Re: incompatible device trees between u-boot and linux

On Wed, Aug 25, 2021 at 11:24:08AM -0400, Tom Rini wrote:

On Wed, Aug 25, 2021 at 06:12:20PM +0300, Vladimir Oltean wrote:

quoted

On Wed, Aug 25, 2021 at 10:26:10AM -0400, Tom Rini wrote:

quoted

On Wed, Aug 25, 2021 at 05:18:16PM +0300, Vladimir Oltean wrote:

quoted

On Wed, Aug 25, 2021 at 10:00:45AM -0400, Tom Rini wrote:

quoted

On Wed, Aug 25, 2021 at 03:58:10PM +0200, Michael Walle wrote:

quoted

Hi,

I noticed that there is a fallback to the u-boot device tree for linux
(esp. EFI boot) if no other device tree was found, see [1]. It seems this
is working fine for imx devices, for example, where you can just boot a
stock installer iso via EFI. It will just work and it is quite a nice
feature as a fallback.

Now for the layerscape architecture, the ls1028a in my case, things are
more difficult because the bindings differ between u-boot and linux - one
which comes to mind is DSA and ethernet.

Which begs the general question, is it encouraged to have both bindings
diverge? To me it seems, that most bindings in u-boot are ad-hoc and there
is no real review or alignment but just added as needed, which is ok if
they are local to u-boot. But since they are nowadays passed to linux
(by default!) I'm not so sure anymore.

OTOH The whole structure around a .dts{,i} and -u-boot.dtsi looks like
they should (could?) be shared between linux and u-boot.

-michael

[1]
https://elixir.bootlin.com/u-boot/v2021.10-rc2/source/common/board_r.c#L471

The U-Boot device tree is supposed to be able to be passed on to Linux
and Just Work.  The bindings are not supposed to be different between
the two (except for when we take the binding while it's being hashed out
upstream BUT THEN RESYNCED).

You might need to spell that out a bit clearer.

You are saying that both U-Boot and Linux are allowed to have their own
custom properties (like 'u-boot,dm-spl' for U-Boot, and 'managed = "in-band-status"'
for Linux), as long as the device tree files themselves are in sync, and
the subset of the device tree blob understood by Linux (i.e. the U-Boot
blob sans the U-Boot specifics) is compatible with the Linux DT blob?

I don't know what about the Linux example makes it Linux specific.  But
yes, 'u-boot,dm-spl' is clearly in our namespace and should be ignored
by Linux.  The whole reason we have the -u-boot.dtsi automatic drop-in
logic (as much as it can be used is that device trees are device trees

        ^
        I don't think this parenthesis ever closes...

Ah, whoops.  Should have been "(as much as it can be used)" because it
does get #included instead in some cases, for reasons.

quoted

quoted

and describe the hardware and developers don't need to write a device
tree for Linux and a device tree for U-Boot and a device tree for
FreeBSD and ...  So yes, you're supposed to use the device tree for a

                ^
                so I never get the answer to "the whole reason is...".

quoted

platform and it works here and there and every where.

The fact that only Linux uses it makes it Linux specific.

quoted

quoted

To expand even further on that, it means we should put 'managed = "in-band-status"'
in U-Boot, which is a Linux phylink device tree property, even if U-Boot
does not use phylink?

We should be able to drop in the device trees from Linux and use them.
Custodians should be re-syncing them periodically.  Some are, even.

Are you ready to take up device tree bindings for PTP timers, PCIe root
complex event collectors, cascaded interrupt controllers, things which
U-Boot will never ever need to support?

At least in Linux there is a policy to not add device tree nodes that do
not have drivers. Is the same policy not true for U-Boot? At least your
./scripts/checkpatch.pl does have the same "check for DT compatible
documentation" section as Linux. You might consider removing it if you
want people to not strip the DTs they submit to U-Boot.

And why do we even maintain the device tree bindings in Linux at all?
It seems rather counter-productive for both ends to do that, if it is
expected that the kernel works with DT blobs provided by third parties too,
and if all third parties need to resync with it (there are other boot
loaders too beyond U-Boot, and other kernels beyond Linux). Somehow it
doesn't feel right for the reference to be the Linux kernel. Maybe this
is something that needs to be brought up with higher-level Linux maintainers.

I have no problem at all with structuring the device tree in the same
way in U-Boot as in Linux, as long as that proves to not be a foolish
endeavor.

DT is ABI is hardware description and OS-agnostic has been the rule for
10+ years.  If that's no longer the case, can someone please tell me?

So if Michael's board with DT provided by U-Boot doesn't work for some
stupid reason like "Linux expects the pcie node to be under /soc", or
"Linux wants all PCIe BARs of a RCIEP ECAM to be spelled out in the
'ranges' property, because it's too dumb to detect them itself", or
something like that, I've got no argument against that, let's go ahead
and resync U-Boot with Linux.

But "DT is ABI is hardware description" is a pretty vague truism that
does not actually help here.

Will you accept device trees with devices for which a driver will never
probe in U-Boot, and will you remove the checkpatch warnings about those,
to not discourage people from submitting them prior to the actual public
review?

If a U-Boot driver will be written for a device that does not have a
driver yet in Linux, then the Linux driver will be written but with
DT bindings incompatible with what was established in U-Boot, will you
wake up the U-Boot developer from the grave and ask them to resync the
driver to follow Linux? Or will you accept drivers at all for hardware
that is not supported by Linux?

I also think there are various degrees of what it means "to work" with a
device tree provided directly by U-Boot. Distros like Arch Linux ARM
have a package for device tree blobs, and it is expected that these are
updated by the distro, and U-Boot just loads them. As Michael points out,
the DT provided by U-Boot is just a fallback. The OS should boot to
prompt and have a network connection to set itself up properly again.
But we need to draw a harder line on what we _actually_ desire to work
beyond that.