[PATCH 0/2] Add clock post-rate-change notifier

[Paul Walmsley <paul@pwsan.com>]

This series adds a clock rate change notifier to the OMAP clock
framework.  Currently only post-rate-change notification is
implemented, although pre-rate-change notification is in the works.

Clock post-rate-change notifiers are intended for drivers that need to
be notified when other code (such as power management code) changes
their clock's rate.  DSPBridge is apparently one such user, but
drivers for UART, MCSPI, etc. are also candidates for these notifiers.

Drivers should use these notifiers by adding function pointers for
clk_notifier_register() and clk_notifier_unregister() to their
platform_data structure.  Ensure that the function pointers are not
NULL before calling, since non-OMAP platforms may not support this
notifier:

  if (pdata->clk_notifier_register)
     (*pdata->clk_notifier_register)(clk, &test_nb);

The callback code must not call back into the clock framework nor
sleep, as the clock framework spinlock is held for the duration of the
callbacks.  Callback functions should be short and lightweight.

...

A few implementation notes.  Upon request, an alternate design was
also investigated based on the existing CPUFreq notifiers.  That
design was set aside, due to the following problems:

- CPUFreq notifier callbacks can sleep.  This will break the clock
  framework, since it calls the notifier inside a spinlock.

- Identifying the changed clock from the callback would require a new
  platform_data-based function in the clock framework, since struct
  cpufreq_freqs has no good way to pass it.

- CPUFreq notifiers depend on CPUFreq, which not everyone uses.

There was also some concern expressed that separate CPUFreq notifiers
and clk rate notifiers could race.  (That would only apply to the
clocks that are changed by CPUFreq, and any clocks downstream from
it.)  This doesn't appear to be a problem in practice, since CPUFreq
post-rate notifiers can only start after the clk post-rate notifiers
have completed and exited the clock framework.

The current implementation has been moderately tested with multiple
clock notifiers on both DPLLs and a downstream clock on 3430SDP.  Also
compile-tested for N800.


- Paul


   text    data     bss     dec     hex filename
3404721  158888  108176 3671785  3806e9 vmlinux.3430sdp.orig
3405361  158896  108176 3672433  380971 vmlinux.3430sdp.patched

 arch/arm/mach-omap2/clock.c       |   37 ++++++++--
 arch/arm/mach-omap2/clock24xx.c   |   42 ++++++++++-
 arch/arm/mach-omap2/clock34xx.c   |   10 +++
 arch/arm/plat-omap/clock.c        |  144 +++++++++++++++++++++++++++++++++++++
 include/asm-arm/arch-omap/clock.h |   21 +++++
 5 files changed, 246 insertions(+), 8 deletions(-)