[v3,1/2] linux: add support for device tree overlays

From: Michael Walle <michael@walle.cc>

The linux kernel can build device tree overlays (.dtbo) itself. Add
support to build and copy them along with the actual device trees.
These can only be in-tree device tree overlays.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
Changes since v2:
-Dropped support for out-of-tree device tree overlays, as Linux 6.12 no
 longer supports building an out-of-tree device tree by simply copying
 it to the kernel source directory (without also modifying the Makefile).

 linux/Config.in | 7 +++++++
 linux/linux.mk  | 3 ++-
 2 files changed, 9 insertions(+), 1 deletion(-)

Hi Niklas,

> -Dropped support for out-of-tree device tree overlays, as Linux 6.12 no
>  longer supports building an out-of-tree device tree by simply copying
>  it to the kernel source directory (without also modifying the Makefile).

Why is that? As it seems, it is not possible to compile it by simply
copying to arch/<arch>/dts (which is buildroot doing currently)
anymore. But copying it to arch/<arch>/dts/<vendor>/ is working.
Correct?

In that case, we should work on getting support for that in
buildroot as well. Esp. buildroot is really lacking copying an
actual device tree into that subdir too and currently it's a PITA to
include a soc.dtsi for example. If you're using a device tree in
buildroot you'd have to use '#include "vendor/soc.dtsi"' while in
the kernel (as well as in yocto) you'll just use '#include
"soc.dtsi"'.

-michael

[replying from an email address that is subscribed this time...]

On Fri, Dec 13, 2024 at 09:53:27AM +0100, Michael Walle wrote:
> Hi Niklas,
> 
> > -Dropped support for out-of-tree device tree overlays, as Linux 6.12 no
> >  longer supports building an out-of-tree device tree by simply copying
> >  it to the kernel source directory (without also modifying the Makefile).
> 
> Why is that? As it seems, it is not possible to compile it by simply
> copying to arch/<arch>/dts (which is buildroot doing currently)
> anymore. But copying it to arch/<arch>/dts/<vendor>/ is working.
> Correct?

I'm assuming that neither works anymore, see:
https://lore.kernel.org/buildroot/CAFOYHZDQmMRuK76TpOSAOFPdqAb4gpw=LNKfmpu=sqAD1EMhFg@mail.gmail.com/T/#u
https://lore.kernel.org/all/CAK7LNAR4h6NZ+D0BK+q4VQBeHWpjzRBQFQ9ovBrftM=6dHRcUg@mail.gmail.com/
https://lore.kernel.org/buildroot/Z1nq_gneO4ORw3EG@curiosity/T/#t


> 
> In that case, we should work on getting support for that in
> buildroot as well. Esp. buildroot is really lacking copying an
> actual device tree into that subdir too and currently it's a PITA to
> include a soc.dtsi for example. If you're using a device tree in
> buildroot you'd have to use '#include "vendor/soc.dtsi"' while in
> the kernel (as well as in yocto) you'll just use '#include
> "soc.dtsi"'.

Yes, it appears that buildroot folks have an idea that it should be possible to
restore BR2_LINUX_KERNEL_CUSTOM_DTS_PATH in newer kernels by sed:ing the
Makefile.

However, I don't see any reason to defer this patch until then.


Once someone has written a patch that restores BR2_LINUX_KERNEL_CUSTOM_DTS_PATH
support for recent kernels, you can submit an incremental path on top of this
that adds support for out-of-tree device tree overlays, if you so wish.

But honestly, considering that the only buildroot configurations that make use of
BR2_LINUX_KERNEL_CUSTOM_DTS_PATH are:
- acmesystems_acqua_a5_256mb_defconfig (v6.6)
- acmesystems_acqua_a5_512mb_defconfig (v6.6)
- octavo_osd32mp1_brk_defconfig (v5.10)   
- octavo_osd32mp1_red_defconfig (v5.10)
- s6lx9_microboard_defconfig (v4.4.7)

are using really old kernel versions, nothing is broken in upstream buildroot,
so I expect that it might take quite a long time before anyone bothers to
restore BR2_LINUX_KERNEL_CUSTOM_DTS_PATH support in newer kernels.


Kind regards,
Niklas

Hi Niklas,

> > > -Dropped support for out-of-tree device tree overlays, as Linux 6.12 no
> > >  longer supports building an out-of-tree device tree by simply copying
> > >  it to the kernel source directory (without also modifying the Makefile).
> > 
> > Why is that? As it seems, it is not possible to compile it by simply
> > copying to arch/<arch>/dts (which is buildroot doing currently)
> > anymore. But copying it to arch/<arch>/dts/<vendor>/ is working.
> > Correct?
>
> I'm assuming that neither works anymore, see:
> https://lore.kernel.org/buildroot/CAFOYHZDQmMRuK76TpOSAOFPdqAb4gpw=LNKfmpu=sqAD1EMhFg@mail.gmail.com/T/#u
> https://lore.kernel.org/all/CAK7LNAR4h6NZ+D0BK+q4VQBeHWpjzRBQFQ9ovBrftM=6dHRcUg@mail.gmail.com/
> https://lore.kernel.org/buildroot/Z1nq_gneO4ORw3EG@curiosity/T/#t

Thanks for the pointers.

