be observed if
225 you use any of the optional configurations together. Using kms +
226 only gdb is generally not a useful combination.</para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500227 <sect3 id="kgdbocArgs1">
228 <title>Using loadable module or built-in</title>
229 <para>
230 <orderedlist>
231 <listitem><para>As a kernel built-in:</para>
232 <para>Use the kernel boot argument: <constant>kgdboc=&lt;tty-device&gt;,[baud]</constant></para></listitem>
233 <listitem>
234 <para>As a kernel loadable module:</para>
235 <para>Use the command: <constant>modprobe kgdboc kgdboc=&lt;tty-device&gt;,[baud]</constant></para>
Jason Wessel65b5ac12010-08-05 09:22:33 -0500236 <para>Here are two examples of how you might format the kgdboc
Jason Wessel84c08fd2010-05-20 21:04:24 -0500237 string. The first is for an x86 target using the first serial port.
238 The second example is for the ARM Versatile AB using the second
239 serial port.
240 <orderedlist>
241 <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
242 <listitem><para><constant>kgdboc=ttyAMA1,115200</constant></para></listitem>
243 </orderedlist>
244 </para>
245 </listitem>
246 </orderedlist></para>
247 </sect3>
248 <sect3 id="kgdbocArgs2">
249 <title>Configure kgdboc at runtime with sysfs</title>
250 <para>At run time you can enable or disable kgdboc by echoing a
251 parameters into the sysfs. Here are two examples:</para>
252 <orderedlist>
253 <listitem><para>Enable kgdboc on ttyS0</para>
254 <para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
255 <listitem><para>Disable kgdboc</para>
256 <para><constant>echo "" &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
257 </orderedlist>
258 <para>NOTE: You do not need to specify the baud if you are
259 configuring the console on tty which is already configured or
260 open.</para>
261 </sect3>
262 <sect3 id="kgdbocArgs3">
263 <title>More examples</title>
Graham Whaleybb485c22015-02-03 16:38:02 +0000264 <para>You can configure kgdboc to use the keyboard, and/or a serial device
265 depending on if you are using kdb and/or kgdb, in one of the
Jason Wessel84c08fd2010-05-20 21:04:24 -0500266 following scenarios.
267 <orderedlist>
268 <listitem><para>kdb and kgdb over only a serial port</para>
269 <para><constant>kgdboc=&lt;serial_device&gt;[,baud]</constant></para>
270 <para>Example: <constant>kgdboc=ttyS0,115200</constant></para>
271 </listitem>
272 <listitem><para>kdb and kgdb with keyboard and a serial port</para>
273 <para><constant>kgdboc=kbd,&lt;serial_device&gt;[,baud]</constant></para>
274 <para>Example: <constant>kgdboc=kbd,ttyS0,115200</constant></para>
275 </listitem>
276 <listitem><para>kdb with a keyboard</para>
277 <para><constant>kgdboc=kbd</constant></para>
278 </listitem>
Jason Wessel65b5ac12010-08-05 09:22:33 -0500279 <listitem><para>kdb with kernel mode setting</para>
280 <para><constant>kgdboc=kms,kbd</constant></para>
281 </listitem>
282 <listitem><para>kdb with kernel mode setting and kgdb over a serial port</para>
283 <para><constant>kgdboc=kms,kbd,ttyS0,115200</constant></para>
284 </listitem>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500285 </orderedlist>
286 </para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500287 <para>NOTE: Kgdboc does not support interrupting the target via the
288 gdb remote protocol. You must manually send a sysrq-g unless you
289 have a proxy that splits console output to a terminal program.
290 A console proxy has a separate TCP port for the debugger and a separate
291 TCP port for the "human" console. The proxy can take care of sending
292 the sysrq-g for you.
293 </para>
294 <para>When using kgdboc with no debugger proxy, you can end up
295 connecting the debugger at one of two entry points. If an
296 exception occurs after you have loaded kgdboc, a message should
297 print on the console stating it is waiting for the debugger. In
298 this case you disconnect your terminal program and then connect the
299 debugger in its place. If you want to interrupt the target system
300 and forcibly enter a debug session you have to issue a Sysrq
301 sequence and then type the letter <constant>g</constant>. Then
302 you disconnect the terminal session and connect gdb. Your options
303 if you don't like this are to hack gdb to send the sysrq-g for you
304 as well as on the initial connect, or to use a debugger proxy that
305 allows an unmodified gdb to do the debugging.
