Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760581AbYF3JXS (ORCPT ); Mon, 30 Jun 2008 05:23:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754774AbYF3JWX (ORCPT ); Mon, 30 Jun 2008 05:22:23 -0400 Received: from smtpeu1.atmel.com ([195.65.72.27]:62550 "EHLO bagnes.atmel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755778AbYF3JWU (ORCPT ); Mon, 30 Jun 2008 05:22:20 -0400 From: Haavard Skinnemoen To: kernel@avr32linux.org Cc: linux-kernel@vger.kernel.org, Haavard Skinnemoen Subject: [PATCH 8/8] avr32: Power Management support ("standby" and "mem" modes) Date: Mon, 30 Jun 2008 11:21:16 +0200 Message-Id: <1214817676-30378-9-git-send-email-haavard.skinnemoen@atmel.com> X-Mailer: git-send-email 1.5.5.4 In-Reply-To: <1214817676-30378-8-git-send-email-haavard.skinnemoen@atmel.com> References: <1214817676-30378-1-git-send-email-haavard.skinnemoen@atmel.com> <1214817676-30378-2-git-send-email-haavard.skinnemoen@atmel.com> <1214817676-30378-3-git-send-email-haavard.skinnemoen@atmel.com> <1214817676-30378-4-git-send-email-haavard.skinnemoen@atmel.com> <1214817676-30378-5-git-send-email-haavard.skinnemoen@atmel.com> <1214817676-30378-6-git-send-email-haavard.skinnemoen@atmel.com> <1214817676-30378-7-git-send-email-haavard.skinnemoen@atmel.com> <1214817676-30378-8-git-send-email-haavard.skinnemoen@atmel.com> X-OriginalArrivalTime: 30 Jun 2008 09:21:08.0211 (UTC) FILETIME=[A10CCC30:01C8DA92] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 18133 Lines: 633 From: Haavard Skinnemoen Implement Standby support. In this mode, we'll suspend all drivers, put the SDRAM in self-refresh mode and switch off the HSB bus ("frozen" mode.) Implement Suspend-to-mem support. In this mode, we suspend all drivers, put the SDRAM into self-refresh mode and switch off all internal clocks except the 32 kHz oscillator ("stop" mode.) The lowest-level suspend code runs from a small portion of SRAM allocated at startup time. This gets rid of a small potential race with the SDRAM where we might try to enter self-refresh mode in the middle of an icache burst. We also relocate all interrupt and exception handlers to SRAM during the small window when we enter and exit the low-power modes. We don't need to do any special tricks to start and stop the PLL. The main clock is automatically gated by hardware until the PLL is stable. Signed-off-by: Haavard Skinnemoen --- arch/avr32/Kconfig | 5 + arch/avr32/mach-at32ap/Makefile | 5 + arch/avr32/mach-at32ap/intc.c | 54 +++++++- arch/avr32/mach-at32ap/pm-at32ap700x.S | 108 ++++++++++++++ arch/avr32/mach-at32ap/pm.c | 245 ++++++++++++++++++++++++++++++++ arch/avr32/mach-at32ap/sdramc.h | 76 ++++++++++ include/asm-avr32/arch-at32ap/pm.h | 3 + include/asm-avr32/thread_info.h | 1 + 8 files changed, 496 insertions(+), 1 deletions(-) create mode 100644 arch/avr32/mach-at32ap/pm.c create mode 100644 arch/avr32/mach-at32ap/sdramc.h diff --git a/arch/avr32/Kconfig b/arch/avr32/Kconfig index 4ff3dea..da86525 100644 --- a/arch/avr32/Kconfig +++ b/arch/avr32/Kconfig @@ -202,6 +202,11 @@ endmenu menu "Power management options" +source "kernel/power/Kconfig" + +config