But I've just tested both compiling .dts and .dtso from within a
vendor directory and it seems to be working on next-20241213 (?).

> > In that case, we should work on getting support for that in
> > buildroot as well. Esp. buildroot is really lacking copying an
> > actual device tree into that subdir too and currently it's a PITA to
> > include a soc.dtsi for example. If you're using a device tree in
> > buildroot you'd have to use '#include "vendor/soc.dtsi"' while in
> > the kernel (as well as in yocto) you'll just use '#include
> > "soc.dtsi"'.
>
> Yes, it appears that buildroot folks have an idea that it should be possible to
> restore BR2_LINUX_KERNEL_CUSTOM_DTS_PATH in newer kernels by sed:ing the
> Makefile.
>
> However, I don't see any reason to defer this patch until then.
>
>
> Once someone has written a patch that restores BR2_LINUX_KERNEL_CUSTOM_DTS_PATH
> support for recent kernels, you can submit an incremental path on top of this
> that adds support for out-of-tree device tree overlays, if you so wish.
>
> But honestly, considering that the only buildroot configurations that make use of
> BR2_LINUX_KERNEL_CUSTOM_DTS_PATH are:
> - acmesystems_acqua_a5_256mb_defconfig (v6.6)
> - acmesystems_acqua_a5_512mb_defconfig (v6.6)
> - octavo_osd32mp1_brk_defconfig (v5.10)   
> - octavo_osd32mp1_red_defconfig (v5.10)
> - s6lx9_microboard_defconfig (v4.4.7)
>
> are using really old kernel versions, nothing is broken in upstream buildroot,
> so I expect that it might take quite a long time before anyone bothers to
> restore BR2_LINUX_KERNEL_CUSTOM_DTS_PATH support in newer kernels.

This is most useful for buildroot externals which you cannot grep
that easily :) Thus I'd imagine it is more common than you think.

-michael

Hello Michael,

On Fri, Dec 13, 2024 at 10:35:37AM +0100, Michael Walle wrote:
> Hi Niklas,
> 
> > > > -Dropped support for out-of-tree device tree overlays, as Linux 6.12 no
> > > >  longer supports building an out-of-tree device tree by simply copying
> > > >  it to the kernel source directory (without also modifying the Makefile).
> > > 
> > > Why is that? As it seems, it is not possible to compile it by simply
> > > copying to arch/<arch>/dts (which is buildroot doing currently)
> > > anymore. But copying it to arch/<arch>/dts/<vendor>/ is working.
> > > Correct?
> >
> > I'm assuming that neither works anymore, see:
> > https://lore.kernel.org/buildroot/CAFOYHZDQmMRuK76TpOSAOFPdqAb4gpw=LNKfmpu=sqAD1EMhFg@mail.gmail.com/T/#u
> > https://lore.kernel.org/all/CAK7LNAR4h6NZ+D0BK+q4VQBeHWpjzRBQFQ9ovBrftM=6dHRcUg@mail.gmail.com/
> > https://lore.kernel.org/buildroot/Z1nq_gneO4ORw3EG@curiosity/T/#t
> 
> Thanks for the pointers.
> 
> But I've just tested both compiling .dts and .dtso from within a
> vendor directory and it seems to be working on next-20241213 (?).

If you say that copying to arch/<arch>/dts/<vendor>/ is still working, then
most likely it is only copying to arch/<arch>/dts that is no longer working.

I still think that this patch can be merged as is.

Once the BR2_LINUX_KERNEL_CUSTOM_DTS_PATH mess has been sorted, someone could
add support for out-of-tree device tree overlays, if anyone wants/needs that.

Adding support for in-tree device tree overlays now, does not make life harder
for anyone if they want to add support for out-of-tree device tree overlays in
the future.


Kind regards,
Niklas

Hi Niklas,

> > But I've just tested both compiling .dts and .dtso from within a
> > vendor directory and it seems to be working on next-20241213 (?).
>
> If you say that copying to arch/<arch>/dts/<vendor>/ is still working, then
> most likely it is only copying to arch/<arch>/dts that is no longer working.

Copying it to dts/ was wrong for arch's which had a subdir structure
anyway.

>
> I still think that this patch can be merged as is.
>
> Once the BR2_LINUX_KERNEL_CUSTOM_DTS_PATH mess has been sorted, someone could
> add support for out-of-tree device tree overlays, if anyone wants/needs that.

Well, that was actually the intention of my patch :)

> Adding support for in-tree device tree overlays now, does not make life harder
> for anyone if they want to add support for out-of-tree device tree overlays in
> the future.

Agreed.

-michael

Hello Thomas,

Any chance of this series getting picked up?


Kind regards,
Niklas

Hello Thomas,

Any chance of this series getting picked up?


Kind regards,
Niklas