306 </para>
Rajaneesh Acharyadd8f30c2014-12-31 17:43:19 +0000307 </sect3>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500308 </sect2>
309 </sect1>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500310 <sect1 id="kgdbwait">
311 <title>Kernel parameter: kgdbwait</title>
312 <para>
313 The Kernel command line option <constant>kgdbwait</constant> makes
314 kgdb wait for a debugger connection during booting of a kernel. You
Graham Whaleybb485c22015-02-03 16:38:02 +0000315 can only use this option if you compiled a kgdb I/O driver into the
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500316 kernel and you specified the I/O driver configuration as a kernel
317 command line option. The kgdbwait parameter should always follow the
318 configuration parameter for the kgdb I/O driver in the kernel
319 command line else the I/O driver will not be configured prior to
320 asking the kernel to use it to wait.
321 </para>
322 <para>
323 The kernel will stop and wait as early as the I/O driver and
Jason Wessel84c08fd2010-05-20 21:04:24 -0500324 architecture allows when you use this option. If you build the
325 kgdb I/O driver as a loadable kernel module kgdbwait will not do
326 anything.
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500327 </para>
328 </sect1>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500329 <sect1 id="kgdbcon">
330 <title>Kernel parameter: kgdbcon</title>
331 <para> The kgdbcon feature allows you to see printk() messages
332 inside gdb while gdb is connected to the kernel. Kdb does not make
333 use of the kgdbcon feature.
334 </para>
335 <para>Kgdb supports using the gdb serial protocol to send console
336 messages to the debugger when the debugger is connected and running.
337 There are two ways to activate this feature.
338 <orderedlist>
339 <listitem><para>Activate with the kernel command line option:</para>
340 <para><constant>kgdbcon</constant></para>
341 </listitem>
342 <listitem><para>Use sysfs before configuring an I/O driver</para>
343 <para>
344 <constant>echo 1 &gt; /sys/module/kgdb/parameters/kgdb_use_con</constant>
345 </para>
346 <para>
347 NOTE: If you do this after you configure the kgdb I/O driver, the
348 setting will not take effect until the next point the I/O is
349 reconfigured.
350 </para>
351 </listitem>
352 </orderedlist>
Rajaneesh Acharyadd8f30c2014-12-31 17:43:19 +0000353 </para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500354 <para>IMPORTANT NOTE: You cannot use kgdboc + kgdbcon on a tty that is an
Graham Whaleybb485c22015-02-03 16:38:02 +0000355 active system console. An example of incorrect usage is <constant>console=ttyS0,115200 kgdboc=ttyS0 kgdbcon</constant>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500356 </para>
357 <para>It is possible to use this option with kgdboc on a tty that is not a system console.
358 </para>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500359 </sect1>
Jason Wesselbec4d622012-03-19 19:35:55 -0500360 <sect1 id="kgdbreboot">
361 <title>Run time parameter: kgdbreboot</title>
362 <para> The kgdbreboot feature allows you to change how the debugger
363 deals with the reboot notification. You have 3 choices for the
364 behavior. The default behavior is always set to 0.</para>
365 <orderedlist>
366 <listitem><para>echo -1 > /sys/module/debug_core/parameters/kgdbreboot</para>
367 <para>Ignore the reboot notification entirely.</para>
368 </listitem>
369 <listitem><para>echo 0 > /sys/module/debug_core/parameters/kgdbreboot</para>
370 <para>Send the detach message to any attached debugger client.</para>
371 </listitem>
372 <listitem><para>echo 1 > /sys/module/debug_core/parameters/kgdbreboot</para>
373 <para>Enter the debugger on reboot notify.</para>
374 </listitem>
375 </orderedlist>
376 </sect1>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500377 </chapter>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500378 <chapter id="usingKDB">
379 <title>Using kdb</title>
380 <para>
381 </para>
382 <sect1 id="quickKDBserial">
383 <title>Quick start for kdb on a serial port</title>
384 <para>This is a quick example of how to use kdb.</para>
385 <para><orderedlist>
Graham Whaleybb485c22015-02-03 16:38:02 +0000386 <listitem><para>Configure kgdboc at boot using kernel parameters:
Jason Wessel84c08fd2010-05-20 21:04:24 -0500387 <itemizedlist>
388 <listitem><para><constant>console=ttyS0,115200 kgdboc=ttyS0,115200</constant></para></listitem>
389 </itemizedlist></para>
390 <para>OR</para>
Graham Whaleybb485c22015-02-03 16:38:02 +0000391 <para>Configure kgdboc after the kernel has booted; assuming you are using a serial port console:
Jason Wessel84c08fd2010-05-20 21:04:24 -0500392 <itemizedlist>
393 <listitem><para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
394 </itemizedlist>
395 </para>
396 </listitem>
