From mboxrd@z Thu Jan 1 00:00:00 1970 From: sboyd@codeaurora.org (Stephen Boyd) Date: Wed, 18 Aug 2010 19:24:00 -0700 Subject: [PATCH 0/3] Fixing udelay() on SMP (and non-SMP too) Message-ID: <1282184643-29860-1-git-send-email-sboyd@codeaurora.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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(). -- 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.