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S390 Debug Feature

==================

files: arch/s390/kernel/debug.c

       include/asm-s390/debug.h

Description:

------------

The goal of this feature is to provide a kernel debug logging API

where log records can be stored efficiently in memory, where each component

(e.g. device drivers) can have one seperate debug log.

One purpose of this is to inspect the debug logs after a production system crash

in order to analyze the reason for the crash.

If the system still runs but only a subcomponent which uses dbf failes,

it is possible to look at the debug logs on a live system via the Linux proc

filesystem.

The debug feature may also very usefull for kernel and driver development.

Design:

-------

Kernel components (e.g. device drivers) can register themselves at the debug

feature with the function call debug_register(). This function initializes a

debug log for the caller. For each debug log exists a number of debug areas

where exactly one is active at one time.  Each debug area consists of contiguous

pages in memory. In the debug areas there are stored debug entries (log records)

which are written by event- and exception-calls.

An event-call writes the specified debug entry to the active debug

area and updates the log pointer for the active area. If the end

of the active debug area is reached, a wrap around is done (ring buffer)

and the next debug entry will be written at the beginning of the active

debug area.

An exception-call writes the specified debug entry to the log and

switches to the next debug area. This is done in order to be sure

that the records which describe the origin of the exception are not

overwritten when a wrap around for the current area occurs.

The debug areas itselve are also ordered in form of a ring buffer.

When an exception is thrown in the last debug area, the following debug

entries are then written again in the very first area.

There are three versions for the event- and exception-calls: One for

logging raw data, one for text and one for numbers.

Each debug entry contains the following data:

- Timestamp

- Cpu-Number of calling task

- Level of debug entry (0...6)

- Return Address to caller

- Flag, if entry is an exception or not

The debug logs can be inspected in a live system through entries in

the proc-filesystem. Under the path /proc/s390dbf there is

a directory for each registered component, which is named like the

corresponding component.

The content of the directories are files which represent different views

to the debug log. Each component can decide which views should be

used through registering them with the function debug_register_view().

Predefined views for hex/ascii, sprintf and raw binary data are provided.

It is also possible to define other views. The content of

a view can be inspected simply by reading the corresponding proc file.

All debug logs have an an actual debug level (range from 0 to 6).

The default level is 3. Event and Exception functions have a 'level'

parameter. Only debug entries with a level that is lower or equal

than the actual level are written to the log. This means that high

priority log entries should have a low level value whereas low priority

entries should have a high one.

The actual debug level can be changed with the help of the proc-filesystem

through writing a number string "x" to the 'level' proc file which is

provided for every debug log. Debugging can be switched off completely

by using "-" on the 'level' proc file.

Example:

> echo "-" > /proc/s390dbf/dasd/level

Kernel Interfaces:

------------------

----------------------------------------------------------------------------

debug_info_t *debug_register(char *name, int pages_index, int nr_areas,

                             int buf_size);

Parameter:    name:        Name of debug log (e.g. used for proc entry)

              pages_index: 2^pages_index pages will be allocated per area

              nr_areas:    number of debug areas

              buf_size:    size of data area in each debug entry

Return Value: Handle for generated debug area

              NULL if register failed

Description:  Allocates memory for a debug log

              Must not be called within an interrupt handler

---------------------------------------------------------------------------

void debug_unregister (debug_info_t * id);

Parameter:     id:   handle for debug log

Return Value:  none

Description:   frees memory for a debug log

               Must not be called within an interrupt handler

---------------------------------------------------------------------------

void debug_set_level (debug_info_t * id, int new_level);

Parameter:     id:        handle for debug log

               new_level: new debug level

Return Value:  none

Description:   Sets new actual debug level if new_level is valid.

---------------------------------------------------------------------------

debug_entry_t* debug_event (debug_info_t* id, int level, void* data,

                            int length);

Parameter:     id:     handle for debug log

               level:  debug level

               data:   pointer to data for debug entry

               length: length of data in bytes

Return Value:  Address of written debug entry

Description:   writes debug entry to active debug area (if level <= actual

               debug level)

---------------------------------------------------------------------------

debug_entry_t* debug_int_event (debug_info_t * id, int level,

                                unsigned int data);

debug_entry_t* debug_long_event(debug_info_t * id, int level,

                                unsigned long data);

Parameter:     id:     handle for debug log

               level:  debug level

               data:   integer value for debug entry

Return Value:  Address of written debug entry

Description:   writes debug entry to active debug area (if level <= actual

               debug level)

---------------------------------------------------------------------------

debug_entry_t* debug_text_event (debug_info_t * id, int level,

                                 const char* data);

Parameter:     id:     handle for debug log

               level:  debug level

               data:   string for debug entry

Return Value:  Address of written debug entry

Description:   writes debug entry in ascii format to active debug area

               (if level <= actual debug level)

---------------------------------------------------------------------------

debug_entry_t* debug_sprintf_event (debug_info_t * id, int level,

                                    char* string,...);

Parameter:     id:    handle for debug log

               level: debug level

               string: format string for debug entry

               ...: varargs used as in sprintf()

Return Value:  Address of written debug entry

Description:   writes debug entry with format string and varargs (longs) to

               active debug area (if level $<=$ actual debug level).

               floats and long long datatypes cannot be used as varargs.