397 <listitem><para>Enter the kernel debugger manually or by waiting for an oops or fault. There are several ways you can enter the kernel debugger manually; all involve using the sysrq-g, which means you must have enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.</para>
398 <itemizedlist>
399 <listitem><para>When logged in as root or with a super user session you can run:</para>
400 <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
401 <listitem><para>Example using minicom 2.2</para>
402 <para>Press: <constant>Control-a</constant></para>
403 <para>Press: <constant>f</constant></para>
404 <para>Press: <constant>g</constant></para>
405 </listitem>
406 <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
407 <para>Press: <constant>Control-]</constant></para>
408 <para>Type in:<constant>send break</constant></para>
409 <para>Press: <constant>Enter</constant></para>
410 <para>Press: <constant>g</constant></para>
411 </listitem>
412 </itemizedlist>
413 </listitem>
414 <listitem><para>From the kdb prompt you can run the "help" command to see a complete list of the commands that are available.</para>
415 <para>Some useful commands in kdb include:
416 <itemizedlist>
417 <listitem><para>lsmod -- Shows where kernel modules are loaded</para></listitem>
418 <listitem><para>ps -- Displays only the active processes</para></listitem>
419 <listitem><para>ps A -- Shows all the processes</para></listitem>
420 <listitem><para>summary -- Shows kernel version info and memory usage</para></listitem>
421 <listitem><para>bt -- Get a backtrace of the current process using dump_stack()</para></listitem>
422 <listitem><para>dmesg -- View the kernel syslog buffer</para></listitem>
423 <listitem><para>go -- Continue the system</para></listitem>
424 </itemizedlist>
425 </para>
426 </listitem>
427 <listitem>
428 <para>When you are done using kdb you need to consider rebooting the
429 system or using the "go" command to resuming normal kernel
430 execution. If you have paused the kernel for a lengthy period of
431 time, applications that rely on timely networking or anything to do
432 with real wall clock time could be adversely affected, so you
433 should take this into consideration when using the kernel
434 debugger.</para>
435 </listitem>
436 </orderedlist></para>
437 </sect1>
438 <sect1 id="quickKDBkeyboard">
439 <title>Quick start for kdb using a keyboard connected console</title>
440 <para>This is a quick example of how to use kdb with a keyboard.</para>
441 <para><orderedlist>
Graham Whaleybb485c22015-02-03 16:38:02 +0000442 <listitem><para>Configure kgdboc at boot using kernel parameters:
Jason Wessel84c08fd2010-05-20 21:04:24 -0500443 <itemizedlist>
444 <listitem><para><constant>kgdboc=kbd</constant></para></listitem>
445 </itemizedlist></para>
446 <para>OR</para>
Graham Whaleybb485c22015-02-03 16:38:02 +0000447 <para>Configure kgdboc after the kernel has booted:
Jason Wessel84c08fd2010-05-20 21:04:24 -0500448 <itemizedlist>
449 <listitem><para><constant>echo kbd &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
450 </itemizedlist>
451 </para>
452 </listitem>
453 <listitem><para>Enter the kernel debugger manually or by waiting for an oops or fault. There are several ways you can enter the kernel debugger manually; all involve using the sysrq-g, which means you must have enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.</para>
454 <itemizedlist>
455 <listitem><para>When logged in as root or with a super user session you can run:</para>
456 <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
457 <listitem><para>Example using a laptop keyboard</para>
458 <para>Press and hold down: <constant>Alt</constant></para>
459 <para>Press and hold down: <constant>Fn</constant></para>
460 <para>Press and release the key with the label: <constant>SysRq</constant></para>
461 <para>Release: <constant>Fn</constant></para>
462 <para>Press and release: <constant>g</constant></para>
463 <para>Release: <constant>Alt</constant></para>
464 </listitem>
465 <listitem><para>Example using a PS/2 101-key keyboard</para>
466 <para>Press and hold down: <constant>Alt</constant></para>
467 <para>Press and release the key with the label: <constant>SysRq</constant></para>
468 <para>Press and release: <constant>g</constant></para>
469 <para>Release: <constant>Alt</constant></para>
470 </listitem>
471 </itemizedlist>
472 </listitem>
473 <listitem>
474 <para>Now type in a kdb command such as "help", "dmesg", "bt" or "go" to continue kernel execution.</para>
475 </listitem>
476 </orderedlist></para>
477 </sect1>
478 </chapter>
479 <chapter id="EnableKGDB">
480 <title>Using kgdb / gdb</title>
481 <para>In order to use kgdb you must activate it by passing
482 configuration information to one of the kgdb I/O drivers. If you
483 do not pass any configuration information kgdb will not do anything
484 at all. Kgdb will only actively hook up to the kernel trap hooks
485 if a kgdb I/O driver is loaded and configured. If you unconfigure
486 a kgdb I/O driver, kgdb will unregister all the kernel hook points.