ARCH_SUSPEND_POSSIBLE + def_bool y + menu "CPU Frequency scaling" source "drivers/cpufreq/Kconfig" diff --git a/arch/avr32/mach-at32ap/Makefile b/arch/avr32/mach-at32ap/Makefile index cb44182..d5018e2 100644 --- a/arch/avr32/mach-at32ap/Makefile +++ b/arch/avr32/mach-at32ap/Makefile @@ -1,3 +1,8 @@ obj-y += pdc.o clock.o intc.o extint.o pio.o hsmc.o obj-$(CONFIG_CPU_AT32AP700X) += at32ap700x.o pm-at32ap700x.o obj-$(CONFIG_CPU_FREQ_AT32AP) += cpufreq.o +obj-$(CONFIG_PM) += pm.o + +ifeq ($(CONFIG_PM_DEBUG),y) +CFLAGS_pm.o += -DDEBUG +endif diff --git a/arch/avr32/mach-at32ap/intc.c b/arch/avr32/mach-at32ap/intc.c index 644a3fb..994c454 100644 --- a/arch/avr32/mach-at32ap/intc.c +++ b/arch/avr32/mach-at32ap/intc.c @@ -22,6 +22,10 @@ struct intc { void __iomem *regs; struct irq_chip chip; struct sys_device sysdev; +#ifdef CONFIG_PM + unsigned long suspend_ipr; + unsigned long saved_ipr[64]; +#endif }; extern struct platform_device at32_intc0_device; @@ -138,8 +142,56 @@ fail: panic("Interrupt controller initialization failed!\n"); } +#ifdef CONFIG_PM +void intc_set_suspend_handler(unsigned long offset) +{ + intc0.suspend_ipr = offset; +} + +static int intc_suspend(struct sys_device *sdev, pm_message_t state) +{ + struct intc *intc = container_of(sdev, struct intc, sysdev); + int i; + + if (unlikely(!irqs_disabled())) { + pr_err("intc_suspend: called with interrupts enabled\n"); + return -EINVAL; + } + + if (unlikely(!intc->suspend_ipr)) { + pr_err("intc_suspend: suspend_ipr not initialized\n"); + return -EINVAL; + } + + for (i = 0; i < 64; i++) { + intc->saved_ipr[i] = intc_readl(intc, INTPR0 + 4 * i); + intc_writel(intc, INTPR0 + 4 * i, intc->suspend_ipr); + } + + return 0; +} + +static int intc_resume(struct sys_device *sdev) +{ + struct intc *intc = container_of(sdev, struct intc, sysdev); + int i; + + WARN_ON(!irqs_disabled()); + + for (i = 0; i < 64; i++) + intc_writel(intc, INTPR0 + 4 * i, intc->saved_ipr[i]); + + return 0; +} +#else +#define intc_suspend NULL +#define intc_resume NULL +#endif + static struct sysdev_class intc_class = { - .name = "intc", + .name = "intc", + .suspend = intc_suspend, + .resume = intc_resume, }; static int __init intc_init_sysdev(void) diff --git a/arch/avr32/mach-at32ap/pm-at32ap700x.S b/arch/avr32/mach-at32ap/pm-at32ap700x.S index 949e248..0a53ad3 100644 --- a/arch/avr32/mach-at32ap/pm-at32ap700x.S +++ b/arch/avr32/mach-at32ap/pm-at32ap700x.S @@ -12,6 +12,12 @@ #include #include +#include "pm.h" +#include "sdramc.h" + +/* Same as 0xfff00000 but fits in a 21 bit signed immediate */ +#define PM_BASE -0x100000 + .section .bss, "wa", @nobits .global disable_idle_sleep .type disable_idle_sleep, @object @@ -64,3 +70,105 @@ cpu_idle_skip_sleep: unmask_interrupts retal r12 .size cpu_idle_skip_sleep, . - cpu_idle_skip_sleep + +#ifdef CONFIG_PM + .section .init.text, "ax", @progbits + + .global pm_exception + .type pm_exception, @function +pm_exception: + /* + * Exceptions are masked when we switch to this handler, so + * we'll only get "unrecoverable" exceptions (offset 0.) + */ + sub r12, pc, . - .Lpanic_msg + lddpc pc, .Lpanic_addr + + .align 2 +.Lpanic_addr: + .long panic +.Lpanic_msg: + .asciz "Unrecoverable exception during suspend\n" + .size pm_exception, . - pm_exception + + .global pm_irq0 + .type pm_irq0, @function +pm_irq0: + /* Disable interrupts and return after the sleep instruction */ + mfsr r9, SYSREG_RSR_INT0 + mtsr SYSREG_RAR_INT0, r8 + sbr r9, SYSREG_GM_OFFSET + mtsr SYSREG_RSR_INT0, r9 + rete + + /* + * void cpu_enter_standby(unsigned long sdramc_base) + * + * Enter PM_SUSPEND_STANDBY mode. At this point, all drivers + * are suspended and interrupts are disabled. Interrupts + * marked as 'wakeup' event sources may still come along and + * get us out of here. + * + * The SDRAM will be put into self-refresh mode (which does + * not require a clock from the CPU), and the CPU will be put + * into "frozen" mode (HSB bus stopped). The SDRAM controller + * will automatically bring the SDRAM into normal mode on the + * first access, and the power manager will automatically + * start the HSB and CPU clocks upon a wakeup event. + * + * This code uses the same "skip sleep" technique as above. + * It is very important that we jump directly to + * cpu_after_sleep after the sleep instruction since that's + * where we'll end up if the interrupt handler decides that we + * need to skip the sleep instruction. + */ + .global pm_standby + .type pm_standby, @function +pm_standby: + /* + * interrupts are already masked at this point, and EVBA + * points to pm_exception above. + */ + ld.w r10, r12[SDRAMC_LPR] + sub r8, pc, . - 1f /* return address for irq handler */ + mov r11, SDRAMC_LPR_LPCB_SELF_RFR + bfins r10, r11, 0, 2 /* LPCB <- self Refresh */ + sync 0 /* flush write buffer */ + st.w r12[SDRAMC_LPR], r11 /* put SDRAM in self-refresh mode */ + ld.w r11, r12[SDRAMC_LPR] + unmask_interrupts + sleep CPU_SLEEP_FROZEN +1: mask_interrupts + retal r12 + .size pm_standby, . - pm_standby + + .global pm_suspend_to_ram + .type pm_suspend_to_ram, @function +pm_suspend_to_ram: + /* + * interrupts are already masked at this point, and EVBA + * points to pm_exception above. + */ + mov r11, 0 + cache r11[2], 8 /* clean all dcache lines */ + sync 0 /* flush write buffer */ + ld.w r10, r12[SDRAMC_LPR] + sub r8, pc, . - 1f /* return address for irq handler */ + mov r11, SDRAMC_LPR_LPCB_SELF_RFR + bfins r10, r11, 0, 2 /* LPCB <- self refresh */ + st.w r12[SDRAMC_LPR], r10 /* put SDRAM in self-refresh mode */ + ld.w r11, r12[SDRAMC_LPR] + + unmask_interrupts + sleep CPU_SLEEP_STOP +1: mask_interrupts + + retal r12 + .size pm_suspend_to_ram, . - pm_suspend_to_ram + + .global pm_sram_end + .type pm_sram_end, @function +pm_sram_end: + .size pm_sram_end, 0 + +#endif /* CONFIG_PM */ diff --git a/arch/avr32/mach-at32ap/pm.c b/arch/avr32/mach-at32ap/pm.c new file mode 100644 index 0000000..0b76432 --- /dev/null +++ b/arch/avr32/mach-at32ap/pm.c @@ -0,0 +1,245 @@ +/* + * AVR32 AP Power Management + * + * Copyright (C) 2008 Atmel Corporation + * + * This program is free software; you can redistribute it and/or + * modify it under the terms of the GNU General Public License + * version 2 as published by the Free Software Foundation. + */ +#include +#include +#include + +#include +#include + +#include +#include + +/* FIXME: This is only valid for AP7000 */ +#define SDRAMC_BASE 0xfff03800 + +#include "sdramc.h" + +#define SRAM_PAGE_FLAGS (SYSREG_BIT(TLBELO_D) | SYSREG_BF(SZ, 1) \ + | SYSREG_BF(AP, 3) | SYSREG_BIT(G)) + + +static unsigned long pm_sram_start; +static size_t pm_sram_size; +static struct vm_struct *pm_sram_area; + +static void (*avr32_pm_enter_standby)(unsigned long sdramc_base); +static void (*avr32_pm_enter_str)(unsigned long sdramc_base); + +/* + * Must be called with interrupts disabled. Exceptions will be masked + * on return (i.e. all exceptions will be "unrecoverable".) + */ +static void *avr32_pm_map_sram(void) +{ + unsigned long vaddr; + unsigned long page_addr; + u32 tlbehi; + u32 mmucr; + + vaddr = (unsigned long)pm_sram_area->addr; + page_addr = pm_sram_start & PAGE_MASK; + + /* + * Mask exceptions and grab the first TLB entry. We won't be + * needing it while sleeping. + */ + asm volatile("ssrf %0" : : "i"(SYSREG_EM_OFFSET) : "memory"); + + mmucr = sysreg_read(MMUCR); + tlbehi = sysreg_read(TLBEHI); + sysreg_write(MMUCR, SYSREG_BFINS(DRP, 0, mmucr)); + + tlbehi = SYSREG_BF(ASID, SYSREG_BFEXT(ASID, tlbehi)); + tlbehi |= vaddr & PAGE_MASK; + tlbehi |= SYSREG_BIT(TLBEHI_V); + + sysreg_write(TLBELO, page_addr | SRAM_PAGE_FLAGS); + sysreg_write(TLBEHI, tlbehi); + __builtin_tlbw(); + + return (void *)(vaddr + pm_sram_start - page_addr); +} + +/* + * Must be called with interrupts disabled. Exceptions will be + * unmasked on return. + */ +static void avr32_pm_unmap_sram(void) +{ + u32 mmucr; + u32 tlbehi; + u32 tlbarlo; + + /* Going to update TLB entry at index 0 */ + mmucr = sysreg_read(MMUCR); + tlbehi = sysreg_read(TLBEHI); + sysreg_write(MMUCR, SYSREG_BFINS(DRP, 0, mmucr)); + + /* Clear the "valid" bit */ + tlbehi = SYSREG_BF(ASID, SYSREG_BFEXT(ASID, tlbehi)); + sysreg_write(TLBEHI, tlbehi); + + /* Mark it as "not accessed" */ + tlbarlo = sysreg_read(TLBARLO); + sysreg_write(TLBARLO, tlbarlo | 0x80000000U); + + /* Update the TLB */ + __builtin_tlbw(); + + /* Unmask exceptions */ + asm volatile("csrf %0" : : "i"(SYSREG_EM_OFFSET) : "memory"); +} + +static int avr32_pm_valid_state(suspend_state_t state) +{ + switch (state) { + case PM_SUSPEND_ON: + case PM_SUSPEND_STANDBY: + case PM_SUSPEND_MEM: + return 1; + + default: + return 0; + } +} + +static int avr32_pm_enter(suspend_state_t state) +{ + u32 lpr_saved; + u32 evba_saved; + void *sram; + + switch (state) { + case PM_SUSPEND_STANDBY: + sram = avr32_pm_map_sram(); + + /* Switch to in-sram exception handlers */ + evba_saved = sysreg_read(EVBA); + sysreg_write(EVBA, (unsigned long)sram); + + /* + * Save the LPR register so that we can re-enable + * SDRAM Low Power mode on resume. + */ + lpr_saved = sdramc_readl(LPR); + pr_debug("%s: Entering standby...\n", __func__); + avr32_pm_enter_standby(SDRAMC_BASE); + sdramc_writel(LPR, lpr_saved); + + /* Switch back to regular exception handlers */ + sysreg_write(EVBA, evba_saved); + + avr32_pm_unmap_sram(); + break; + + case PM_SUSPEND_MEM: + sram = avr32_pm_map_sram(); + + /* Switch to in-sram exception handlers */ + evba_saved = sysreg_read(EVBA); + sysreg_write(EVBA, (unsigned long)sram); + + /* + * Save the LPR register so that we can re-enable + * SDRAM Low Power mode on resume. + */ + lpr_saved = sdramc_readl(LPR); + pr_debug("%s: Entering suspend-to-ram...