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Date: Mon, 20 Dec 2010 14:20:29 -0800 From: Stephen Boyd <sboyd@...eaurora.org> To: Russell King - ARM Linux <linux@....linux.org.uk> Cc: linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, Saravana Kannan <skannan@...eaurora.org>, Daniel Walker <dwalker@...eaurora.org>, Nicolas Pitre <nicolas.pitre@...aro.org>, John Stultz <johnstul@...ibm.com>, Brian Swetland <swetland@...gle.com>, Arve Hjønnevåg <arve@...roid.com>, Andrew Morton <akpm@...ux-foundation.org> Subject: [PATCHv4 0/4] ARM: Fixing udelay() for SMP and non-SMP systems 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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