Annotation of wikisrc/ports/evbarm/raspberry_pi.mdwn, revision 1.120

1.1       jakllsch    1: [[!meta title="NetBSD/evbarm on Raspberry Pi"]]
                      2: 
1.120   ! gdt         3: This page describes the NetBSD/evbarm port on [Raspberry Pi](http://www.raspberrypi.org) hardware.  All [board variants](https://en.wikipedia.org/wiki/Raspberry_Pi#Specifications) earlier than the RPI4 are believed supported, and specific boards known to work are listed.  We use e.g. "RPI2" to refer to "Raspberry Pi 2" to save precious bytes on this page.  This web page is 32-bit (aarch32) centric, as that has been until mid-2018 the only approach.
1.39      wiki        4: 
1.104     gdt         5: Initial, limited, Raspberry Pi support was introduced in NetBSD 6.0.  NetBSD 7.0 adds complete support for the board, along with introducing support for the quad-core Raspberry Pi 2 board.  Raspberry Pi 3 support was added for NetBSD 8, and backported to NetBSD 7 in July of 2017.  (This page assumes those using NetBSD 7 are using 7.2 or later, but note that everyone uses 8 or -current.)
1.39      wiki        6: 
1.120   ! gdt         7: In mid-2018, -current gained support for 64-bit ARM (aarch64) support.  This will be in NetBSD 9.
1.114     gdt         8: 
1.3       wiki        9: [[images/raspberrypi.jpg]]
                     10: 
1.26      wiki       11: [[!toc levels=2]]
                     12: 
1.14      wiki       13: <small>([Raspberry Pi image](http://www.flickr.com/photos/42325803@N07/8118758647/) by Christopher Lee used under CC-By-2.0 license)</small>
1.3       wiki       14: 
1.54      gdt        15: # What works (and what doesn't yet)
1.53      gdt        16: 
1.70      gdt        17: ## NetBSD 7 and NetBSD 8
1.53      gdt        18: 
1.102     gdt        19:  - RPI1, RPI2, RPI2-1.2, RPI3, RPI3+ (except RPI3 builtin WiFi and bluetooth)
                     20:  - RPI0 and RPI0W are expected to work (without WiFi, and one needs fdt files \todo where from?)
1.101     gdt        21:  - multiple processors on RPI2/RPI3
1.74      gdt        22:  - boots normally to multiuser, with FAT32 boot partition on uSD
                     23:  - root filesystem can be uSD or USB-attached mass storage
1.53      gdt        24:  - serial or graphics console (with EDID query / parsing)
1.74      gdt        25:  - X11 via HDMI
                     26:  - GPU (VCHIQ) - 3D and video decode. man page missing.
                     27:  - USB host controller - dwctwo(4) and most devices work
                     28:  - USB Ethernet - usmsc(4)
1.53      gdt        29:  - DMA controller driver and sdhc(4) support
1.74      gdt        30:  - RNG
1.53      gdt        31:  - Audio: works. man page missing.
1.74      gdt        32:  - GPIO
1.53      gdt        33:  - I²C: works, could use enhancements, man page
                     34:  - SPI: could use enhancements, man page
                     35: 
                     36: ## NetBSD current
                     37: 
1.105     gdt        38:  - RPI3+ 
1.101     gdt        39:  - RPI3 builtin bluetooth
                     40:  - RPI3 new SD host controller driver
1.114     gdt        41:  - \todo Verify: RPI2-1.2 and RPI3 aarch64
1.53      gdt        42: 
1.102     gdt        43: ## What needs documenting if it works
                     44: 
                     45:  - CM1
                     46:  - CM3
                     47:  - CM3lite
                     48: 
1.54      gdt        49: ## What needs work
1.53      gdt        50: 
1.120   ! gdt        51:  - RPI4 (almost certainly; not clear)
1.53      gdt        52:  - USB (host); isochronous transfers.
1.105     gdt        53:  - RPI3, RPI0W builtin WiFi
                     54:  - RPI0W Bluetooth Low Energy (probably)
1.113     gdt        55:  - aarch64 support is evolving very rapidly, and not yet recommended for production, largely because one must run -current.
1.53      gdt        56: 
1.57      gdt        57: # CPU types
                     58: 
1.101     gdt        59:  - RPI1 uses "earmv6hf".
