[PATCH v2 0/3] Support OpenSBI firmware header

[Zong Li]

This series adds OpenSBI firmware header. The motivation is when a
system where the previous booting stage runs on a dedicated boot
processor, it can load and patch the OpenSBI image in DRAM and then
release the application hart that runs OpenSBI, but it can never execute
on that hart and therefore cannot place anything in its registers, such
as $a0, $a1 and $a2

Every OpenSBI firmware image, regardless of its type (FW_DYNAMIC,
FW_JUMP or FW_PAYLOAD), starts with a fixed 128-byte header. The header
serves two purposes:

1. it lets the previous booting stage recognize an OpenSBI firmware
   image and find out what it was built for, and
2. it gives the previous booting stage a well-known place inside the
   image to hand over the boot arguments by patching a few words,
   instead of setting up the *a0*, *a1* and *a2* registers.

This series also provides a document 'docs/firmware/fw_header.md' to
describe the layout of the 128-byte OpenSBI firmware header, the
a0/a1/a2 override flags, what a previous booting stage is expected to put
in each register, and an example of how to patch the header.

Thanks Anup for discussing it and providing the ideas

Changed in v1:
- Remove _fw_header_override_a0
- Use mhartid for a0 when FW_HEADER_FLAGS_OVERRIDE_A0 is set
- Update fw.md for the changes above

Zong Li (3):
  firmware: fw_base.S: add a formatted OpenSBI firmware header
  firmware: fw_base.S: override a0, a1 and a2 from the firmware header
  docs: firmware: document the OpenSBI firmware header

 docs/firmware/fw.md        |   9 +++
 docs/firmware/fw_header.md | 137 +++++++++++++++++++++++++++++++++++++
 firmware/fw_base.S         |  94 +++++++++++++++++++++++++
 3 files changed, 240 insertions(+)
 create mode 100644 docs/firmware/fw_header.md

-- 
2.43.7