Constant udelay() for SMP and non-SMP systems

Note: I will submit this to the patch tracker in 48 hours. Tested by are appreciated.

These patches fix the udelay() issue pointed out on arm-lkml[1][2]. A quick recap: some SMP machines can scale their CPU frequencies independent of one another. loops_per_jiffy is calibrated globally and used in __const_udelay(). If one CPU is running faster than what the loops_per_jiffy is calculated (or scaled) for, udelay() will be incorrect and not wait long enough (or too long). A similar problem occurs if the cpu frequency is scaled during a udelay() call.

We could fix this issue a couple ways, wholesale replacement of __udelay() and __const_udelay() (see [2] for that approach), or replacement of __delay() (this series). Option 1 can fail if anybody uses udelay() before memory is mapped and also duplicates most of the code in asm/delay.h. It also needs to hardcode the timer tick frequency, which can sometimes be inaccurate. The benefit is that loops_per_jiffy stays the same and thus BogoMIPS is unchanged. Option 2 cannot fail since the __delay() loop is repointed after memory is mapped in, but it suffers from a low BogoMIPS when timers are clocked slowly. It also more accurately calculates the timer tick frequency through the use of calibrate_delay_direct().

-- Reference --
[1] http://article.gmane.org/gmane.linux.kernel/977567
[2] http://article.gmane.org/gmane.linux.ports.arm.kernel/78496

Changes since v4:
* Rebased against changes to udelay.S

Changes since v3:
* Inlined set_delay_fn()

Changes since v2:
* Additional patch using the timer based delay

Changes since v1:
* likely() in delay.c
* comment fixup for read_current_timer_delay_loop()
* cosmetic improvements to commit text

Stephen Boyd (3):
ARM: Translate delay.S into (mostly) C
ARM: Allow machines to override __delay()
ARM: Implement a timer based __delay() loop

 arch/arm/include/asm/delay.h |   11 +++++-
 arch/arm/kernel/armksyms.c   |    4 --
 arch/arm/lib/delay.S         |   69 ---------------------------------
 arch/arm/lib/delay.c         |   87 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 97 insertions(+), 74 deletions(-)
 delete mode 100644 arch/arm/lib/delay.S
 create mode 100644 arch/arm/lib/delay.c

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