1.109     gdt        60:  - RPI0 uses "earmv6hf".
                     61:  - RPI0W uses "earmv6hf".
1.101     gdt        62:  - RPI2 uses "earmv7hf".
1.113     gdt        63:  - RPI2-1.2 uses "earmv7hf" or "aarch64" (armv8 CPU hardware)
                     64:  - RPI3 uses "earmv7hf" or "aarch64" (armv8 CPU hardware)
1.57      gdt        65: 
1.101     gdt        66: Note that one can run a build of earmv6hf on the 2 and 3.  There will still be a kernel7, built to use the 2/3 hardware, but with the armv6 instruction set.
1.96      gdt        67: 
1.101     gdt        68: In theory the code compiled for earmv7hf will be faster, but anecdotal experience is that it doesn't matter that much.
1.97      gdt        69: 
1.101     gdt        70: While the evbarm port has "eb" variants (for big-endian mode), the RPI systems do not support eb and these variants will not work.  Systems built with older CPU architectures (earm, earmv4, earmv5) are not expected to work on RPI.
1.96      gdt        71: 
1.113     gdt        72: The RPI2-1.2 and RPI3 have an armv8 CPU that supports aarch64 (64-bit
                     73: mode) in addition to aarch32 (regular 32-bit ARM).  This is supported,
                     74: in -current only, by the "aarch64" MACHINE_ARCH of evbarm, also
                     75: available in build.sh via the alias evbarm64.  This is sometimes
                     76: referred to as [[NetBSD/aarch64|aarch64]].
1.70      gdt        77: 
1.7       wiki       78: # Installation
1.53      gdt        79: 
1.62      gdt        80: ## SD card structure
                     81: 
1.86      gdt        82: The Raspberry Pi looks for firmware and kernel.img on the first FAT32 MBR partition of the uSD card.  A separate kernel (kernel7.img) is used on RPI2 and RPI3.
                     83: The NetBSD kernel will then find NetBSD MBR partition and within that the root disklabel partition, and use that FFS partition as the root filesystem.
1.62      gdt        84: 
1.86      gdt        85: A 2 GB card is the smallest workable size, and the installation image will fit.  After the first boot, the system resizes the NetBSD root partition to fill the card.  Note that swap is after /boot and before /, and not contained in the NetBSD fdisk partition.  However, if you don't try to change the partition structure, this should not cause you any trouble.
1.63      gdt        86: 
1.91      gdt        87: Note that SD cards generally have limited write tolerance, so you may wish to disable atime updates via the noatime option, as is done by the default installation.
                     88: 
1.62      gdt        89: ## Choosing a version
                     90: 
1.95      gdt        91: First, decide if you want to install a formal release (7.2 or 8.0), a stable branch build (netbsd-7, netbsd-8), or NetBSD-current.  For people who don't know how to choose among those, a recent build of netbsd-8 is probably best, with 8.0 the choice for those who value being at exactly a formal release.
1.65      gdt        92: 
                     93: See also "ebijun's image", below, which is NetBSD-current and includes packages.
1.58      gdt        94: 
                     95: ## Getting bits to install
                     96: 
1.96      gdt        97: You can either build a release yourself with build.sh, or get a release from the NetBSD HTTPS/FTP servers.  The bits from both sources should match, except for things like  timestamps, or because the sources are from slightly different points along branches.
1.58      gdt        98: 
                     99: ### Building yourself
                    100: 
1.95      gdt       101: Getting sources and building a release with build.sh is not special for evbarm.  However, the evbarm port has a very large number of CPU types, compared to i386 and amd64 which have one each.  The standard approach is to use -m to define MACHINE and -a to define MACHINE_ARCH.  build.sh supports aliases that can be passed as a MACHINE value, but denote both MACHINE and a MACHINE_ARCH.   The third line uses an alias and is equal to the second, for RPI2/3.  Note that the aliases start with "evb" while the MACHINE_ARCH values do not, and that aliases have "-el" or "-eb", while the MACHINE_ARCH values have no suffix or "eb".