487 </para>
488 <para> All kgdb I/O drivers can be reconfigured at run time, if
489 <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
490 are enabled, by echo'ing a new config string to
491 <constant>/sys/module/&lt;driver&gt;/parameter/&lt;option&gt;</constant>.
492 The driver can be unconfigured by passing an empty string. You cannot
493 change the configuration while the debugger is attached. Make sure
494 to detach the debugger with the <constant>detach</constant> command
495 prior to trying to unconfigure a kgdb I/O driver.
496 </para>
497 <sect1 id="ConnectingGDB">
498 <title>Connecting with gdb to a serial port</title>
499 <orderedlist>
500 <listitem><para>Configure kgdboc</para>
Graham Whaleybb485c22015-02-03 16:38:02 +0000501 <para>Configure kgdboc at boot using kernel parameters:
Jason Wessel84c08fd2010-05-20 21:04:24 -0500502 <itemizedlist>
503 <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
504 </itemizedlist></para>
505 <para>OR</para>
Graham Whaleybb485c22015-02-03 16:38:02 +0000506 <para>Configure kgdboc after the kernel has booted:
Jason Wessel84c08fd2010-05-20 21:04:24 -0500507 <itemizedlist>
508 <listitem><para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
509 </itemizedlist></para>
510 </listitem>
511 <listitem>
512 <para>Stop kernel execution (break into the debugger)</para>
513 <para>In order to connect to gdb via kgdboc, the kernel must
514 first be stopped. There are several ways to stop the kernel which
515 include using kgdbwait as a boot argument, via a sysrq-g, or running
516 the kernel until it takes an exception where it waits for the
517 debugger to attach.
518 <itemizedlist>
519 <listitem><para>When logged in as root or with a super user session you can run:</para>
520 <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
521 <listitem><para>Example using minicom 2.2</para>
522 <para>Press: <constant>Control-a</constant></para>
523 <para>Press: <constant>f</constant></para>
524 <para>Press: <constant>g</constant></para>
525 </listitem>
526 <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
527 <para>Press: <constant>Control-]</constant></para>
528 <para>Type in:<constant>send break</constant></para>
529 <para>Press: <constant>Enter</constant></para>
530 <para>Press: <constant>g</constant></para>
531 </listitem>
532 </itemizedlist>
533 </para>
534 </listitem>
535 <listitem>
Graham Whaleybb485c22015-02-03 16:38:02 +0000536 <para>Connect from gdb</para>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500537 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500538 Example (using a directly connected port):
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500539 </para>
540 <programlisting>
541 % gdb ./vmlinux
542 (gdb) set remotebaud 115200
543 (gdb) target remote /dev/ttyS0
544 </programlisting>
545 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500546 Example (kgdb to a terminal server on TCP port 2012):
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500547 </para>
548 <programlisting>
549 % gdb ./vmlinux
Jason Wessela606b5e2008-06-24 10:52:55 -0500550 (gdb) target remote 192.168.2.2:2012
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500551 </programlisting>
552 <para>
553 Once connected, you can debug a kernel the way you would debug an
554 application program.
