[PATCH 0/5] Add support for resolving path references in overlays

[Ayush Singh <ayush@beagleboard.org>]

dts allows references both in integer context:
	foo = <&bar>;
in which case it resolves to a phandle, but also in string/bytestring
context:
	foo = &bar;
In which case it resolves to a path.

Runtime overlays, only support the former, but not the latter. The
following patch attempts to solve this asymmetry.

Additionally, `__symbols__` does not support phandles, which
makes overlays modifying `__symbols__` rather limiting. More context
regarding this patch can be found here[0].

Implementation
**************

Overlay
=======

Properties to path references in overlays are left empty in the compiled
binary blob. Only `__fixups__` entry is generated for such properties.
This makes it simple to distinguish between phandles and paths when
applying the overlay (and also makes overlays small).

Application
===========

I have divided the overlay application into 2 stages.

1. Overlay prepare (`fdt_overlay_prepare`)

This step prepares the overlay for merging with the base device tree. In
At this stage, the base device tree is passed as read-only and only the
overlay is modified and resized.

Additionally, since any resizing will invalidate the offsets and
property references, I am creating a read-only copy of the table while
resolving the path references.

2. Overlay application (`fdt_overlay_apply`)

Performs the actual merging to base tree. The overlay is read-only at this
stage.

Limitations
***********

1. Local Path references

Currently, this patch series only implements path references to base
devicetree. This means local path references are still broken. I am
working on adding support for that using `__local_fixups__` but it is
not ready yet.

2. Breaking change for utilities

This is a breaking change for any utilities using `fdt_overlay_apply`
since I have moved some of it's functionality to `fdt_overlay_prepare`.
Not really sure how important this is.

If it is not desirable, I do have 2 ways to avoid it.

  a. Just expand both overlay and base device tree when `FDT_ERR_NOSPACE`
     is returned. A bit wasteful, but probably not a big deal.

  b. Add new error variant `FDT_ERR_OVERLAY_NOSPACE`.

Alternatives
************

Some alternative approaches that were considered:

1. Using aliases.

Currently, it is not possible to update aliases in device tree overlays.
I sent a patch a few weeks ago to add this support [1]. However, as was
outlined by Rob, this can break existing drivers that used the unused
indexes for devices not present in the aliases list.

2. Add support for phandles in `__symbols__`

This has been discussed in the following patch series [2]. However,
since there is no way to distinguish between strings and phandles in
devicetree (everything is bytestring), the type guessing is awkward.

[0]: https://lore.kernel.org/devicetree-compiler/44bfc9b3-8282-4cc7-8d9a-7292cac663ef@ti.com/T/#mf0f6ae4db0848f725ec6e2fb625291fa0d4eec71
[1]: https://lore.kernel.org/all/20241110-of-alias-v2-0-16da9844a93e@beagleboard.org/T/#t
[2]: https://lore.kernel.org/devicetree-compiler/44bfc9b3-8282-4cc7-8d9a-7292cac663ef@ti.com/T/#mbbc181b0ef394b85b76b2024d7e209ebe70f7003

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

---
Ayush Singh (5):
      dtc: Allow path fixups in overlays
      libfdt: Add namelen variants for setprop
      fdtoverlay: Implement resolving path references
      tests: Fix overlay tests
      tests: Add path tests for overlay

 checks.c                                |   6 +-
 fdtoverlay.c                            |  34 +++++--
 libfdt/fdt_overlay.c                    | 174 +++++++++++++++++++-------------
 libfdt/fdt_rw.c                         |  19 ++++
 libfdt/libfdt.h                         |  93 ++++++++++++++++-
 libfdt/version.lds                      |   1 +
 livetree.c                              |  19 +++-
 tests/overlay.c                         |   1 +
 tests/overlay_bad_fixup.c               |   2 +-
 tests/overlay_overlay.dts               |  11 ++
 tests/overlay_overlay_manual_fixups.dts |  26 ++++-
 tests/overlay_overlay_nosugar.dts       |  19 ++++
 util.h                                  |   1 +
 13 files changed, 321 insertions(+), 85 deletions(-)
---
base-commit: 2d10aa2afe35527728db30b35ec491ecb6959e5c
change-id: 20241114-overlay-path-d9980477f76a

Best regards,
-- 
Ayush Singh <ayush@beagleboard.org>