=====================================================================
A sketch on what to consider when implementing the new time framework
on new architectures (like ia64, mips64, parisc, sh).
=====================================================================
(See
http://ftp.kernel.org/pub/linux/daemons/ntp/PPS/PPS-2.4.0-pre3.tar.bz2
for an implementation for i386)

* Add new config variables to `config.in' and `defconfig' (CONFIG_NTP,
  CONFIG_NTP_PPS, CONFIG_NTP_PPS_SERIAL)

* use `<time.h>' instead of `<timex.h>' or `<sched.h>' to access
  kernel time.

* The kernel knows how to convert kernel time to CMOS time, don't mess
  with time zones yourself

* time is kept in nanoseconds. `do_fast_gettimeoffset()' is replaced
  with `do_exact_nanotime()' that returns nanoseconds passed since
  occurrence of the last timer interrupt. `do_slow_gettimeoffset()' is
  replaced with `do_poor_nanotime()' accordingly.

* `do_gettimeofday()' and `do_settimeofday()' are implemented in the
  architecture-independent module, messing with all the status
  updates.  The common code uses the `do_nanotime()' callback to call
  the architectures' code (allowing code selection during runtime or
  boot-up).

* `set_rtc_mmss()' is called `update_rtc()' now, and it sets the
  complete date and time (not just minutes). A new `ktime_to_rtc()'
  converts kernel time to broken down time components suitable to
  write to CMOS RTC.  `mktime()' is also architecture-independent
  now. The new `rtc_to_ktime()' is used after reading the RTC to get
  kernel time.

* a new `timevar_init()' initializes all the time variables.

* `struct timex' has been changed significantly while trying to
  preserve binary compatibility as far as possible.

* time routines are in `kernel/time.c' now, and `xtime', the kernel's
  representation of time, is protected by `rwlock_t xtime_lock'.  A
  new `rtc_runs_localtime' determines if time-zone corrections have to
  be made for RTC time updates. A new data type `l_fp', a 64bit
  quantity, is used for some internal time variables (needed by the
  NTP clock model).

* a new sysctl interface allows controlling of some time variables,
  most notably the time zone and `rtc_runs_localtime'.  While
  adjusting `time_tick' (the former `tick') is deprecated for NTP
  applications, it allows fine compensation of systematic clock
  errors.

* When the kernel time is set, the RTC update procedure is triggered.

* Old routines are implemented using POSIX-alike `do_clock_gettime()'
  and `do_clock_settime()'. There's also a `do_clock_getres()' that
  gives quite realistic (not optimistic) estimates.

* `adjtimex()' has been significantly reworked, just as most of the
  other time-keeping routines.

* Updating the RTC is controlled by new variables: `rtc_update_slave',
  when non-zero, controls after how many seconds the RTC has to be
  updated. Internally `last_rtc_update' keeps the time of the last
  update.  Upon update the `rtc_update_slave' is cleared on success.