555 </para>
556 <para>
557 If you are having problems connecting or something is going
558 seriously wrong while debugging, it will most often be the case
559 that you want to enable gdb to be verbose about its target
560 communications. You do this prior to issuing the <constant>target
Frank Rowandd34a7922009-05-15 07:56:25 -0500561 remote</constant> command by typing in: <constant>set debug remote 1</constant>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500562 </para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500563 </listitem>
564 </orderedlist>
565 <para>Remember if you continue in gdb, and need to "break in" again,
566 you need to issue an other sysrq-g. It is easy to create a simple
567 entry point by putting a breakpoint at <constant>sys_sync</constant>
568 and then you can run "sync" from a shell or script to break into the
569 debugger.</para>
570 </sect1>
571 </chapter>
572 <chapter id="switchKdbKgdb">
573 <title>kgdb and kdb interoperability</title>
574 <para>It is possible to transition between kdb and kgdb dynamically.
575 The debug core will remember which you used the last time and
576 automatically start in the same mode.</para>
577 <sect1>
578 <title>Switching between kdb and kgdb</title>
579 <sect2>
580 <title>Switching from kgdb to kdb</title>
581 <para>
582 There are two ways to switch from kgdb to kdb: you can use gdb to
583 issue a maintenance packet, or you can blindly type the command $3#33.
Graham Whaleybb485c22015-02-03 16:38:02 +0000584 Whenever the kernel debugger stops in kgdb mode it will print the
Jason Wessel84c08fd2010-05-20 21:04:24 -0500585 message <constant>KGDB or $3#33 for KDB</constant>. It is important
586 to note that you have to type the sequence correctly in one pass.
587 You cannot type a backspace or delete because kgdb will interpret
588 that as part of the debug stream.
589 <orderedlist>
590 <listitem><para>Change from kgdb to kdb by blindly typing:</para>
591 <para><constant>$3#33</constant></para></listitem>
592 <listitem><para>Change from kgdb to kdb with gdb</para>
593 <para><constant>maintenance packet 3</constant></para>
594 <para>NOTE: Now you must kill gdb. Typically you press control-z and
595 issue the command: kill -9 %</para></listitem>
596 </orderedlist>
597 </para>
598 </sect2>
599 <sect2>
600 <title>Change from kdb to kgdb</title>
601 <para>There are two ways you can change from kdb to kgdb. You can
602 manually enter kgdb mode by issuing the kgdb command from the kdb
603 shell prompt, or you can connect gdb while the kdb shell prompt is
604 active. The kdb shell looks for the typical first commands that gdb
605 would issue with the gdb remote protocol and if it sees one of those
606 commands it automatically changes into kgdb mode.</para>
607 <orderedlist>
608 <listitem><para>From kdb issue the command:</para>
609 <para><constant>kgdb</constant></para>
610 <para>Now disconnect your terminal program and connect gdb in its place</para></listitem>
611 <listitem><para>At the kdb prompt, disconnect the terminal program and connect gdb in its place.</para></listitem>
612 </orderedlist>
613 </sect2>
614 </sect1>
615 <sect1>
616 <title>Running kdb commands from gdb</title>
617 <para>It is possible to run a limited set of kdb commands from gdb,
618 using the gdb monitor command. You don't want to execute any of the
619 run control or breakpoint operations, because it can disrupt the
620 state of the kernel debugger. You should be using gdb for
621 breakpoints and run control operations if you have gdb connected.
622 The more useful commands to run are things like lsmod, dmesg, ps or
623 possibly some of the memory information commands. To see all the kdb
624 commands you can run <constant>monitor help</constant>.</para>
625 <para>Example:
626 <informalexample><programlisting>
627(gdb) monitor ps
6281 idle process (state I) and
62927 sleeping system daemon (state M) processes suppressed,
630use 'ps A' to see all.
631Task Addr Pid Parent [*] cpu State Thread Command
632
6330xc78291d0 1 0 0 0 S 0xc7829404 init
6340xc7954150 942 1 0 0 S 0xc7954384 dropbear
6350xc78789c0 944 1 0 0 S 0xc7878bf4 sh
636(gdb)
637 </programlisting></informalexample>
638 </para>
639 </sect1>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500640 </chapter>
641 <chapter id="KGDBTestSuite">
642 <title>kgdb Test Suite</title>
643 <para>
644 When kgdb is enabled in the kernel config you can also elect to
645 enable the config parameter KGDB_TESTS. Turning this on will
646 enable a special kgdb I/O module which is designed to test the
647 kgdb internal functions.
648 </para>
649 <para>
650 The kgdb tests are mainly intended for developers to test the kgdb
651 internals as well as a tool for developing a new kgdb architecture
652 specific implementation. These tests are not really for end users
653 of the Linux kernel. The primary source of documentation would be
654 to look in the drivers/misc/kgdbts.c file.
