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Summary of the Arm ports BoF at DC17




[ Please note the cross-post and Reply-To ]

Hi folks,

As promised, here's a quick summary of what was discussed at the Arm
ports BoF session in Montréal. Apologies for the delay in posting...

Thanks to the awesome efforts of our video team, the session is
already online [1]. I've taken a copy of the Gobby notes too,
alongside my small set of slides for the session. [2]

We spoke about the past/present/future state of Arm ports in Debian.

arm64
=====

Most recent Arm port, first released with Jessie. We're starting to
see more devices becoming available that will run Debian arm64
well. We're also starting to get real server hardware turning
up. arm64 hardware often follows better standards than some of the
older generations of Arm hardware, meaning that most of the machines
coming (assuming kernel support!) should work well out of the box
using device tree or ACPI. Please use ACPI if you can - the two are
mostly equivalent in terms of what they describe, but ACPI is a better
choice for the future where possible, with better standards.

armhf
=====

Much as in previous BoFs in terms of support and future!

Current port, first released with Wheezy. Due to cross-distro effort,
this setup (ARMv7 EABI using VFPv3D16) is the default supported 32-bit
Arm architecture in all distros now. This means that there's even a
fair chance of binaries built for one distro running on others!

We've got a couple of multi-platform kernel variants (armmp and
armmp-lpae for large-memory systems) that will support just about any
new devices shipping, given updated drivers and device tree support.

UEFI is now a thing for some ARMv7 platforms. Some people are looking
into using UEFI by default, plus there's also the ongoing work by
Alexander Graf on the UEFI shim layer in modern U-Boot. This allows
for easier, consistent boot support (e.g. booting off USB
automatically for installation).

armel
=====

armel is the longest-running of the three current Arm ports in Debian,
starting in 2007 with Lenny.

We've been debating the future of armel for some time in Debian
now. It's typically still supported by most upstreams (Linux kernel,
gcc, etc.), but we're more frequently finding issues with support for
it in newer tools and languages and applications further up the
stack. It's difficult (in some cases almost impossible) to run many
larger applications like browsers on ARMv4t, as the upstream
developers just don't support it any more.

Based on this, I described a plan to drop armel from testing and *not*
release it with Buster. This is *not* a done deal, and there's still
scope for interested developers to step up and make sure that armel is
supported in future. We'd been asking for that for the last couple of
years and it had not happened.

We'd been talking in the past few years about options for armel,
including a partial armel port, dropping support for some packages
that are difficult to port or not considered useful. While that might
sound attractive as a possible way to go, nobody appears to have done
any of the work needed to make it possible.

************************************************************************
  In the background, almost in parallel there has been some discussion
  on the mailing list about this topic and it seems we now *do* have
  some developers wanting to keep armel alive. See the thread starting
  at [3] for more information, or if you'd like to help. I still have
  personal concerns about the viability of armel in the future, but I
  wish any other volunteers luck in keeping it alive.
************************************************************************

Whether we continue with armel or not, it's probably worthwhile for
users to think about migrating to newer hardware if they care about it
in the longer term.

Buildds and hardware
====================

The existing build machines for armel and armhf are all still ARMv7
based, using donated Marvell Armada XP machines. They're working very
well as build machines - they're quite fast and take a reasonable
amount of memory. Scaleway [4] use similar hardware for Arm-based
hosting. We also still have an older Freescale imx53 porter box for
people on armhf who might need to debug NEON issues, as the Armada XP
machines don't include it.

arm64 gives us more options for sensible build machines: AMD Seattle,
APM Mustang, Arm Junos. We're still looking at more and better
hardware in the future, including potential offers from arm64 server
hardware developers.

We're wanting to move on to more and more arm64 hardware in the future
for build machines, including for building armhf. Using real server
hardware will help us, as the machines will then fit in a rack, with
remote management options too. Unfortunately, you can't rely on these
for building for armel. There are old deprecated instructions (mainly
barriers and atomics) that are no longer supported in the ARMv8
hardware. There is optional kernel support to trap the exceptions here
and emulate the instructions, but it's really not recommended for
serious use (e.g. on a build machine!).

Using nice big arm64 boxes also makes it feasible to run build
machines and porter boxes in VMs, which is very attractive for
separation, security etc.

Vagrant reported that he's seen some issues even building (and
running) armhf code on ARMv8 hardware. This was the old barrier code
from ARMv4 days; many software projects seem to be using *really* old
embedded libraries (e.g. libgc) that desperately need updating to fix
this kind of problem. Please file bugs if you find any other similar
issues, these need to be fixed.

Further discussion
==================

If we *did* drop armel as a release architecture, we'd be the first
port in Debian to voluntarily do so. Do we need a new term for that?
("Vancouver" is already in use; "Montréal" maybe? *grin*).

We always need more porters - join us on the mailing list or in IRC!

