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docs: s390: restore important non-kdoc parts of s390dbf.rst
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Complements previous ("s390: include/asm/debug.h add kerneldoc markups")
which seemed to have dropped important non-kdoc parts such as
user space interface (level, size, flush)
as well as views and caution regarding strings in the sprintf view.

Signed-off-by: Steffen Maier <[email protected]>
Acked-by: Christian Borntraeger <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>
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steffen-maier authored and Vasily Gorbik committed Jul 5, 2019
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339 changes: 339 additions & 0 deletions Documentation/s390/s390dbf.rst
View file
Original file line number Diff line number Diff line change
Expand Up @@ -112,6 +112,345 @@ Kernel Interfaces:
Predefined views:
-----------------

extern struct debug_view debug_hex_ascii_view;

extern struct debug_view debug_raw_view;

extern struct debug_view debug_sprintf_view;

Examples
--------

::

/*
* hex_ascii- + raw-view Example
*/

#include <linux/init.h>
#include <asm/debug.h>

static debug_info_t* debug_info;

static int init(void)
{
/* register 4 debug areas with one page each and 4 byte data field */

debug_info = debug_register ("test", 1, 4, 4 );
debug_register_view(debug_info,&debug_hex_ascii_view);
debug_register_view(debug_info,&debug_raw_view);

debug_text_event(debug_info, 4 , "one ");
debug_int_exception(debug_info, 4, 4711);
debug_event(debug_info, 3, &debug_info, 4);

return 0;
}

static void cleanup(void)
{
debug_unregister (debug_info);
}

module_init(init);
module_exit(cleanup);

::

/*
* sprintf-view Example
*/

#include <linux/init.h>
#include <asm/debug.h>

static debug_info_t* debug_info;

static int init(void)
{
/* register 4 debug areas with one page each and data field for */
/* format string pointer + 2 varargs (= 3 * sizeof(long)) */

debug_info = debug_register ("test", 1, 4, sizeof(long) * 3);
debug_register_view(debug_info,&debug_sprintf_view);

debug_sprintf_event(debug_info, 2 , "first event in %s:%i\n",__FILE__,__LINE__);
debug_sprintf_exception(debug_info, 1, "pointer to debug info: %p\n",&debug_info);

return 0;
}

static void cleanup(void)
{
debug_unregister (debug_info);
}

module_init(init);
module_exit(cleanup);

Debugfs Interface
-----------------
Views to the debug logs can be investigated through reading the corresponding
debugfs-files:

Example::

> ls /sys/kernel/debug/s390dbf/dasd
flush hex_ascii level pages raw
> cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort -k2,2 -s
00 00974733272:680099 2 - 02 0006ad7e 07 ea 4a 90 | ....
00 00974733272:682210 2 - 02 0006ade6 46 52 45 45 | FREE
00 00974733272:682213 2 - 02 0006adf6 07 ea 4a 90 | ....
00 00974733272:682281 1 * 02 0006ab08 41 4c 4c 43 | EXCP
01 00974733272:682284 2 - 02 0006ab16 45 43 4b 44 | ECKD
01 00974733272:682287 2 - 02 0006ab28 00 00 00 04 | ....
01 00974733272:682289 2 - 02 0006ab3e 00 00 00 20 | ...
01 00974733272:682297 2 - 02 0006ad7e 07 ea 4a 90 | ....
01 00974733272:684384 2 - 00 0006ade6 46 52 45 45 | FREE
01 00974733272:684388 2 - 00 0006adf6 07 ea 4a 90 | ....

See section about predefined views for explanation of the above output!

Changing the debug level
------------------------

Example::


> cat /sys/kernel/debug/s390dbf/dasd/level
3
> echo "5" > /sys/kernel/debug/s390dbf/dasd/level
> cat /sys/kernel/debug/s390dbf/dasd/level
5

Flushing debug areas
--------------------
Debug areas can be flushed with piping the number of the desired
area (0...n) to the debugfs file "flush". When using "-" all debug areas
are flushed.

Examples:

1. Flush debug area 0::

> echo "0" > /sys/kernel/debug/s390dbf/dasd/flush

2. Flush all debug areas::

> echo "-" > /sys/kernel/debug/s390dbf/dasd/flush

Changing the size of debug areas
------------------------------------
It is possible the change the size of debug areas through piping
the number of pages to the debugfs file "pages". The resize request will
also flush the debug areas.

Example:

Define 4 pages for the debug areas of debug feature "dasd"::

> echo "4" > /sys/kernel/debug/s390dbf/dasd/pages

Stooping the debug feature
--------------------------
Example:

1. Check if stopping is allowed::

> cat /proc/sys/s390dbf/debug_stoppable

2. Stop debug feature::

> echo 0 > /proc/sys/s390dbf/debug_active

lcrash Interface
----------------
It is planned that the dump analysis tool lcrash gets an additional command
's390dbf' to display all the debug logs. With this tool it will be possible
to investigate the debug logs on a live system and with a memory dump after
a system crash.