655 </para>
656 <para>
657 The kgdb test suite can also be configured at compile time to run
658 the core set of tests by setting the kernel config parameter
659 KGDB_TESTS_ON_BOOT. This particular option is aimed at automated
660 regression testing and does not require modifying the kernel boot
661 config arguments. If this is turned on, the kgdb test suite can
662 be disabled by specifying "kgdbts=" as a kernel boot argument.
663 </para>
664 </chapter>
665 <chapter id="CommonBackEndReq">
Jason Wessel84c08fd2010-05-20 21:04:24 -0500666 <title>Kernel Debugger Internals</title>
Jason Wessel225a4422008-04-01 16:55:26 -0500667 <sect1 id="kgdbArchitecture">
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500668 <title>Architecture Specifics</title>
669 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500670 The kernel debugger is organized into a number of components:
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500671 <orderedlist>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500672 <listitem><para>The debug core</para>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500673 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500674 The debug core is found in kernel/debugger/debug_core.c. It contains:
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500675 <itemizedlist>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500676 <listitem><para>A generic OS exception handler which includes
677 sync'ing the processors into a stopped state on an multi-CPU
678 system.</para></listitem>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500679 <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500680 <listitem><para>The API to make calls to the arch-specific kgdb implementation</para></listitem>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500681 <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
682 <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500683 <listitem><para>The API to invoke either the kdb or kgdb frontend to the debug core.</para></listitem>
Jason Wessel65b5ac12010-08-05 09:22:33 -0500684 <listitem><para>The structures and callback API for atomic kernel mode setting.</para>
685 <para>NOTE: kgdboc is where the kms callbacks are invoked.</para></listitem>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500686 </itemizedlist>
687 </para>
688 </listitem>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500689 <listitem><para>kgdb arch-specific implementation</para>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500690 <para>
691 This implementation is generally found in arch/*/kernel/kgdb.c.
692 As an example, arch/x86/kernel/kgdb.c contains the specifics to
693 implement HW breakpoint as well as the initialization to
694 dynamically register and unregister for the trap handlers on
Jason Wessel84c08fd2010-05-20 21:04:24 -0500695 this architecture. The arch-specific portion implements:
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500696 <itemizedlist>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500697 <listitem><para>contains an arch-specific trap catcher which
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500698 invokes kgdb_handle_exception() to start kgdb about doing its
699 work</para></listitem>
700 <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
701 <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
702 <listitem><para>Any special exception handling and cleanup</para></listitem>
703 <listitem><para>NMI exception handling and cleanup</para></listitem>
Graham Whaleybb485c22015-02-03 16:38:02 +0000704 <listitem><para>(optional) HW breakpoints</para></listitem>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500705 </itemizedlist>
706 </para>
707 </listitem>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500708 <listitem><para>gdbstub frontend (aka kgdb)</para>
709 <para>The gdbstub is located in kernel/debug/gdbstub.c. It contains:</para>
710 <itemizedlist>
711 <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
712 </itemizedlist>
713 </listitem>
714 <listitem><para>kdb frontend</para>
715 <para>The kdb debugger shell is broken down into a number of
716 components. The kdb core is located in kernel/debug/kdb. There
717 are a number of helper functions in some of the other kernel
718 components to make it possible for kdb to examine and report
719 information about the kernel without taking locks that could
720 cause a kernel deadlock. The kdb core contains implements the following functionality.</para>
721 <itemizedlist>
722 <listitem><para>A simple shell</para></listitem>
723 <listitem><para>The kdb core command set</para></listitem>
724 <listitem><para>A registration API to register additional kdb shell commands.</para>
Jason Wessel4aad8f52010-10-25 10:14:05 -0500725 <itemizedlist>
726 <listitem><para>A good example of a self-contained kdb module
727 is the "ftdump" command for dumping the ftrace buffer. See:
728 kernel/trace/trace_kdb.c</para></listitem>
729 <listitem><para>For an example of how to dynamically register
730 a new kdb command you can build the kdb_hello.ko kernel module
731 from samples/kdb/kdb_hello.c. To build this example you can
732 set CONFIG_SAMPLES=y and CONFIG_SAMPLE_KDB=m in your kernel
733 config. Later run "modprobe kdb_hello" and the next time you
734 enter the kdb shell, you can run the "hello"
735 command.</para></listitem>
736 </itemizedlist></listitem>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500737 <listitem><para>The implementation for kdb_printf() which
