s390dbf.txt

linux 内核源代码

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 separate 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 fails,
it is possible to look at the debug logs on a live system via the Linux
debugfs filesystem.
The debug feature may also very useful 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 themselves 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 debugfs-filesystem. Under the toplevel directory "s390dbf" there is
a directory for each registered component, which is named like the
corresponding component. The debugfs normally should be mounted to
/sys/kernel/debug therefore the debug feature can be accessed under
/sys/kernel/debug/s390dbf.

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 debugfs file.

All debug logs have 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, when
writing events, 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 debugfs-filesystem
through writing a number string "x" to the 'level' debugfs file which is
provided for every debug log. Debugging can be switched off completely
by using "-" on the 'level' debugfs file.

Example:

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

It is also possible to deactivate the debug feature globally for every
debug log. You can change the behavior using  2 sysctl parameters in
/proc/sys/s390dbf:
There are currently 2 possible triggers, which stop the debug feature
globally. The first possibility is to use the "debug_active" sysctl. If
set to 1 the debug feature is running. If "debug_active" is set to 0 the
debug feature is turned off.
The second trigger which stops the debug feature is a kernel oops.
That prevents the debug feature from overwriting debug information that
happened before the oops. After an oops you can reactivate the debug feature
by piping 1 to /proc/sys/s390dbf/debug_active. Nevertheless, its not
suggested to use an oopsed kernel in a production environment.
If you want to disallow the deactivation of the debug feature, you can use
the "debug_stoppable" sysctl. If you set "debug_stoppable" to 0 the debug
feature cannot be stopped. If the debug feature is already stopped, it
will stay deactivated.

Kernel Interfaces:
------------------

----------------------------------------------------------------------------
debug_info_t *debug_register(char *name, int pages, int nr_areas,
                             int buf_size);

Parameter:    name:        Name of debug log (e.g. used for debugfs entry)
              pages:       number of pages, which 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. 

---------------------------------------------------------------------------
void debug_stop_all(void);

Parameter:     none

Return Value:  none

Description:   stops the debug feature if stopping is allowed. Currently
               used in case of a kernel oops.

---------------------------------------------------------------------------
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 debugfs 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 debugfs 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 <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);