199 lines
6.6 KiB
Plaintext
199 lines
6.6 KiB
Plaintext
README for MIPS time services
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Jun Sun
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jsun@mvista.com or jsun@junsun.net
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ABOUT
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-----
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This file describes the new arch/mips/kernel/time.c, related files and the
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services they provide.
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If you are short in patience and just want to know how to use time.c for a
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new board or convert an existing board, go to the last section.
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FILES, COMPATABILITY AND CONFIGS
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---------------------------------
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The old arch/mips/kernel/time.c is renamed to old-time.c.
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A new time.c is put there, together with include/asm-mips/time.h.
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Two configs variables are introduced, CONFIG_OLD_TIME_C and CONFIG_NEW_TIME_C.
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So we allow boards using
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1) old time.c (CONFIG_OLD_TIME_C)
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2) new time.c (CONFIG_NEW_TIME_C)
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3) neither (their own private time.c)
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However, it is expected every board will move to the new time.c in the near
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future.
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WHAT THE NEW CODE PROVIDES?
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---------------------------
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The new time code provide the following services:
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a) Implements functions required by Linux common code:
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time_init
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do_gettimeofday
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do_settimeofday
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b) provides an abstraction of RTC and null RTC implementation as default.
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extern unsigned long (*rtc_get_time)(void);
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extern int (*rtc_set_time)(unsigned long);
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c) a set of gettimeoffset functions for different CPUs and different
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needs.
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d) high-level and low-level timer interrupt routines where the timer
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interrupt source may or may not be the CPU timer. The high-level
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routine is dispatched through do_IRQ() while the low-level is
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dispatched in assemably code (usually int-handler.S)
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WHAT THE NEW CODE REQUIRES?
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---------------------------
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For the new code to work properly, each board implementation needs to supply
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the following functions or values:
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a) board_time_init - a function pointer. Invoked at the beginnig of
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time_init(). It is optional.
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1. (optional) set up RTC routines
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2. (optional) calibrate and set the mips_counter_frequency
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b) board_timer_setup - a function pointer. Invoked at the end of time_init()
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1. (optional) over-ride any decisions made in time_init()
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2. set up the irqaction for timer interrupt.
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3. enable the timer interrupt
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c) (optional) board-specific RTC routines.
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d) (optional) mips_counter_frequency - It must be definied if the board
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is using CPU counter for timer interrupt or it is using fixed rate
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gettimeoffset().
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PORTING GUIDE
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-------------
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Step 1: decide how you like to implement the time services.
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a) does this board have a RTC? If yes, implement the two RTC funcs.
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b) does the CPU have counter/compare registers?
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If the answer is no, you need a timer to provide the timer interrupt
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at 100 HZ speed.
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You cannot use the fast gettimeoffset functions, i.e.,
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unsigned long fixed_rate_gettimeoffset(void);
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unsigned long calibrate_div32_gettimeoffset(void);
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unsigned long calibrate_div64_gettimeoffset(void);
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You can use null_gettimeoffset() will gives the same time resolution as
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jiffy. Or you can implement your own gettimeoffset (probably based on
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some ad hoc hardware on your machine.)
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c) The following sub steps assume your CPU has counter register.
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Do you plan to use the CPU counter register as the timer interrupt
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or use an exnternal timer?
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In order to use CPU counter register as the timer interrupt source, you
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must know the counter speed (mips_counter_frequency). It is usually the
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same as the CPU speed or an integral divisor of it.
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d) decide on whether you want to use high-level or low-level timer
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interrupt routines. The low-level one is presumably faster, but should
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not make too mcuh difference.
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Step 2: the machine setup() function
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If you supply board_time_init(), set the function poointer.
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Set the function pointer board_timer_setup() (mandatory)
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Step 3: implement rtc routines, board_time_init() and board_timer_setup()
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if needed.
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board_time_init() -
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a) (optional) set up RTC routines,
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b) (optional) calibrate and set the mips_counter_frequency
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(only needed if you intended to use fixed_rate_gettimeoffset
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or use cpu counter as timer interrupt source)
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board_timer_setup() -
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a) (optional) over-write any choices made above by time_init().
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b) machine specific code should setup the timer irqaction.
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c) enable the timer interrupt
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If the RTC chip is a common chip, I suggest the routines are put under
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arch/mips/libs. For example, for DS1386 chip, one would create
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rtc-ds1386.c under arch/mips/lib directory. Add the following line to
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the arch/mips/lib/Makefile:
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obj-$(CONFIG_DDB5476) += rtc-ds1386.o
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Step 4: if you are using low-level timer interrupt, change your interrupt
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dispathcing code to check for timer interrupt and jump to
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ll_timer_interrupt() directly if one is detected.
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Step 5: Modify arch/mips/config.in and add CONFIG_NEW_TIME_C to your machine.
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Modify the appropriate defconfig if applicable.
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Final notes:
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For some tricky cases, you may need to add your own wrapper functions
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for some of the functions in time.c.
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For example, you may define your own timer interrupt routine, which does
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some of its own processing and then calls timer_interrupt().
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You can also over-ride any of the built-in functions (gettimeoffset,
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RTC routines and/or timer interrupt routine).
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PORTING NOTES FOR SMP
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----------------------
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If you have a SMP box, things are slightly more complicated.
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The time service running every jiffy is logically divided into two parts:
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1) the one for the whole system (defined in timer_interrupt())
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2) the one that should run for each CPU (defined in local_timer_interrupt())
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You need to decide on your timer interrupt sources.
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case 1) - whole system has only one timer interrupt delivered to one CPU
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In this case, you set up timer interrupt as in UP systems. In addtion,
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you need to set emulate_local_timer_interrupt to 1 so that other
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CPUs get to call local_timer_interrupt().
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THIS IS CURRENTLY NOT IMPLEMNETED. However, it is rather easy to write
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one should such a need arise. You simply make a IPI call.
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case 2) - each CPU has a separate timer interrupt
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In this case, you need to set up IRQ such that each of them will
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call local_timer_interrupt(). In addition, you need to arrange
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one and only one of them to call timer_interrupt().
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You can also do the low-level version of those interrupt routines,
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following similar dispatching routes described above.
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Note about do_gettimeoffset():
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It is very likely the CPU counter registers are not sync'ed up in a SMP box.
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Therefore you cannot really use the many of the existing routines that
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are based on CPU counter. You should wirte your own gettimeoffset rouinte
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if you want intra-jiffy resolution.
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