738 emits messages directly to I/O drivers, bypassing the kernel
739 log.</para></listitem>
740 <listitem><para>SW / HW breakpoint management for the kdb shell</para></listitem>
741 </itemizedlist>
742 </listitem>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500743 <listitem><para>kgdb I/O driver</para>
744 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500745 Each kgdb I/O driver has to provide an implementation for the following:
Jason Wessel225a4422008-04-01 16:55:26 -0500746 <itemizedlist>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500747 <listitem><para>configuration via built-in or module</para></listitem>
Jason Wessel225a4422008-04-01 16:55:26 -0500748 <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
749 <listitem><para>read and write character interface</para></listitem>
750 <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
751 <listitem><para>(optional) Early debug methodology</para></listitem>
752 </itemizedlist>
753 Any given kgdb I/O driver has to operate very closely with the
754 hardware and must do it in such a way that does not enable
755 interrupts or change other parts of the system context without
756 completely restoring them. The kgdb core will repeatedly "poll"
757 a kgdb I/O driver for characters when it needs input. The I/O
758 driver is expected to return immediately if there is no data
759 available. Doing so allows for the future possibility to touch
Graham Whaleybb485c22015-02-03 16:38:02 +0000760 watchdog hardware in such a way as to have a target system not
Jason Wessel225a4422008-04-01 16:55:26 -0500761 reset when these are enabled.
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500762 </para>
763 </listitem>
764 </orderedlist>
765 </para>
766 <para>
767 If you are intent on adding kgdb architecture specific support
768 for a new architecture, the architecture should define
769 <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
770 Kconfig file. This will enable kgdb for the architecture, and
771 at that point you must create an architecture specific kgdb
772 implementation.
773 </para>
774 <para>
775 There are a few flags which must be set on every architecture in
776 their &lt;asm/kgdb.h&gt; file. These are:
777 <itemizedlist>
778 <listitem>
Graham Whaleybb485c22015-02-03 16:38:02 +0000779 <para>
780 NUMREGBYTES: The size in bytes of all of the registers, so
781 that we can ensure they will all fit into a packet.
782 </para>
783 </listitem>
784 <listitem>
785 <para>
786 BUFMAX: The size in bytes of the buffer GDB will read into.
787 This must be larger than NUMREGBYTES.
788 </para>
789 </listitem>
790 <listitem>
791 <para>
792 CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
793 flush_cache_range or flush_icache_range. On some architectures,
794 these functions may not be safe to call on SMP since we keep other
795 CPUs in a holding pattern.
796 </para>
797 </listitem>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500798 </itemizedlist>
799 </para>
800 <para>
801 There are also the following functions for the common backend,
802 found in kernel/kgdb.c, that must be supplied by the
803 architecture-specific backend unless marked as (optional), in
804 which case a default function maybe used if the architecture
805 does not need to provide a specific implementation.
806 </para>
807!Iinclude/linux/kgdb.h
Jason Wessel225a4422008-04-01 16:55:26 -0500808 </sect1>
809 <sect1 id="kgdbocDesign">
810 <title>kgdboc internals</title>
Jason Wessel65b5ac12010-08-05 09:22:33 -0500811 <sect2>
812 <title>kgdboc and uarts</title>
Jason Wessel225a4422008-04-01 16:55:26 -0500813 <para>
814 The kgdboc driver is actually a very thin driver that relies on the
815 underlying low level to the hardware driver having "polling hooks"
Graham Whaleybb485c22015-02-03 16:38:02 +0000816 to which the tty driver is attached. In the initial
817 implementation of kgdboc the serial_core was changed to expose a
Jason Wessel84c08fd2010-05-20 21:04:24 -0500818 low level UART hook for doing polled mode reading and writing of a
Jason Wessel225a4422008-04-01 16:55:26 -0500819 single character while in an atomic context. When kgdb makes an I/O
Jason Wessel65b5ac12010-08-05 09:22:33 -0500820 request to the debugger, kgdboc invokes a callback in the serial
821 core which in turn uses the callback in the UART driver.</para>
Jason Wessel225a4422008-04-01 16:55:26 -0500822 <para>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500823 When using kgdboc with a UART, the UART driver must implement two callbacks in the <constant>struct uart_ops</constant>. Example from drivers/8250.c:<programlisting>
Jason Wessel225a4422008-04-01 16:55:26 -0500824#ifdef CONFIG_CONSOLE_POLL
825 .poll_get_char = serial8250_get_poll_char,
826 .poll_put_char = serial8250_put_poll_char,
827#endif
828 </programlisting>
829 Any implementation specifics around creating a polling driver use the
830 <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
831 Keep in mind that polling hooks have to be implemented in such a way
832 that they can be called from an atomic context and have to restore
Jason Wessel84c08fd2010-05-20 21:04:24 -0500833 the state of the UART chip on return such that the system can return
Jason Wessel225a4422008-04-01 16:55:26 -0500834 to normal when the debugger detaches. You need to be very careful
Jason Wessel65b5ac12010-08-05 09:22:33 -0500835 with any kind of lock you consider, because failing here is most likely
Jason Wessel225a4422008-04-01 16:55:26 -0500836 going to mean pressing the reset button.
