efi_timer.c

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/*
 * Copyright (C) 2008 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );

#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <ipxe/timer.h>
#include <ipxe/init.h>
#include <ipxe/efi/efi.h>

/** @file
 *
 * iPXE timer API for EFI
 *
 */

/**
 * Number of jiffies per second
 *
 * This is a policy decision.
 */
#define EFI_JIFFIES_PER_SEC 32

/** Current tick count */
static unsigned long efi_jiffies;

/** Timer tick event */
static EFI_EVENT efi_tick_event;

/** Colour for debug messages */
#define colour &efi_jiffies

/**
 * Delay for a fixed number of microseconds
 *
 * @v usecs             Number of microseconds for which to delay
 */
static void efi_udelay ( unsigned long usecs ) {
        EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
        EFI_STATUS efirc;
        int rc;

        if ( ( efirc = bs->Stall ( usecs ) ) != 0 ) {
                rc = -EEFI ( efirc );
                DBGC ( colour, "EFI could not delay for %ldus: %s\n",
                       usecs, strerror ( rc ) );
                /* Probably screwed */
        }
}

/**
 * Get current system time in ticks
 *
 * @ret ticks           Current time, in ticks
 */
static unsigned long efi_currticks ( void ) {
        EFI_BOOT_SERVICES *bs = efi_systab->BootServices;

        /* UEFI manages to ingeniously combine the worst aspects of
         * both polling and interrupt-driven designs.  There is no way
         * to support proper interrupt-driven operation, since there
         * is no way to hook in an interrupt service routine.  A
         * mockery of interrupts is provided by UEFI timers, which
         * trigger at a preset rate and can fire at any time.
         *
         * We therefore have all of the downsides of a polling design
         * (inefficiency and inability to sleep until something
         * interesting happens) combined with all of the downsides of
         * an interrupt-driven design (the complexity of code that
         * could be preempted at any time).
         *
         * The UEFI specification expects us to litter the entire
         * codebase with calls to RaiseTPL() as needed for sections of
         * code that are not reentrant.  Since this doesn't actually
         * gain us any substantive benefits (since even with such
         * calls we would still be suffering from the limitations of a
         * polling design), we instead choose to run at TPL_CALLBACK
         * almost all of the time, dropping to a lower TPL to allow
         * timer ticks to occur.
         *
         * We record the external TPL at the point of entry into iPXE,
         * and drop back only as far as this external TPL.  This
         * avoids the unexpected behaviour that may arise from having
         * iPXE temporarily drop to TPL_APPLICATION in the middle of
         * an entry point invoked at TPL_CALLBACK.  The side effect is
         * that iPXE's view of the system time is effectively frozen
         * for the duration of any call made in to iPXE at
         * TPL_CALLBACK or higher.
         *
         *
         * For added excitement, UEFI provides no clean way for device
         * drivers to shut down in preparation for handover to a
         * booted operating system.  The platform firmware simply
         * doesn't bother to call the drivers' Stop() methods.
         * Instead, all non-trivial drivers must register an
         * EVT_SIGNAL_EXIT_BOOT_SERVICES event to be signalled when
         * ExitBootServices() is called, and clean up without any
         * reference to the EFI driver model.
         *
         * Unfortunately, all timers silently stop working when
         * ExitBootServices() is called.  Even more unfortunately, and
         * for no discernible reason, this happens before any
         * EVT_SIGNAL_EXIT_BOOT_SERVICES events are signalled.  The
         * net effect of this entertaining design choice is that any
         * timeout loops on the shutdown path (e.g. for gracefully
         * closing outstanding TCP connections) may wait indefinitely.
         *
         * There is no way to report failure from currticks(), since
         * the API lazily assumes that the host system continues to
         * travel through time in the usual direction.  Work around
         * EFI's violation of this assumption by falling back to a
         * simple free-running monotonic counter during shutdown.
         */
        if ( efi_shutdown_in_progress ) {
                efi_jiffies++;
        } else {
                bs->RestoreTPL ( efi_external_tpl );
                bs->RaiseTPL ( efi_internal_tpl );
        }

        return ( efi_jiffies * ( TICKS_PER_SEC / EFI_JIFFIES_PER_SEC ) );
}

/**
 * Timer tick
 *
 * @v event             Timer tick event
 * @v context           Event context
 */
static EFIAPI void efi_tick ( EFI_EVENT event __unused,
                              void *context __unused ) {

        /* Increment tick count */
        efi_jiffies++;
}

/**
 * Start timer tick
 *
 */
static void efi_tick_startup ( void ) {
        EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
        EFI_STATUS efirc;
        int rc;

        /* Create timer tick event */
        if ( ( efirc = bs->CreateEvent ( ( EVT_TIMER | EVT_NOTIFY_SIGNAL ),
                                         TPL_CALLBACK, efi_tick, NULL,
                                         &efi_tick_event ) ) != 0 ) {
                rc = -EEFI ( efirc );
                DBGC ( colour, "EFI could not create timer tick: %s\n",
                       strerror ( rc ) );
                /* Nothing we can do about it */
                return;
        }

        /* Start timer tick */
        if ( ( efirc = bs->SetTimer ( efi_tick_event, TimerPeriodic,
                                      ( 10000000 / EFI_JIFFIES_PER_SEC ) ))!=0){
                rc = -EEFI ( efirc );
                DBGC ( colour, "EFI could not start timer tick: %s\n",
                       strerror ( rc ) );
                /* Nothing we can do about it */
                return;
        }
        DBGC ( colour, "EFI timer started at %d ticks per second\n",
               EFI_JIFFIES_PER_SEC );
}

/**
 * Stop timer tick
 *
 * @v booting           System is shutting down in order to boot
 */
static void efi_tick_shutdown ( int booting __unused ) {
        EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
        EFI_STATUS efirc;
        int rc;

        /* Stop timer tick */
        if ( ( efirc = bs->SetTimer ( efi_tick_event, TimerCancel, 0 ) ) != 0 ){
                rc = -EEFI ( efirc );
                DBGC ( colour, "EFI could not stop timer tick: %s\n",
                       strerror ( rc ) );
                /* Self-destruct initiated */
                return;
        }
        DBGC ( colour, "EFI timer stopped\n" );

        /* Destroy timer tick event */
        if ( ( efirc = bs->CloseEvent ( efi_tick_event ) ) != 0 ) {
                rc = -EEFI ( efirc );
                DBGC ( colour, "EFI could not destroy timer tick: %s\n",
                       strerror ( rc ) );
                /* Probably non-fatal */
                return;
        }
}

/** Timer tick startup function */
struct startup_fn efi_tick_startup_fn __startup_fn ( STARTUP_EARLY ) = {
        .name = "efi_tick",
        .startup = efi_tick_startup,
        .shutdown = efi_tick_shutdown,
};

/** EFI timer */
struct timer efi_timer __timer ( TIMER_NORMAL ) = {
        .name = "efi",
        .currticks = efi_currticks,
        .udelay = efi_udelay,
};