The Reboot Command - History

The reboot command in Linux is a 'soft' reboot which means that the processor is restarted but not actually powered down and powered back on.

Invariably there are situations which sometimes can cause a typical 'soft restart' via the Linux reboot command to hang or not properly restart. There are a variety of reasons for this occurring including:

• Peripherals left in an unexpected state which don't have a reset line to them
• Capacitive load issues where certain peripheral combinations can cause the primary power supply to drain slowly resulting in need for a minimum 'power off' time to clear
• SoC errata

Because of these situations with a soft reboot, Gateworks has provided reliable ways to reboot the single board computer.

Reboot Command

In order to have a high reliability Gateworks has designed most of its products with a GSC and a on-board primary power supply which the GSC has the ability to disable. This design provides the following capabilities:

• Capability of powering the board off, and on again at a specific time (GSC Sleep mode)
• Providing a sane alternative to soft reset which power-cycles the board (with a user specified 'off' time)

Our latest BSP's have been updated to so that the reboot command will perform a 1 second GSC sleep essentially creating a 'hard' or 'machine' restart:

• Yocto 1.8
  • Kernel (flsc-3.14 kernel) - ​commit 061ef9
  • Yocto 1.8 OS ​commit 22f181
• Android L5 (3.14.28 kernel) - ​commit 7cb45f3
• OpenWrt 16.02 (4.4 kernel) -​commit 3d57ef0
  • Example
    • Note the line that states it is using a GSC powerdown
    • Note that the Reset cause shown in example below shows POR, because the GSC actually cycled the board power. If the Reset cause shows WDOG, then the CPU watchdog kicked in, which is not the desired result.

      root@OpenWrt:/# reboot
      root@OpenWrt:/# 
      [  273.301224] imx2-wdt 20bc000.wdog: Device shutdown: Expect reboot!
      [  273.307825] reboot: Restarting system
      [  273.311504] gsc 0-0020: GSC powerdown for 1 seconds
      U-Boot 2015.04-g040377a (Apr 11 2017 - 10:05:57)
      
      CPU:   Freescale i.MX6DL rev1.1 at 792MHz
      CPU:   Industrial temperature grade (-40C to 105C) at 57C
      Reset cause: POR
      
      

Manual Reboot via GSC Sleep

Older BSP's with the GSC driver (OpenWrt 14.08, Yocto 1.7, Android 4.4.3 (KitKat)) that have an earlier version of the GSC driver can use use the drivers powerdown sysfs hook to request a 1 second powerdown and therefore perform a safe restart as:

1. Stop any applications or services which may be writing files that you care about
2. sync the filesystems
3. Write the number of seconds to power the board down for to the gsc powerdown sysfs hook (recommend 2 seconds as this garuntees '1 to 2 seconds')

   sync && echo 2 > /sys/bus/i2c/devices/i2c-0/0-0020/powerdown
   

• Note : If performing this reset without a coin cell battery, a maximum of 4 seconds should be used for resetting the board. Anything longer may put the board to 'sleep' and not allow it to wake up without pressing the pushbutton or unplugging and plugging in the main power supply.

Older Manual Method

If for some reason you are using a BSP that does not have the GSC kernel drivers you can do this manually with i2cget/i2cset:

#!/bin/sh
#
# usage: gsc_sleep [seconds]
#

# default 2 second sleep ensures between 1 and 2 seconds pass before powerup
SECS=${1:-2}

# syncs the filesystem
echo "Syncing filesystems"
sync

# perform i2c transactions in loop to retry on i2c timeouts
echo "Powering down for $SECS seconds"
while [ 1 ]; do
  # set R6-R9 with 32bit number of seconds to wake
  i2cset -f -y 0 0x20 6 $((SECS % 256)) || continue
  SECS=$((SECS >> 8))
  i2cset -f -y 0 0x20 7 $((SECS % 256)) || continue
  SECS=$((SECS >> 8))
  i2cset -f -y 0 0x20 8 $((SECS % 256)) || continue
  SECS=$((SECS >> 8))
  i2cset -f -y 0 0x20 9 $((SECS % 256)) || continue

  # latch sleep time - taking care to not disturb other bits
  R1=$(i2cget -f -y 0 0x20 1) || continue
  R1=$((R1 | 0x04)) # set R1.2 LATCH_SLEEP
  i2cset -f -y 0 0x20 1 $R1 || continue

  # set R1.0 SLEEP_ENABLE and R1.1 ACTIVATE_SLEEP - taking care not to disturb other bits
  R1=$(i2cget -f -y 0 0x20 1) || continue
  R1=$((R1 | 0x03)) # set R1.0 and R1.1
  i2cset -f -y 0 0x20 1 $R1 || continue
done

See the complete GSC reference here