Brief discussion about Arm being bought last year by Softbank - very
few major changes for Arm folks and things are all looking
positive. More work ongoing for future IoT developments.

packet.net have offered us VM time in their ARM-based cloud. We're not
too happy about running official build machines in such a setup - we
much prefer to control the base hardware for such machines. However,
use of these for porter boxes or reproducible builds sounds like a
great option. packet.net are using Cavium ThunderX machines, 48 cores
per socket and bigger with lots of memory. They're great for parallel
workloads, but they're *loud* and there's a bigger wrinkle for some
folks - they don't support A32 (the 32-bit Arm ISA) natively.

Clarification of ARM naming for CPUs and instruction sets, to help
explain this limitation and what it means. We would not be able to
run/build armhf (A32) code on the packet.net machines, but we could do
arm64 happily. Definitely we will need to think about security and
trust with respect to our buildds in future as we start doing Secure
Boot and maybe further secure code signing.

So, what machines would be recommended for users to buy today for
arm64 development? 48-core ThunderX machines are all well and good,
but a little pricey for hobbyist developers at home! What's available
with 4 or 8 cores, 4-8 GiB RAM, etc.? Lots of options around, but
there's a gap in the market. There are a fair number of cheap dev
boards with 1-2 GiB RAM (based on mobile CPUs), but bigger stuff is
typically much more expensive - "demo boards for server hardware",
which are much more expensive than comparable x86-based machines.

To do serious development on a *computer* rather than an embedded
device, we want something more than what's available on the cheap
mobile dev boards. Mention (again) of Softbank machines, Mustangs,
Gigabyte motherboards etc. but there are problems getting hold of
these. It's still difficult to buy these from normal reselllers.

Which bootloader is the better choice for these computers? That's a
common subject for discussion...! I personally would really push
towards UEFI, while others prefer U-Boot. IMHO U-Boot is fine for
mobile devices and dev boards etc., but there's so much arbitrary
variation in different vendor-provided versions of U-Boot that it's
quite common to find missing support for filesystems or network
devices (or whatever). It's possible to replace the installed U-Boot
on many boards, but that depends on hardware support being properly
upstreamed; lack of that upstreaming work is another common bugbear in
vendor U-Boot binaries. If you want to use your arm64 machine as a
*computer*, my recommended route is UEFI; this is why all the server
vendors have gone that route.

My current affordable arm64 machine of choice is the Marvell
8040-based Macchiatobin [5]. It's designed for networking / NAS
devices, and mostly works well. Work is ongoing to upstream things
fully, but there are issues in the first cut of the board that means
the onboard PCI is broken. :-( It comes with U-Boot out of the box,
but a UEFI port is in development. It's 4-core Cortex-A72 and supports
up to 16G of RAM. It has sufficient onboard devices to be a useful box
for many things (USB, SATA, network) if the lack of PCI is not a
problem. It *will* want a kernel newer than what's in Stretch to make
things work well.

There's a smaller/simpler Marvell-based Espressobin which may work for
people.

The Pine64 [6] is another alternative, based on a mobile CPU. It's
therefore got limited RAM and I/O. Upstreaming has taken a while, but
is getting there in current kernel releases. U-Boot head will work on
the board, including the UEFI implementation mentioned earlier.
There's the related PineBook project [7] too - a small laptop-style
machine based around the Pine64 board.

Vagrant mentioned other options:

 * Nvidia Jetson TX-1 (pretty good, but needs some non-mainline patches yet)
 * Odroid C2 (hed early troubles, but working on it)

Answering this question ("I want something that just works, what
should I buy?") is always much harder than it should be... :-(

Hector asked about the current state of the less well-known Arm ports:
big-endian versions and arm64ilp32. Lots of different BE ports have
happened over the years, for various reasons. arm64ilp32 [8] is the
arm64 equivalent of x32 on x64-64; IMHO there is no need for it in
Debian as a mainstream port. Wookey is working on it in Linaro at the
moment. There is a BE version too. While x32 *might* have some
performance wins over i386 (depending on code), it's not clear that
arm64ilp32 is likely to be any better than armhf (A32) in most cases.

Last question: what should people do to keep armel? Main things:

 * deal with armel-specific bug reports
 * get involved in toolchains and make sure they continue to work
 * *mail the debian-arm list and commit to working on stuff*

[1] http://meetings-archive.debian.net/pub/debian-meetings/2017/debconf17/debian-arm-bof.vp8.webm
[2] http2://www.einval.com/~steve/talks/Debconf17-arm-BoF/
[3] https://lists.debian.org/debian-arm/2017/08/msg00015.html
[4] https://www.scaleway.com/
[5] https://www.solid-run.com/product/armada-8040-networking-community-board/
[6] https://www.pine64.org/
[7] https://www.pine64.org/?page_id=3707
[8] https://wiki.debian.org/Arm64ilp32Port

-- 
Steve McIntyre, Cambridge, UK.                                steve@xxxxxxxxxx
"Managing a volunteer open source project is a lot like herding
 kittens, except the kittens randomly appear and disappear because they
 have day jobs." -- Matt Mackall

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