power_manager/udev/optional/set_blkdev_pm

#!/bin/sh

# Copyright 2018 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

# This script configures power management for block devices. It is executed
# at boot time and whenever power is plugged or unplugged (through udev).
#
# If supported by the block device APM (Advanced Power Management) is enabled,
# otherwise the standby timeout of the device is configured. The specific
# settings vary depending on whether the device runs on battery or AC.

if dump_power_status | grep -q 'line_power_connected 1'; then
  on_ac="1"
else
  on_ac="0"
fi

for dev_sysfs in /sys/block/sd[a-z]; do
  # handle glob without matches
  [ -e "${dev_sysfs}" ] || break

  is_removable=$(cat "${dev_sysfs}/removable")
  if [ "${is_removable}" = "1" ]; then
    continue
  fi

  dev_node="/dev/${dev_sysfs##*/}"

  if hdparm -B "${dev_node}" | grep -q 'APM_level.*=.*not supported'; then
    has_apm="0"
  else
    has_apm="1"
  fi

  # Skip APM for SanDisk SSDs, it is too aggressive (b/35524329)
  if grep -q "SanDisk.*SSD" "${dev_sysfs}/device/model"; then
    has_apm="0"
  fi

  if [ "${has_apm}" = "1" ]; then
    if [ "${on_ac}" = "1" ]; then
      # max performance
      apm_level=254
    else
      # max power savings
      apm_level=1
    fi

    hdparm -B "${apm_level}" "${dev_node}"
  else
    # APM not supported
    if [ "${on_ac}" = "1" ]; then
      # set standby timeout to 2h
      standby_timeout=120
    else
      # set standby timeout to 5s
      standby_timeout=1
    fi

    hdparm -S "${standby_timeout}" "${dev_node}"
  fi
done