1.66      gdt       102: 
1.59      gdt       103:  - ./build.sh -m evbarm -a earmv6hf -u release
                    104:  - ./build.sh -m evbarm -a earmv7hf -u release
1.95      gdt       105:  - ./build.sh -m evbearmv7hf-el -u release
1.81      gdt       106: 
1.94      gdt       107: Consider setting RELEASEMACHINEDIR if you wish to build multiple MACHINE_ARCH values for a MACHINE; see build.sh.  Use something like "evbarm-earmv7hf", so that 1) earvm6 and earmv7 don't collide and 2) anita will recognize it as a type of evbarm.
1.58      gdt       108: 
1.85      gdt       109: ### NetBSD autobuild HTTPS/FTP servers
1.58      gdt       110: 
1.115     sevan     111: NetBSD provides nightly builds on [nycdn.netbsd.org](https://nycdn.netbsd.org/pub/NetBSD-daily/).  The next directory level is the branch being built (netbsd-7, netbsd-8, HEAD, and more), plus optionally things like compiler type.  It is followed by date/time, e.g. "HEAD/201811051650Z"; once a build is complete the symlink "latest" is adjusted to point to it.  The next level is "${MACHINE}-${MACHINE_ARCH}", e.g. "evbarm-earmv7hf", and multiple combinations are provided.
1.58      gdt       112: 
1.115     sevan     113: An example URL, arguably the standard approach for first-time NetBSD/RPI users, is https://nycdn.netbsd.org/pub/NetBSD-daily/netbsd-8/latest/evbarm-earmv7hf/binary/gzimg/
1.95      gdt       114: 
                    115: ### release layout
                    116: 
1.96      gdt       117: Once you get to the releasedir, self-built and autobuild releases have the same structure.
                    118: 
1.85      gdt       119:  - The 'evbarm-earmv6hf/binary/gzimg/' directory contains an rpi.img file that will run on any of the RPI boards.
1.103     gdt       120:  - The 'evbarm-earmv7hf/binary/gzimg/' directory contains an armv7.img file that uses the armv7 instruction set, and thus can run only on the RPI2 and RPI3 (and perhaps the CM3).  It also supports systems other than the RPI family.
1.85      gdt       121: 
1.95      gdt       122: \todo Explain why there is no armv7_inst.gz.
1.58      gdt       123: 
1.65      gdt       124: ## Preparing a uSD card
1.10      wiki      125: 
1.107     gdt       126: Once you have rpi.img.gz (or rpi_inst for earmv6 boards), put it on a uSD card using gunzip and dd, for example:
1.14      wiki      127: 
1.60      gdt       128:  - gunzip rpi.img.gz
1.67      ryoon     129:  - dd if=rpi.img of=/dev/disk1
1.14      wiki      130: 
1.89      gdt       131: ## Console approaches
                    132: 
                    133: The standard approach is to use a USB keyboard and an HDMI monitor for installation.
                    134: 
1.58      gdt       135: ### Serial Console
                    136: 
1.89      gdt       137: By default the rpi.img is set to use the HDMI output.  If you wish to use a serial console, mount the FAT32 partition on another system and edit cmdline.txt and remove '"console=fb"'.
1.14      wiki      138: 
1.109     gdt       139:  - Most (all?) USB-to-TTL serial adapters have wires for TX, RX and ground, and not RTS/CTS or other flow control lines.   Thus, your terminal program (or terminal) must be configured to not require flow control; a symptom of misconfiguration is that you see console output, but cannot type anything.  If so, adjust your serial console application's flow control settings to "none".
1.41      wiki      140: 
1.89      gdt       141:    - In Kermit, the command is "set flow none".
                    142:    - In minicom, run "minicom -s" and set hardware flow control to "no".
1.41      wiki      143: 
1.89      gdt       144: ### Enabling ssh for installation without any console
1.41      wiki      145: 
1.89      gdt       146: If you want to enable ssh with the standard image, so that you can log in over the net without either a serial or HDMI console, you can edit the configuration of a uSD card before booting.   On another computer, mount the ffs partition, place /root/.ssh/authorized_keys, uncomment PermitRootLogin in /etc/ssh/sshd_config, and comment out the rc_configure=NO in /etc/rc.conf.  Besides having to find the IP address (e.g. from DHCP server logs), you will have to wait for the partition resizing and reboot.
