Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753558Ab1DEX4p (ORCPT ); Tue, 5 Apr 2011 19:56:45 -0400 Received: from wolverine02.qualcomm.com ([199.106.114.251]:33983 "EHLO wolverine02.qualcomm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751954Ab1DEX4o (ORCPT ); Tue, 5 Apr 2011 19:56:44 -0400 X-IronPort-AV: E=McAfee;i="5400,1158,6307"; a="83965972" From: Stephen Boyd To: Russell King Cc: linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Saravana Kannan , Nicolas Pitre , Andrew Morton , Mattias Wallin , Linus Walleij Subject: [PATCHv5 0/3] Constant udelay() for SMP and non-SMP systems Date: Tue, 5 Apr 2011 16:56:37 -0700 Message-Id: <1302047800-26720-1-git-send-email-sboyd@codeaurora.org> X-Mailer: git-send-email 1.7.5.rc0.131.gfa38c Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2620 Lines: 65 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-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/