\n", __func__); + avr32_pm_enter_str(SDRAMC_BASE); + sdramc_writel(LPR, lpr_saved); + + /* Switch back to regular exception handlers */ + sysreg_write(EVBA, evba_saved); + + avr32_pm_unmap_sram(); + break; + + case PM_SUSPEND_ON: + pr_debug("%s: Entering idle...\n", __func__); + cpu_enter_idle(); + break; + + default: + pr_debug("%s: Invalid suspend state %d\n", __func__, state); + goto out; + } + + pr_debug("%s: wakeup\n", __func__); + +out: + return 0; +} + +static struct platform_suspend_ops avr32_pm_ops = { + .valid = avr32_pm_valid_state, + .enter = avr32_pm_enter, +}; + +static unsigned long avr32_pm_offset(void *symbol) +{ + extern u8 pm_exception[]; + + return (unsigned long)symbol - (unsigned long)pm_exception; +} + +static int __init avr32_pm_init(void) +{ + extern u8 pm_exception[]; + extern u8 pm_irq0[]; + extern u8 pm_standby[]; + extern u8 pm_suspend_to_ram[]; + extern u8 pm_sram_end[]; + void *dst; + + /* + * To keep things simple, we depend on not needing more than a + * single page. + */ + pm_sram_size = avr32_pm_offset(pm_sram_end); + if (pm_sram_size > PAGE_SIZE) + goto err; + + pm_sram_start = sram_alloc(pm_sram_size); + if (!pm_sram_start) + goto err_alloc_sram; + + /* Grab a virtual area we can use later on. */ + pm_sram_area = get_vm_area(pm_sram_size, VM_IOREMAP); + if (!pm_sram_area) + goto err_vm_area; + pm_sram_area->phys_addr = pm_sram_start; + + local_irq_disable(); + dst = avr32_pm_map_sram(); + memcpy(dst, pm_exception, pm_sram_size); + flush_dcache_region(dst, pm_sram_size); + invalidate_icache_region(dst, pm_sram_size); + avr32_pm_unmap_sram(); + local_irq_enable(); + + avr32_pm_enter_standby = dst + avr32_pm_offset(pm_standby); + avr32_pm_enter_str = dst + avr32_pm_offset(pm_suspend_to_ram); + intc_set_suspend_handler(avr32_pm_offset(pm_irq0)); + + suspend_set_ops(&avr32_pm_ops); + + printk("AVR32 AP Power Management enabled\n"); + + return 0; + +err_vm_area: + sram_free(pm_sram_start, pm_sram_size); +err_alloc_sram: +err: + pr_err("AVR32 Power Management initialization failed\n"); + return -ENOMEM; +} +arch_initcall(avr32_pm_init); diff --git a/arch/avr32/mach-at32ap/sdramc.h b/arch/avr32/mach-at32ap/sdramc.h new file mode 100644 index 0000000..66eeaed --- /dev/null +++ b/arch/avr32/mach-at32ap/sdramc.h @@ -0,0 +1,76 @@ +/* + * Register definitions for the AT32AP SDRAM Controller + * + * Copyright (C) 2008 Atmel Corporation + * + * This program is free software; you can redistribute it and/or + * modify it under the terms of the GNU General Public License + * version 2 as published by the Free Software Foundation. + */ + +/* Register offsets */ +#define SDRAMC_MR 0x0000 +#define SDRAMC_TR 0x0004 +#define SDRAMC_CR 0x0008 +#define SDRAMC_HSR 0x000c +#define SDRAMC_LPR 0x0010 +#define SDRAMC_IER 0x0014 +#define SDRAMC_IDR 0x0018 +#define SDRAMC_IMR 0x001c +#define SDRAMC_ISR 0x0020 +#define SDRAMC_MDR 0x0024 + +/* MR - Mode Register */ +#define SDRAMC_MR_MODE_NORMAL ( 0 << 0) +#define SDRAMC_MR_MODE_NOP ( 1 << 0) +#define SDRAMC_MR_MODE_BANKS_PRECHARGE ( 2 << 0) +#define SDRAMC_MR_MODE_LOAD_MODE ( 3 << 0) +#define SDRAMC_MR_MODE_AUTO_REFRESH ( 4 << 0) +#define SDRAMC_MR_MODE_EXT_LOAD_MODE ( 5 << 0) +#define SDRAMC_MR_MODE_POWER_DOWN ( 6 << 0) + +/* CR - Configuration Register */ +#define SDRAMC_CR_NC_8_BITS ( 0 << 0) +#define SDRAMC_CR_NC_9_BITS ( 1 << 0) +#define SDRAMC_CR_NC_10_BITS ( 2 << 0) +#define SDRAMC_CR_NC_11_BITS ( 3 << 0) +#define SDRAMC_CR_NR_11_BITS ( 0 << 2) +#define SDRAMC_CR_NR_12_BITS ( 1 << 2) +#define SDRAMC_CR_NR_13_BITS ( 2 << 2) +#define SDRAMC_CR_NB_2_BANKS ( 0 << 4) +#define SDRAMC_CR_NB_4_BANKS ( 1 << 4) +#define SDRAMC_CR_CAS(x) ((x) << 5) +#define SDRAMC_CR_DBW_32_BITS ( 0 << 7) +#define SDRAMC_CR_DBW_16_BITS ( 1 << 7) +#define SDRAMC_CR_TWR(x) ((x) << 8) +#define SDRAMC_CR_TRC(x) ((x) << 12) +#define SDRAMC_CR_TRP(x) ((x) << 16) +#define SDRAMC_CR_TRCD(x) ((x) << 20) +#define SDRAMC_CR_TRAS(x) ((x) << 24) +#define SDRAMC_CR_TXSR(x) ((x) << 28) + +/* HSR - High Speed Register */ +#define SDRAMC_HSR_DA ( 1 << 0) + +/* LPR - Low Power Register */ +#define SDRAMC_LPR_LPCB_INHIBIT ( 0 << 0) +#define SDRAMC_LPR_LPCB_SELF_RFR ( 1 << 0) +#define SDRAMC_LPR_LPCB_PDOWN ( 2 << 0) +#define SDRAMC_LPR_LPCB_DEEP_PDOWN ( 3 << 0) +#define SDRAMC_LPR_PASR(x) ((x) << 4) +#define SDRAMC_LPR_TCSR(x) ((x) << 8) +#define SDRAMC_LPR_DS(x) ((x) << 10) +#define SDRAMC_LPR_TIMEOUT(x) ((x) << 12) + +/* IER/IDR/IMR/ISR - Interrupt Enable/Disable/Mask/Status Register */ +#define SDRAMC_ISR_RES ( 1 << 0) + +/* MDR - Memory Device Register */ +#define SDRAMC_MDR_MD_SDRAM ( 0 << 0) +#define SDRAMC_MDR_MD_LOW_PWR_SDRAM ( 1 << 0) + +/* Register access macros */ +#define sdramc_readl(reg) \ + __raw_readl((void __iomem __force *)SDRAMC_BASE + SDRAMC_##reg) +#define sdramc_writel(reg, value) \ + __raw_writel(value, (void __iomem __force *)SDRAMC_BASE + SDRAMC_##reg) diff --git a/include/asm-avr32/arch-at32ap/pm.h b/include/asm-avr32/arch-at32ap/pm.h index 356e430..979b355 100644 --- a/include/asm-avr32/arch-at32ap/pm.h +++ b/include/asm-avr32/arch-at32ap/pm.h @@ -19,6 +19,7 @@ #ifndef __ASSEMBLY__ extern void cpu_enter_idle(void); +extern void cpu_enter_standby(unsigned long sdramc_base); extern bool disable_idle_sleep; @@ -43,6 +44,8 @@ static inline void cpu_idle_sleep(void) else cpu_enter_idle(); } + +void intc_set_suspend_handler(unsigned long offset); #endif #endif /* __ASM_AVR32_ARCH_PM_H */ diff --git a/include/asm-avr32/thread_info.h b/include/asm-avr32/thread_info.h index 07049f6..df68631 100644 --- a/include/asm-avr32/thread_info.h +++ b/include/asm-avr32/thread_info.h @@ -88,6 +88,7 @@ static inline struct thread_info *current_thread_info(void) #define TIF_MEMDIE 6 #define TIF_RESTORE_SIGMASK 7 /* restore signal mask in do_signal */ #define TIF_CPU_GOING_TO_SLEEP 8 /* CPU is entering sleep 0 mode */ +#define TIF_FREEZE 29 #define TIF_DEBUG 30 /* debugging enabled */ #define TIF_USERSPACE 31 /* true if FS sets userspace */ -- 1.5.5.4 -- 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/