[PATCHv2 0/3] ARM: Fixing udelay() for SMP and non-SMP systems

[Stephen Boyd]

These patches are another attempt at fixing 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 can't fail since the __delay() loop is
replaced 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().

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

I didn't add any more comments to delay.c's functions since I
couldn't come up with anything worthwhile to say.

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

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 |    5 ++-
 arch/arm/kernel/armksyms.c   |    4 --
 arch/arm/lib/delay.S         |   65 ------------------------------
 arch/arm/lib/delay.c         |   90 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 94 insertions(+), 70 deletions(-)
 delete mode 100644 arch/arm/lib/delay.S
 create mode 100644 arch/arm/lib/delay.c

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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