1.65      gdt       147: 
1.89      gdt       148: ### Installation with sshramdisk image
1.65      gdt       149: 
1.89      gdt       150: build.sh (and hence the FTP site) also creates an image 'rpi_inst.img.gz' specifically for installation without HDMI or a serial console, when built for earmv6hf.  Note that this image is much smaller and that you will need to fetch the sets over the network.  To use this method, write that image to a uSD card as above, and then:
1.58      gdt       151: 
1.89      gdt       152:  - Connect an Ethernet cable from the RPI to a LAN with a DHCP server, and another host you can use for ssh.
                    153:  - Power on the RPI, and wait.  Watch the logs on the DHCP server, and find the IP address assigned to the RPI.
                    154:  - Use ssh to login to the address you found with user "sysinst", and password "netbsd".
                    155:  - When installing, ensure that you enable DHCP and ssh, so that you can log in again after the system is installed.
1.53      gdt       156: 
1.107     gdt       157: The rpi_inst.img.gz image will only work for systems that use earmv6hf kernels (so not RPI2/3).  See [this port-arm message](https://mail-index.netbsd.org/port-arm/2017/08/18/msg004374.html) for details.
                    158: 
1.55      gdt       159: ## Installation via ebijun's image
                    160: 
1.58      gdt       161: As an alternative to the standard installation images, Jun Ebihara
                    162: provides an install image for Raspberry Pi that includes packages.  It
                    163: is based on NetBSD-current and is built for earmv6hf, and thus will
                    164: work on Raspberry Pi 1, 2 and 3.  This image is typically updated
                    165: every few weeks.
1.55      gdt       166: 
1.56      gdt       167:  - [https://github.com/ebijun/NetBSD/blob/master/RPI/RPIimage/Image/README](https://github.com/ebijun/NetBSD/blob/master/RPI/RPIimage/Image/README)
1.55      gdt       168: 
1.98      gdt       169: ## Configuring 802.11
                    170: 
                    171: After installation, the Ethernet will function as on any other NetBSD system; simply enable dhcpcd or configure a static address.  USB WiFi devices will also function as on any other NetBSD system; in addition to dhcpcd or static, configure and enable wpa_supplicant.
                    172: 
                    173: Note that the built-in WiFi in the RPI3 is not yet supported.   USB WiFi interfaces (that work on NetBSD in general) should all work.  In particular, the following are known to work:
                    174: 
                    175:  - urtwn0: Realtek (0xbda) 802.11n WLAN Adapter (0x8176), rev 2.00/2.00, addr 5, MAC/BB RTL8188CUS, RF 6052 1T1R
                    176: 
1.90      gdt       177: ## Links
                    178: 
                    179: The following pages have been published by NetBSD community members.  (Note that some of them are old.)
                    180: 
                    181:  - https://www.cambus.net/netbsd-on-the-raspberry-pi/
                    182: 
1.74      gdt       183: # Maintaining a system
                    184: 
1.78      gdt       185: ## vcgencmd
                    186: 
1.80      gdt       187: The program vcgencmd, referenced in the boot section,  can be found in pkgsrc/misc/raspberrypi-userland.
1.78      gdt       188: 
1.53      gdt       189: ## Updating the kernel
1.46      schmonz   190: 
1.42      wiki      191:  - Build a new kernel, e.g. using build.sh. It will tell you where the ELF version of the kernel is, e.g.
                    192: 
                    193:          ...
                    194:          Kernels built from RPI2:
                    195:           /Users/feyrer/work/NetBSD/cvs/src-current/obj.evbarm-Darwin-XXX/sys/arch/evbarm/compile/RPI2/netbsd
                    196:          ...
                    197: 
1.69      rin       198:  - Besides the "netbsd" kernel in ELF format, there is also a "netbsd.img" (for current) or "netbsd.bin" (for 7 and 8) kernel that is in a format that the Raspberry can boot.
1.48      sevan     199:  - Depending on your hardware version, copy this either to /boot/kernel.img (First generation Pi, Pi Zero hardware) or to /boot/kernel7.img (Pi 2, Pi 3 hardware)
1.42      wiki      200:  - reboot
                    201: 
1.73      gdt       202: ## Updating the firmware
                    203: 
                    204: A section below describes the process of updating NetBSD's copy of the firmware from upstream, with testing, by NetBSD developers.  This section is about updating a system's firmware from the firmware in a version of NetBSD.
                    205: 
1.88      gdt       206: \todo Explain where the firmware is in the source tree, and if it is in the installed system image (such as /usr/mdec).  Explain how to update a system (presumably /boot) from either an installed system's new firmware files, or the source tree.  Explain any particular cautions.