837 </para>
Jason Wessel65b5ac12010-08-05 09:22:33 -0500838 </sect2>
839 <sect2 id="kgdbocKbd">
840 <title>kgdboc and keyboards</title>
841 <para>The kgdboc driver contains logic to configure communications
842 with an attached keyboard. The keyboard infrastructure is only
843 compiled into the kernel when CONFIG_KDB_KEYBOARD=y is set in the
844 kernel configuration.</para>
845 <para>The core polled keyboard driver driver for PS/2 type keyboards
846 is in drivers/char/kdb_keyboard.c. This driver is hooked into the
847 debug core when kgdboc populates the callback in the array
848 called <constant>kdb_poll_funcs[]</constant>. The
849 kdb_get_kbd_char() is the top-level function which polls hardware
850 for single character input.
851 </para>
852 </sect2>
853 <sect2 id="kgdbocKms">
854 <title>kgdboc and kms</title>
855 <para>The kgdboc driver contains logic to request the graphics
856 display to switch to a text context when you are using
857 "kgdboc=kms,kbd", provided that you have a video driver which has a
858 frame buffer console and atomic kernel mode setting support.</para>
859 <para>
860 Every time the kernel
861 debugger is entered it calls kgdboc_pre_exp_handler() which in turn
862 calls con_debug_enter() in the virtual console layer. On resuming kernel
863 execution, the kernel debugger calls kgdboc_post_exp_handler() which
864 in turn calls con_debug_leave().</para>
865 <para>Any video driver that wants to be compatible with the kernel
866 debugger and the atomic kms callbacks must implement the
867 mode_set_base_atomic, fb_debug_enter and fb_debug_leave operations.
868 For the fb_debug_enter and fb_debug_leave the option exists to use
869 the generic drm fb helper functions or implement something custom for
870 the hardware. The following example shows the initialization of the
871 .mode_set_base_atomic operation in
872 drivers/gpu/drm/i915/intel_display.c:
873 <informalexample>
874 <programlisting>
875static const struct drm_crtc_helper_funcs intel_helper_funcs = {
876[...]
877 .mode_set_base_atomic = intel_pipe_set_base_atomic,
878[...]
879};
880 </programlisting>
881 </informalexample>
882 </para>
883 <para>Here is an example of how the i915 driver initializes the fb_debug_enter and fb_debug_leave functions to use the generic drm helpers in
884 drivers/gpu/drm/i915/intel_fb.c:
885 <informalexample>
886 <programlisting>
887static struct fb_ops intelfb_ops = {
888[...]
889 .fb_debug_enter = drm_fb_helper_debug_enter,
890 .fb_debug_leave = drm_fb_helper_debug_leave,
891[...]
892};
893 </programlisting>
894 </informalexample>
895 </para>
896 </sect2>
Jason Wessel225a4422008-04-01 16:55:26 -0500897 </sect1>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500898 </chapter>
899 <chapter id="credits">
900 <title>Credits</title>
901 <para>
902 The following people have contributed to this document:
903 <orderedlist>
904 <listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
905 <listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500906 </orderedlist>
Jason Wessel225a4422008-04-01 16:55:26 -0500907 In March 2008 this document was completely rewritten by:
908 <itemizedlist>
909 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
910 </itemizedlist>
Jason Wessel84c08fd2010-05-20 21:04:24 -0500911 In Jan 2010 this document was updated to include kdb.
912 <itemizedlist>
913 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
914 </itemizedlist>
Jason Wessele3e2aaf2008-03-20 13:43:45 -0500915 </para>
916 </chapter>
917</book>
918