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AP_HAL_FLYMAPLE: added documentation about safety of noInterrupts()/interrupts()

This commit is contained in:
Mike McCauley 2013-09-24 21:55:30 +10:00 committed by Andrew Tridgell
parent ab71f2e535
commit 4cd952446d
2 changed files with 31 additions and 21 deletions
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23
libraries/AP_HAL_FLYMAPLE/FlymaplePortingNotes.txt
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@ -110,6 +110,29 @@ LIBMAPLE_PATH = $(HOME)/libmaple
# Also, the ARM compiler tools MUST be in your current PATH
#config.mk END
Interrupt disabling on ARM
On AVR, ISRs run by default with the global interrupt enable flag disabled,
whereas mainline code runs by default with global interrupt enable flag
*enabled*. Which means that cli()/sei() in an ISR will have a different effect
to cli()sei() in mainline code. Thats why code that *might* run in an ISR must
use the special idiom: so that it restores the flag to the state it was before
the critical block
On ARM, the global interrupt disable flag PRIMASK is not altered behind your
back by hardware. By default its always clear (ie enabled) even in ISRs. A
different mechanism prevents ISRs from being reinterrupted. This means that
non-nested noInterrupts()/interrupts() will always leave the PRIMASK as it was
(interrupts enabled) when the critical block started, whether in ISRs or
mainline code.
Conclusion:
On AVR, cli()/sei() is dangerous both in ISRs *and* when nested.
On ARM, noInterrupts()/interrupts() is only dangerous when nested.
Remaining issues:
1. For reasons I do not yet understand, the magnetic heading reported by

29
libraries/AP_HAL_FLYMAPLE/Scheduler.cpp
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@ -1,30 +1,17 @@
/*
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 3 of the License, or
(at your option) 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, see <http://www.gnu.org/licenses/>.
*/
/*
Flymaple port by Mike McCauley
*/
#include <AP_HAL.h>
#if CONFIG_HAL_BOARD == HAL_BOARD_FLYMAPLE
// Scheduler.cpp
//
// Flymaple Scheduler.
// We use systick interrupt for the 1kHz ordinary timers.
// We use a slightly higher priority HardwareTimer 2 for the failsafe callbacks
// so a hung timer wont prevent the failsafe timer interrupt running
//
// Use of noInterrupts()/interrupts() on FLymaple ARM processor.
// Please see the notes in FlymaplePortingNotes.txt in this directory for
// information about disabling interrupts on Flymaple
#include <AP_HAL.h>
#if CONFIG_HAL_BOARD == HAL_BOARD_FLYMAPLE
#include "Scheduler.h"