1.73      gdt       207: 
1.105     gdt       208: Probably, for the RPI3+, one needs to use -current, or use -8 with firmware from -current.  \todo Defuzz.
1.101     gdt       209: 
1.99      gdt       210: \todo Explain if using updated firmware from one branch (e.g. netbsd-current) on a system using a different branch (e.g. netbsd-8) is safe.  Explain if pullups are done to release branches with new firmware.
                    211: 
1.75      gdt       212: ## Booting
                    213: 
1.79      gdt       214: The device boots by finding a file "bootcode.bin".   The primary location is a FAT32 partition on the uSD card, and an additional location is on a USB drive.  See the [upstream documentation on booting](https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/) and read all the subpages.
1.75      gdt       215: 
1.112     gdt       216: The standard approach is to use a uSD card, with a fdisk partition table containing a FAT32 partition marked active, and a NetBSD partition.  The NetBSD partition will then contain a disklabel, pointing to an FFS partition (a), a swap partition (b) and the FAT32 boot partition mounted as /boot (e).  The file /boot/cmdline.txt has a line to set the root partition.
1.75      gdt       217: 
1.112     gdt       218: One wrinkle in the standard approach is that the disk layout is "boot swap /", but the NetBSD fdisk partition starts at the location of /, so the swap partition is not within the NetBSD fdisk partition.  The / partition can hold a disklabel, while swap cannot.   It is normal to have swap after / (and thus within the fdisk partition), but the arrangement used permits growing / on first boot, for the typical case where a larger uSD is used, compared to the minimum image size.
1.75      gdt       219: 
1.77      gdt       220: An alternate approach is to have the boot FAT32 partition as above, but to have the entire system including root on an external disk.  This is configured by changing root=ld0a to root=sd0a or root=dk0 (depending on disklabel/GPT).  Besides greater space, part of the point is to avoid writing to the uSD card.
1.75      gdt       221: 
1.80      gdt       222: A third approach, workable on the Pi 3 only, is to configure USB host booting (already enableed on the 3+; see the upstream documentation) and have the boot partition also on the external device.  In this case the external device must have an MBR because the hardware's first-stage boot does not have GPT support. In theory the [procedure to program USB host boot mode](https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/msd.md) will function on a NetBSD system because the programming is done by bootcode.bin.
                    223: \todo Confirm that putting program_usb_boot_mode=1 in config.txt and booting works to program the OTP bit.  Confirm that one can then boot NetBSD from external USB.
1.75      gdt       224: 
1.101     gdt       225: There is no well-defined USB enumeration order, so the preferred approach if one has multiple USB mass storage devices is to use named wedges in both fstab and cmdline.txt.
1.75      gdt       226: 
1.113     gdt       227: ## Split-mode aarch32/aarch64
                    228: 
                    229: \todo Verify this, and add any necessary cautions about boot code.
                    230: 
                    231: The aarch64 kernel can run aarch32 binaries, so one can boot a aarch64 kernel on a system with aarch32 userland.
                    232: 
1.93      gdt       233: # X11 and GPU
                    234: 
1.27      wiki      235: ## Video playback
1.29      wiki      236: Accelerated video playback is supported in NetBSD 7 with the [OMXPlayer](http://pkgsrc.se/multimedia/omxplayer) application and through GStreamer with the [omx](http://pkgsrc.se/multimedia/gst-plugins1-omx) plugin.
1.27      wiki      237: 
                    238: ## OpenGL ES
                    239: Accelerated OpenGL ES is supported in NetBSD 7. The GL ES client libraries are included with the [misc/raspberrypi-userland](http://pkgsrc.se/misc/raspberrypi-userland) package.
                    240: 
1.28      wiki      241: ## Quake 3
1.27      wiki      242: A Raspberry Pi optimized build of *ioquake3* is available in the [games/ioquake3-raspberrypi](http://pkgsrc.se/games/ioquake3-raspberrypi) package. To use it, the following additional resources are required:
                    243: 
                    244:  - pak0.pk3 from Quake 3 CD
1.31      snj       245:  - additional pak files from the [games/ioquake3-pk3](http://pkgsrc.se/games/ioquake3-pk3) package
1.27      wiki      246:  - read/write permissions on /dev/vchiq and /dev/wsmouse
                    247: 
1.31      snj       248: Place the pak0.pk3 file in the /usr/pkg/lib/ioquake3/baseq3 directory.
