Documentation/mips/time.README

   1 README for MIPS time services
   2
   3 Jun Sun
   4 jsun@mvista.com or jsun@junsun.net
   5
   6
   7 ABOUT
   8 -----
   9 This file describes the new arch/mips/kernel/time.c, related files and the
  10 services they provide.
  11
  12 If you are short in patience and just want to know how to use time.c for a
  13 new board or convert an existing board, go to the last section.
  14
  15
  16 FILES, COMPATABILITY AND CONFIGS
  17 ---------------------------------
  18
  19 The old arch/mips/kernel/time.c is renamed to old-time.c.
  20
  21 A new time.c is put there, together with include/asm-mips/time.h.
  22
  23 Two configs variables are introduced, CONFIG_OLD_TIME_C and CONFIG_NEW_TIME_C.
  24 So we allow boards using
  25
  26         1) old time.c (CONFIG_OLD_TIME_C)
  27         2) new time.c (CONFIG_NEW_TIME_C)
  28         3) neither (their own private time.c)
  29
  30 However, it is expected every board will move to the new time.c in the near
  31 future.
  32
  33
  34 WHAT THE NEW CODE PROVIDES?
  35 ---------------------------
  36
  37 The new time code provide the following services:
  38
  39   a) Implements functions required by Linux common code:
  40         time_init
  41         do_gettimeofday
  42         do_settimeofday
  43
  44   b) provides an abstraction of RTC and null RTC implementation as default.
  45         extern unsigned long (*rtc_get_time)(void);
  46         extern int (*rtc_set_time)(unsigned long);
  47
  48   c) a set of gettimeoffset functions for different CPUs and different
  49      needs.
  50
  51   d) high-level and low-level timer interrupt routines where the timer
  52      interrupt source  may or may not be the CPU timer.  The high-level
  53      routine is dispatched through do_IRQ() while the low-level is
  54      dispatched in assemably code (usually int-handler.S)
  55
  56
  57 WHAT THE NEW CODE REQUIRES?
  58 ---------------------------
  59
  60 For the new code to work properly, each board implementation needs to supply
  61 the following functions or values:
  62
  63   a) board_time_init - a function pointer.  Invoked at the beginnig of
  64      time_init().  It is optional.
  65         1. (optional) set up RTC routines
  66         2. (optional) calibrate and set the mips_counter_frequency
  67
  68   b) plat_timer_setup - a function pointer.  Invoked at the end of time_init()
  69         1. (optional) over-ride any decisions made in time_init()
  70         2. set up the irqaction for timer interrupt.
  71         3. enable the timer interrupt
  72
  73   c) (optional) board-specific RTC routines.
  74
  75   d) (optional) mips_counter_frequency - It must be definied if the board
  76      is using CPU counter for timer interrupt or it is using fixed rate
  77      gettimeoffset().
  78
  79
  80 PORTING GUIDE
  81 -------------
  82
  83 Step 1: decide how you like to implement the time services.
  84
  85   a) does this board have a RTC?  If yes, implement the two RTC funcs.
  86
  87   b) does the CPU have counter/compare registers?
  88
  89      If the answer is no, you need a timer to provide the timer interrupt
  90      at 100 HZ speed.
  91
  92      You cannot use the fast gettimeoffset functions, i.e.,
  93
  94         unsigned long fixed_rate_gettimeoffset(void);
  95         unsigned long calibrate_div32_gettimeoffset(void);
  96         unsigned long calibrate_div64_gettimeoffset(void);
  97
  98     You can use null_gettimeoffset() will gives the same time resolution as
  99     jiffy.  Or you can implement your own gettimeoffset (probably based on
 100     some ad hoc hardware on your machine.)
 101
 102   c) The following sub steps assume your CPU has counter register.
 103      Do you plan to use the CPU counter register as the timer interrupt
 104      or use an exnternal timer?
 105
 106      In order to use CPU counter register as the timer interrupt source, you
 107      must know the counter speed (mips_counter_frequency).  It is usually the
 108      same as the CPU speed or an integral divisor of it.
 109
 110   d) decide on whether you want to use high-level or low-level timer
 111      interrupt routines.  The low-level one is presumably faster, but should
 112      not make too mcuh difference.
 113
 114
 115 Step 2:  the machine setup() function
 116
 117   If you supply board_time_init(), set the function poointer.
 118
 119
 120 Step 3: implement rtc routines, board_time_init() and plat_timer_setup()
 121   if needed.
 122
 123   board_time_init() -
 124         a) (optional) set up RTC routines,
 125         b) (optional) calibrate and set the mips_counter_frequency
 126             (only needed if you intended to use fixed_rate_gettimeoffset
 127              or use cpu counter as timer interrupt source)
 128
 129   plat_timer_setup() -
 130         a) (optional) over-write any choices made above by time_init().
 131         b) machine specific code should setup the timer irqaction.
 132         c) enable the timer interrupt
 133
 134
 135   If the RTC chip is a common chip, I suggest the routines are put under
 136   arch/mips/libs.  For example, for DS1386 chip, one would create
 137   rtc-ds1386.c under arch/mips/lib directory.  Add the following line to
 138   the arch/mips/lib/Makefile:
 139
 140         obj-$(CONFIG_DDB5476) += rtc-ds1386.o
 141
 142 Step 4: if you are using low-level timer interrupt, change your interrupt
 143   dispathcing code to check for timer interrupt and jump to
 144   ll_timer_interrupt() directly  if one is detected.
 145
 146 Step 5: Modify arch/mips/config.in and add CONFIG_NEW_TIME_C to your machine.
 147   Modify the appropriate defconfig if applicable.
 148
 149 Final notes:
 150
 151 For some tricky cases, you may need to add your own wrapper functions
 152 for some of the functions in time.c.
 153
 154 For example, you may define your own timer interrupt routine, which does
 155 some of its own processing and then calls timer_interrupt().
 156
 157 You can also over-ride any of the built-in functions (gettimeoffset,
 158 RTC routines and/or timer interrupt routine).
 159
 160
 161 PORTING NOTES FOR SMP
 162 ----------------------
 163
 164 If you have a SMP box, things are slightly more complicated.
 165
 166 The time service running every jiffy is logically divided into two parts:
 167
 168   1) the one for the whole system  (defined in timer_interrupt())
 169   2) the one that should run for each CPU (defined in local_timer_interrupt())
 170
 171 You need to decide on your timer interrupt sources.
 172
 173   case 1) - whole system has only one timer interrupt delivered to one CPU
 174
 175         In this case, you set up timer interrupt as in UP systems.  In addtion,
 176         you need to set emulate_local_timer_interrupt to 1 so that other
 177         CPUs get to call local_timer_interrupt().
 178
 179         THIS IS CURRENTLY NOT IMPLEMNETED.  However, it is rather easy to write
 180         one should such a need arise.  You simply make a IPI call.
 181
 182   case 2) - each CPU has a separate timer interrupt
 183
 184         In this case, you need to set up IRQ such that each of them will
 185         call local_timer_interrupt().  In addition, you need to arrange
 186         one and only one of them to call timer_interrupt().
 187
 188         You can also do the low-level version of those interrupt routines,
 189         following similar dispatching routes described above.
 190
 191 Note about do_gettimeoffset():
 192
 193   It is very likely the CPU counter registers are not sync'ed up in a SMP box.
 194   Therefore you cannot really use the many of the existing routines that
 195   are based on CPU counter.  You should wirte your own gettimeoffset rouinte
 196   if you want intra-jiffy resolution.