[PATCH 0/2] [SRU] [X/raspi2] Overlay files don't get applied

[Paolo Pisati]

BugLink: https://bugs.launchpad.net/bugs/1703802

Impact:

While working on a bug that involved loading an overlay dtb files upon boot, i
noticed that, no matter what i put in config.txt::dtoverlay=..., no overlay was
loaded at all.

After further investigation i found that recently (~April 2017), the raspberry
BSP changed the overlay files suffix from -overlay.dtb to .dtbo, and while the
upstream bootloader was updated to accept the new scheme, we keep shipping an
older bootloader in the the archive that didn't know about this change.

To make this bug more annoying, the bootloader doesn't emit an error or a
warning if it fails to load an overlay, it silently ignores it and keep going.

Ubuntu core #stable isn't affected by this bug since it's shipping it's own
overlay files in a static tarball (using the old scheme), and #edge (that is
where new daily images are built) is shipping an older kernel with the old
scheme.

While #beta and #candidates should be affected.

Fix:

A way to fix this bug would be to update the raspi2 bootloader to a more recent
version, but since updating it has proven challenging in the past (multiple
images from multiple sources, #updates vs #security, different boot configs,
etc), i'm taking a more straightforward approach by reverting the dtb overlay
files rename back to the previous scheme.

With that said, i'm concurrently opening another LP bug (1703799) because we
must update the bootloader and revert this fix as soon as the new bootloader
lands in the archive.

How to test:

The are different ways you can test this (like modifying the
config.txt::dtoverlay=... parameter and then checking that the corresponding
hardware started to work, but then you would never know if it's a kernel driver
problem, a dtb merge problem, wrong wiring, failing hardware, etc), but the best
way is to use the dtc to dump out the dts structure before and after the
dtoverlay modification and then diff the two:

vanilla image:

$ dtc -I fs -O dts /proc/device-tree > pi3-vanilla.dts

now install the new kernel with the revert (dpkg -i ... or snap install ...),
modify dtoverlay, reboot and dump again:

# echo "dtoverlay=vc4-kms-v3d" >> /boot/firmware/config.txt
# reboot

$ dtc -I fs -O dts /proc/device-tree > pi3-vc4.dts

check if the dtb passed to the kernel has changed:

$ diff -du pi3-vanilla.dts pi3-vc4.dts
[check that there's actually a diff and something sensible was applied]

if you are doing this on ubuntu-core, config.txt is located in
/boot/uboot/config.txt, while if you are doing this in ubuntu classic, remember
to manually copy the overlay files that are shipped with the new kernel:

# rm -rf /boot/firmware/overlays
# cp /lib/firmware/`uname -r`/device-tree/overlays /boot/firmware

Paolo Pisati (2):
  Revert "dts, kbuild: dtbs_install installs .dtbo files too"
  UBUNTU: [SAUCE] Revert "BCM270X_DT: Add a .dtbo target, use for
    overlays" and subsequent

 arch/arm/boot/.gitignore            |   2 +-
 arch/arm/boot/dts/overlays/Makefile | 207 ++++++++++++++++++------------------
 scripts/Makefile.dtbinst            |  10 +-
 scripts/Makefile.lib                |  10 --
 4 files changed, 110 insertions(+), 119 deletions(-)

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2.7.4