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Subject[PATCHv4 0/4] ARM: Fixing udelay() for SMP and non-SMP systems
Date
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 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().

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

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 (4):
ARM: Translate delay.S into (mostly) C
ARM: Allow machines to override __delay()
ARM: Implement a timer based __delay() loop
msm: timer: Migrate to timer based __delay()

arch/arm/include/asm/delay.h | 11 ++++-
arch/arm/kernel/armksyms.c | 4 --
arch/arm/lib/delay.S | 65 -------------------------
arch/arm/lib/delay.c | 81 ++++++++++++++++++++++++++++++++
arch/arm/mach-msm/include/mach/timex.h | 1 +
arch/arm/mach-msm/timer.c | 16 ++++++-
6 files changed, 107 insertions(+), 71 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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