Re: /dev/disk0 vs /dev/mmc0

[Lucas Stach <l.stach@pengutronix.de>]

Am Dienstag, den 08.10.2013, 11:13 +0200 schrieb David Jander:
[...]
> > > You are right. Sorry for the confusion. There were aliases in barebox's
> > > version of imx53.dtsi which went missing somehow in my tree. I have been
> > > merging stuff from mainline Linux....
> > > That leads me to the question: What is the intended relationship between
> > > barebox and Linux kernel DTS files? Which one is supposed to be the
> > > "master" or "upstream" version? Aren't they supposed to be kept in sync
> > > somehow? At least the basic SoC-specific .dtsi files?
> > > 
> > Missing other options we regard the Linux DTs as the upstream. Ideally
> > DTs should be stored in a separate repo, so they don't get broken
> > unnecessarily.
> > 
> > We will try to keep the Barebox DTs in sync with Linux as much as
> > possible. But even then there is no guarantee that Barebox and Linux DTs
> > don't drift away from each other over time.
> > If a binding gets broken (let's hope that everyone gets their act
> > together and don't do this anymore), it will require code changes and
> > maintenance work to get things in sync again.
> > 
> > Another real-life use-case where you might end up with different DTs is
> > if you update your Linux kernel more regularly than the Barebox on your
> > device. So please if ever possible don't boot the Linux kernel with the
> > builtin Barebox DT, but design your boot concept in a way that you
> > always deliver a DTB fitting your kernel.
> 
> Our boot-scheme checks for a DTB on the boot-medium. If it finds one matching
> the board, it is used, otherwise it boots with barebox's built-in DTB, and
> that should be the default situation, unless it becomes necessary to overrule
> the internal DTB for whatever regrettable reason.
> 
Yeah that's the conclusion you may be inclined to take if you try to
live in an ideal world. (Including a dot-shaped earth in a vacuum.)

Experience with the device tree shows that there are a lot more of those
regrettable reasons than one might imagine. So in order to get around
all those real world problems always boot your kernel with a devicetree
matching exactly that kernel version. Take this as a well-meaning advise
from someone who hit the hard wall of the real-world device tree more
than one.

> I have had strong discussions (almost flame-war) long time ago on alkml about
> the issue of hardware initialization. IMNSHO, hardware initialization that has
> to do with the electrical layout of the board (PAD settings, drive-strength,
> etc...) must always be done in the boot-loader. Back in the time where there
> was no DT for ARM, Linux kernel board support code was a bloody mess with this
> sort of HW initialization everywhere, and everywhere done differently. Only
> the hardware designer know what drive-strength and other PAD settings need to
> be chosen for each PAD. That is where your design is based on. Kernel
> (-developers) do not know, and should not need to care.
> Now there is this "invention" of putting all this information in the DT. This
> actually seems like a good compromise between what I said above and the fact
> that the kernel sometimes needs to reconfigure PADs due to PM, pluggable
> devices and such. But still the knowledge about PAD settings (and what
> hardware the board happens to have for that matter) belongs to the
> boot-loader. Having more than one source for this is dangerous.
> 
No, that is utterly wrong. Linux should always know everything about
your hardware and _not_ expect anything to be set up by the bootloader.
There are some corner cases like chip errata fixes here, but generally
don't assume the bootloader to set things up for you. The bootloader
should only do the minimal needed init for loading the kernel and
booting. The kernel is your hardware abstraction for your running
system, not the bootloader.

Ideally both the bootloader and kernel would source their needed
information from a shared DT that's decoupled from kernel development
and stable, once again reality strikes in here. I agree with you that
not a kernel developer, but board designer should write the DT and we
might actually get this separation in a clean way, if we manage to split
out the DTs from the kernel some day.

Regards,
Lucas
-- 
Pengutronix e.K.                           | Lucas Stach                 |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-5076 |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |


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