---------------------------------------------------------------------------

debug_entry_t* debug_exception (debug_info_t* id, int level, void* data,

                                int length);

Parameter:     id:     handle for debug log

               level:  debug level

               data:   pointer to data for debug entry

               length: length of data in bytes

Return Value:  Address of written debug entry

Description:   writes debug entry to active debug area (if level <= actual

               debug level) and switches to next debug area

---------------------------------------------------------------------------

debug_entry_t* debug_int_exception (debug_info_t * id, int level,

                                    unsigned int data);

debug_entry_t* debug_long_exception(debug_info_t * id, int level,

                                    unsigned long data);

Parameter:     id:     handle for debug log

               level:  debug level

               data:   integer value for debug entry

Return Value:  Address of written debug entry

Description:   writes debug entry to active debug area (if level <= actual

               debug level) and switches to next debug area

---------------------------------------------------------------------------

debug_entry_t* debug_text_exception (debug_info_t * id, int level,

                                     const char* data);

Parameter:     id:     handle for debug log

               level:  debug level

               data:   string for debug entry

Return Value:  Address of written debug entry

Description:   writes debug entry in ascii format to active debug area

               (if level <= actual debug level) and switches to next debug

               area

---------------------------------------------------------------------------

debug_entry_t* debug_sprintf_exception (debug_info_t * id, int level,

                                        char* string,...);

Parameter:     id:    handle for debug log

               level: debug level

               string: format string for debug entry

               ...: varargs used as in sprintf()

Return Value:  Address of written debug entry

Description:   writes debug entry with format string and varargs (longs) to

               active debug area (if level $<=$ actual debug level) and

               switches to next debug area.

               floats and long long datatypes cannot be used as varargs.

---------------------------------------------------------------------------

int debug_register_view (debug_info_t * id, struct debug_view *view);

Parameter:     id:    handle for debug log

               view:  pointer to debug view struct

Return Value:  0  : ok

               < 0: Error

Description:   registers new debug view and creates proc dir entry

---------------------------------------------------------------------------

int debug_unregister_view (debug_info_t * id, struct debug_view *view);

Parameter:     id:    handle for debug log

               view:  pointer to debug view struct

Return Value:  0  : ok

               < 0: Error

Description:   unregisters debug view and removes proc dir entry

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 

#include 

static debug_info_t* debug_info;

int init_module(void)

{

    /* register 4 debug areas with one page each and 4 byte data field */

    debug_info = debug_register ("test", 0, 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;

}

void cleanup_module(void)

{

    debug_unregister (debug_info);

}

---------------------------------------------------------------------------

/*

 * sprintf-view Example

 */

#include 

#include 

static debug_info_t* debug_info;

int init_module(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", 0, 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;

}

void cleanup_module(void)

{

    debug_unregister (debug_info);

}

ProcFS Interface

----------------

Views to the debug logs can be investigated through reading the corresponding

proc-files:

Example:

> ls /proc/s390dbf/dasd

flush  hex_ascii  level      raw

> cat /proc/s390dbf/dasd/hex_ascii | sort +1

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 /proc/s390dbf/dasd/level

3

> echo "5" > /proc/s390dbf/dasd/level

> cat /proc/s390dbf/dasd/level

5

Flushing debug areas

--------------------

Debug areas can be flushed with piping the number of the desired

area (0...n) to the proc file "flush". When using "-" all debug areas

are flushed.

Examples:

1. Flush debug area 0:

> echo "0" > /proc/s390dbf/dasd/flush

2. Flush all debug areas:

> echo "-" > /proc/s390dbf/dasd/flush

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/expection fuctions 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.

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 proc 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 debug-proc 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 proc fs, 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" > /proc/s390dbf/dasd/level).

For header_proc there can be used the default function

debug_dflt_header_fn() which is defined in 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 

#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;

...

debug_info = debug_register ("test", 0, 4, 4 ));

debug_register_view(debug_info, &debug_test_view);

for(i = 0; i < 10; i ++) debug_int_event(debug_info, 1, i);

> cat /proc/s390dbf/test/myview

00 00964419734:611402 1 - 00 88042ca   This error...........

00 00964419734:611405 1 - 00 88042ca   That error...........

00 00964419734:611408 1 - 00 88042ca   Problem..............

00 00964419734:611411 1 - 00 88042ca   Something went wrong.

00 00964419734:611414 1 - 00 88042ca   Everything ok........

00 00964419734:611417 1 - 00 88042ca   data: 00000005

00 00964419734:611419 1 - 00 88042ca   data: 00000006

00 00964419734:611422 1 - 00 88042ca   data: 00000007

00 00964419734:611425 1 - 00 88042ca   data: 00000008

00 00964419734:611428 1 - 00 88042ca   data: 00000009