1.27      wiki      249: 
1.32      wiki      250: ## RetroArch / Libretro
                    251: Using [emulators/retroarch](http://pkgsrc.se/emulators/retroarch) it is possible to run many emulators at full speed the Raspberry Pi. Emulator cores for various gaming consoles are available in the [emulators/libretro-*](http://pkgsrc.se/search.php?so=libretro-) packages. To begin using retroarch:
                    252: 
                    253:  - Install [emulators/retroarch](http://pkgsrc.se/emulators/retroarch)
                    254:  - Install the libretro core for the system you would like to emulate (lets take [emulators/libretro-gambatte](http://pkgsrc.se/emulators/libretro-gambatte), a GameBoy Color emulator, as an example).
1.118     nia       255:  - Make sure your user has read and write permissions on `/dev/vchiq`.
                    256:  - Plug in a USB HID compatible Gamepad, such as the Logitech F710 in "DirectInput" mode (set "D/X" switch to "D"). Note that since the framebuffer GL driver will not allow for keyboard input in RetroArch, you will have to copy your joypad configuration from another system.
                    257:  - Configure retroarch by editing $HOME/.config/retroarch/retroarch.cfg:
                    258:        video_driver = "gl"
                    259:        input_driver = "null"
                    260:        joypad_driver = "sdl2"
1.116     nia       261:        menu_driver = "rgui"
1.32      wiki      262: 
1.53      gdt       263: # Developer notes
1.50      gdt       264: 
1.53      gdt       265: These notes are for people working on improvements to RPI support in NetBSD.
1.50      gdt       266: 
1.72      gdt       267: ## Updating the firmware version in the NetBSD sources
1.50      gdt       268: 
1.72      gdt       269: (Note that trying new firmware may result in a non-bootable system, so
                    270: be prepared to recover the bootable media with another system.)
1.50      gdt       271: 
1.72      gdt       272: Upstream firmware releases are
                    273: [on GitHub](https://github.com/raspberrypi/firmware/releases).
                    274: Copy all files except `kernel*.img` into `/boot` and reboot.
                    275: 
                    276: New firmware should pass all of the following tests before being committed to NetBSD.
1.50      gdt       277: 
1.53      gdt       278: - Audio
                    279: - OMXPlayer (and [[!template id=man name="vchiq"]])
                    280: - Serial/framebuffer console
                    281: - CPU frequency scaling
1.50      gdt       282: 
1.92      gdt       283: Tests should be run on all of `rpi[0123]`.
1.94      gdt       284: 
                    285: ## Testing with anita and qemu
                    286: 
1.111     gdt       287: See the anita section in the evbarm page.
1.94      gdt       288: 
1.110     gdt       289: It is not currently known how to emulate a RPI in qemu, and therefore anita does not yet have support for this.  \todo Add a command-line example to run qemu emulating some RPI model.
1.119     leot      290: 
                    291: 
                    292: # Misc notes
                    293: Miscellaneous notes about Raspberry PI.
                    294: 
                    295: ## Power supply needed (or: why there is a little rainbow square in the top-right corner?)
                    296: Raspberry Pi devices are powered by 5V micro USB and a 2.5A (2500mA)
                    297: power supply is reccomended.  For more information please read:
                    298: 
                    299:  <https://www.raspberrypi.org/documentation/faqs/#pi-power>
                    300: 
                    301: Power glitches can also manifest in other ways, e.g. with an USB
                    302: disk plugged:
                    303: 
                    304: [[!template id=programlisting text="""
                    305: sd0(umass0:0:0:0): generic HBA error
                    306: sd0: cache synchronization failed
                    307: """]]
                    308: 
                    309: Using a reccomended power supply avoid such issues.
                    310: 
                    311: ## Xenon death flash (Raspberry Pi 2 is camera-shy)
                    312: When using laser pointers or xenon flashes in cameras (or other
                    313: flashes of high-intensity long-wave light) against a Raspberry Pi
                    314: 2 the Pi can power itself off.
                    315: For more information please read:
                    316: 
                    317:  <https://www.raspberrypi.org/blog/xenon-death-flash-a-free-physics-lesson/>

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