[M] Change in coreboot[main]: device_tree: Add function to get top of memory from a FDT blob

[Alper Nebi Yasak (Code Review)]

Attention is currently required from: Alper Nebi Yasak, Jakub Czapiga, Maximilian Brune, Nico Huber.

Hello Jakub Czapiga, Julius Werner, Maximilian Brune, Nico Huber, build bot (Jenkins),

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/80322?usp=email

to look at the new patch set (#6).

The following approvals got outdated and were removed:
Verified+1 by build bot (Jenkins)

Change subject: device_tree: Add function to get top of memory from a FDT blob
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device_tree: Add function to get top of memory from a FDT blob

coreboot needs to figure out top of memory to place CBMEM data. On some
non-x86 QEMU virtual machines, this is achieved by probing the RAM space
to find where the VM starts discarding data since it's not backed by
actual RAM. This behaviour seems to have changed on the QEMU side since
then, VMs using the "virt" model have started raising exceptions/errors
instead of silently discarding data (likely [1] for example) which has
previously broken coreboot on these emulation boards.

The qemu-aarch64 and qemu-riscv mainboards are intended for the "virt"
models and had this issue, which were mostly fixed by using exception
handlers in the RAM detection process [2][3]. But on 32-bit RISC-V we
fail to initialize CBMEM if we have 2048 MiB or more of RAM, and on
64-bit RISC-V we had to limit probing to 16383 MiB because it can run
into MMIO regions otherwise.

The qemu-armv7 mainboard code is intended for the "vexpress-a9" model VM
which doesn't appear to suffer from this issue. Still, the issue can be
observed on the ARMv7 "virt" model via a port based on qemu-aarch64.

QEMU docs for ARM and RISC-V "virt" models [4][5] recommend reading the
device tree blob it provides for device information (incl. RAM size).
Implement functions that parse the device tree blob to find described
memory regions and calculate the top of memory in order to use it in
mainboard code as an alternative to probing RAM space. ARM64 code
initializes CBMEM in romstage where malloc isn't available, so take care
to do parsing without unflattening the blob and make the code available
in romstage as well.

[1] https://lore.kernel.org/qemu-devel/1504626814-23124-1-git-send-email-peter.m...
[2] https://review.coreboot.org/c/coreboot/+/34774
[3] https://review.coreboot.org/c/coreboot/+/36486
[4] https://qemu-project.gitlab.io/qemu/system/arm/virt.html
[5] https://qemu-project.gitlab.io/qemu/system/riscv/virt.html

Change-Id: I8bef09bc1bc4e324ebeaa37f78d67d3aa315f52c
Signed-off-by: Alper Nebi Yasak 
---
M src/include/device_tree.h
M src/lib/Makefile.mk
M src/lib/device_tree.c
3 files changed, 102 insertions(+), 1 deletion(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/22/80322/6