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 -- Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum. -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