Investigating raw memory
------------------------
One last possibility to investigate the debug logs at a live
system and after a system crash is to look at the raw memory
under VM or at the Service Element.
It is possible to find the anker of the debug-logs through
the 'debug_area_first' symbol in the System map. Then one has
to follow the correct pointers of the data-structures defined
in debug.h and find the debug-areas in memory.
Normally modules which use the debug feature will also have
a global variable with the pointer to the debug-logs. Following
this pointer it will also be possible to find the debug logs in
memory.

For this method it is recommended to use '16 * x + 4' byte (x = 0..n)
for the length of the data field in debug_register() in
order to see the debug entries well formatted.


Predefined Views
----------------

There are three predefined views: hex_ascii, raw and sprintf.
The hex_ascii view shows the data field in hex and ascii representation
(e.g. '45 43 4b 44 | ECKD').
The raw view returns a bytestream as the debug areas are stored in memory.

The sprintf view formats the debug entries in the same way as the sprintf
function would do. The sprintf event/exception functions write to the
debug entry a pointer to the format string (size = sizeof(long))
and for each vararg a long value. So e.g. for a debug entry with a format
string plus two varargs one would need to allocate a (3 * sizeof(long))
byte data area in the debug_register() function.

IMPORTANT:
Using "%s" in sprintf event functions is dangerous. You can only
use "%s" in the sprintf event functions, if the memory for the passed string
is available as long as the debug feature exists. The reason behind this is
that due to performance considerations only a pointer to the string is stored
in the debug feature. If you log a string that is freed afterwards, you will
get an OOPS when inspecting the debug feature, because then the debug feature
will access the already freed memory.

NOTE:
If using the sprintf view do NOT use other event/exception functions
than the sprintf-event and -exception functions.

The format of the hex_ascii and sprintf view is as follows:

- Number of area
- Timestamp (formatted as seconds and microseconds since 00:00:00 Coordinated
Universal Time (UTC), January 1, 1970)
- level of debug entry
- Exception flag (* = Exception)
- Cpu-Number of calling task
- Return Address to caller
- data field

The format of the raw view is:

- Header as described in debug.h
- datafield

A typical line of the hex_ascii view will look like the following (first line
is only for explanation and will not be displayed when 'cating' the view):

area time level exception cpu caller data (hex + ascii)
--------------------------------------------------------------------------
00 00964419409:440690 1 - 00 88023fe


Defining views
--------------

Views are specified with the 'debug_view' structure. There are defined
callback functions which are used for reading and writing the debugfs files::

struct debug_view {
char name[DEBUG_MAX_PROCF_LEN];
debug_prolog_proc_t* prolog_proc;
debug_header_proc_t* header_proc;
debug_format_proc_t* format_proc;
debug_input_proc_t* input_proc;
void* private_data;
};

where::

typedef int (debug_header_proc_t) (debug_info_t* id,
struct debug_view* view,
int area,
debug_entry_t* entry,
char* out_buf);

typedef int (debug_format_proc_t) (debug_info_t* id,
struct debug_view* view, char* out_buf,
const char* in_buf);
typedef int (debug_prolog_proc_t) (debug_info_t* id,
struct debug_view* view,
char* out_buf);
typedef int (debug_input_proc_t) (debug_info_t* id,
struct debug_view* view,
struct file* file, const char* user_buf,
size_t in_buf_size, loff_t* offset);


The "private_data" member can be used as pointer to view specific data.
It is not used by the debug feature itself.

The output when reading a debugfs file is structured like this::

"prolog_proc output"

"header_proc output 1" "format_proc output 1"
"header_proc output 2" "format_proc output 2"
"header_proc output 3" "format_proc output 3"
...

When a view is read from the debugfs, the Debug Feature calls the
'prolog_proc' once for writing the prolog.
Then 'header_proc' and 'format_proc' are called for each
existing debug entry.

The input_proc can be used to implement functionality when it is written to
the view (e.g. like with 'echo "0" > /sys/kernel/debug/s390dbf/dasd/level).

For header_proc there can be used the default function
debug_dflt_header_fn() which is defined in debug.h.
and which produces the same header output as the predefined views.
E.g::

00 00964419409:440761 2 - 00 88023ec

In order to see how to use the callback functions check the implementation
of the default views!

Example::

#include <asm/debug.h>

#define UNKNOWNSTR "data: %08x"

const char* messages[] =
{"This error...........\n",
"That error...........\n",
"Problem..............\n",
"Something went wrong.\n",
"Everything ok........\n",
NULL
};

static int debug_test_format_fn(
debug_info_t * id, struct debug_view *view,
char *out_buf, const char *in_buf
)
{
int i, rc = 0;

if(id->buf_size >= 4) {
int msg_nr = *((int*)in_buf);
if(msg_nr < sizeof(messages)/sizeof(char*) - 1)
rc += sprintf(out_buf, "%s", messages[msg_nr]);
else
rc += sprintf(out_buf, UNKNOWNSTR, msg_nr);
}
out:
return rc;
}

struct debug_view debug_test_view = {
"myview", /* name of view */
NULL, /* no prolog */
&debug_dflt_header_fn, /* default header for each entry */
&debug_test_format_fn, /* our own format function */
NULL, /* no input function */
NULL /* no private data */
};

test:
=====

::

debug_info_t *debug_info;
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