README for MIPS time services

Jun Sun
jsun@mvista.com or jsun@junsun.net


ABOUT
-----
This file describes the new arch/mips/kernel/time.c, related files and the 
services they provide. 

If you are short in patience and just want to know how to use time.c for a 
new board or convert an existing board, go to the last section.


FILES, COMPATABILITY AND CONFIGS
---------------------------------

The old arch/mips/kernel/time.c is renamed to old-time.c.

A new time.c is put there, together with include/asm-mips/time.h.

Two configs variables are introduced, CONFIG_OLD_TIME_C and CONFIG_NEW_TIME_C.
So we allow boards using 

	1) old time.c (CONFIG_OLD_TIME_C)
	2) new time.c (CONFIG_NEW_TIME_C)
	3) neither (their own private time.c)

However, it is expected every board will move to the new time.c in the near
future.

In Linux 2.5 and Linux 2.4.26 CONFIG_OLD_TIME_C was removed.

WHAT THE NEW CODE PROVIDES?
--------------------------- 

The new time code provide the following services:

  a) Implements functions required by Linux common code:
	time_init
	do_gettimeofday
	do_settimeofday

  b) provides an abstraction of RTC and null RTC implementation as default.
	extern unsigned long (*rtc_get_time)(void);
	extern int (*rtc_set_time)(unsigned long);

  c) a set of gettimeoffset functions for different CPUs and different
     needs.

  d) high-level and low-level timer interrupt routines where the timer 
     interrupt source  may or may not be the CPU timer.  The high-level 
     routine is dispatched through do_IRQ() while the low-level is 
     dispatched in assemably code (usually int-handler.S)


WHAT THE NEW CODE REQUIRES?
---------------------------

For the new code to work properly, each board implementation needs to supply
the following functions or values:

  a) board_time_init - a function pointer.  Invoked at the beginnig of
     time_init().  It is optional.
	1. (optional) set up RTC routines
	2. (optional) calibrate and set the mips_counter_frequency

  b) board_timer_setup - a function pointer.  Invoked at the end of time_init()
	1. (optional) over-ride any decisions made in time_init()
	2. set up the irqaction for timer interrupt.
	3. enable the timer interrupt

  c) (optional) board-specific RTC routines.

  d) (optional) mips_counter_frequency - It must be definied if the board
     is using CPU counter for timer interrupt or it is using fixed rate
     gettimeoffset().


PORTING GUIDE
-------------

Step 1: decide how you like to implement the time services.

  a) does this board have a RTC?  If yes, implement the two RTC funcs.

  b) does the CPU have counter/compare registers? 

     If the answer is no, you need a timer to provide the timer interrupt
     at 100 HZ speed.

     You cannot use the fast gettimeoffset functions, i.e.,

	unsigned long fixed_rate_gettimeoffset(void);
	unsigned long calibrate_div32_gettimeoffset(void);
	unsigned long calibrate_div64_gettimeoffset(void);

    You can use null_gettimeoffset() will gives the same time resolution as
    jiffy.  Or you can implement your own gettimeoffset (probably based on 
    some ad hoc hardware on your machine.)

  c) The following sub steps assume your CPU has counter register.
     Do you plan to use the CPU counter register as the timer interrupt
     or use an exnternal timer?

     In order to use CPU counter register as the timer interrupt source, you 
     must know the counter speed (mips_counter_frequency).  It is usually the
     same as the CPU speed or an integral divisor of it.

  d) decide on whether you want to use high-level or low-level timer
     interrupt routines.  The low-level one is presumably faster, but should
     not make too mcuh difference.


Step 2:  the machine setup() function

  If you supply board_time_init(), set the function poointer.

  Set the function pointer board_timer_setup() (mandatory)


Step 3: implement rtc routines, board_time_init() and board_timer_setup()
  if needed.

  board_time_init() - 
  	a) (optional) set up RTC routines, 
        b) (optional) calibrate and set the mips_counter_frequency
 	    (only needed if you intended to use fixed_rate_gettimeoffset
 	     or use cpu counter as timer interrupt source)

  board_timer_setup() - 
 	a) (optional) over-write any choices made above by time_init().
 	b) machine specific code should setup the timer irqaction.
 	c) enable the timer interrupt


  If the RTC chip is a common chip, I suggest the routines are put under
  arch/mips/libs.  For example, for DS1386 chip, one would create
  rtc-ds1386.c under arch/mips/lib directory.  Add the following line to
  the arch/mips/lib/Makefile:

	obj-$(CONFIG_DDB5476) += rtc-ds1386.o

Step 4: if you are using low-level timer interrupt, change your interrupt
  dispathcing code to check for timer interrupt and jump to 
  ll_timer_interrupt() directly  if one is detected.

Step 5: Modify arch/mips/config.in and add CONFIG_NEW_TIME_C to your machine.
  Modify the appropriate defconfig if applicable.

Final notes: 

For some tricky cases, you may need to add your own wrapper functions 
for some of the functions in time.c.  

For example, you may define your own timer interrupt routine, which does
some of its own processing and then calls timer_interrupt().

You can also over-ride any of the built-in functions (gettimeoffset,
RTC routines and/or timer interrupt routine).


PORTING NOTES FOR SMP
----------------------

If you have a SMP box, things are slightly more complicated.  

The time service running every jiffy is logically divided into two parts:

  1) the one for the whole system  (defined in timer_interrupt())
  2) the one that should run for each CPU (defined in local_timer_interrupt())

You need to decide on your timer interrupt sources.

  case 1) - whole system has only one timer interrupt delivered to one CPU

	In this case, you set up timer interrupt as in UP systems.  In addtion,
	you need to set emulate_local_timer_interrupt to 1 so that other
	CPUs get to call local_timer_interrupt().

	THIS IS CURRENTLY NOT IMPLEMNETED.  However, it is rather easy to write
	one should such a need arise.  You simply make a IPI call.

  case 2) - each CPU has a separate timer interrupt

	In this case, you need to set up IRQ such that each of them will
	call local_timer_interrupt().  In addition, you need to arrange
	one and only one of them to call timer_interrupt().

	You can also do the low-level version of those interrupt routines,
	following similar dispatching routes described above.

Note about do_gettimeoffset():
  
  It is very likely the CPU counter registers are not sync'ed up in a SMP box.
  Therefore you cannot really use the many of the existing routines that
  are based on CPU counter.  You should wirte your own gettimeoffset rouinte
  if you want intra-jiffy resolution.
