[RFC PATCH 00/17] Switch RISC-V to declarative entry points

[Ahmad Fatoum <a.fatoum@barebox.org>]

Every board currently open-codes its PBL entry point: set up the stack,
relocate, call setup_c(), do the lowlevel init and so on, all in C code
that runs before the C environment is ready.

This is error-prone, because board code authors need to be aware of some
very subtle details: global_variable_offset vs get_runtime_offset,
not accessing global variables before relocation, using noinline
functions as compiler barriers, no spilling to stack when ENTRY_FUNCTION
is used with no stack set up...

For the last point, we already had a solution in the form of
ENTRY_FUNCTION_WITHSTACK, which takes a stack pointer and places it
into a well-known location that a generic assembly entry point can read
and use to set up the stack before branching into C code.

That approach is not generic enough however: It only accepts a single
stack pointer when we really want one per execution level and even on
ARM32, it's still implemented using __naked, which precludes use with
clang as clang doesn't allow arbitrary C code there.

Address all of this by generalizing the idea behind
ENTRY_FUNCTION_WITHSTACK into a new ENTRY_POINT macro:
Instead of a board having one board-specific entry point function and a
single stack, it now has a struct of different callbacks and metadata:

struct pbl_stage {
	unsigned long stacktop;		/* stack top or PBL_STACKTOP_* */
	unsigned long relocaddr;	/* 0: don't relocate to specific addr */

	/*
	 * Memory the stage runs from: barebox proper starts out with
	 * mem_minsz, mem_maxsz (default: mem_minsz) is only used to find
	 * the stage by PC.
	 */
	unsigned long mem_start;
	unsigned long mem_minsz;
	unsigned long mem_maxsz;

	void *board_data;

	pbl_board_fn_t prereloc_cpu_init;
	pbl_board_fn_t soc_init;
	void (*board_init)(unsigned long entry_data);
	void (*soc_chainload)(void *board_data);
};

struct pbl_board_info {
	unsigned int soc_type;		/* see pbl_set_soc_type() */

	/* Called without a stack, so it must not spill */
	pbl_locate_pc_fn_t assembly_locate_pc;

	struct pbl_stage rom_stage;	/* PBL_STAGE_ROM */
	struct pbl_stage first_stage;	/* PBL_STAGE_OCRAM */
	struct pbl_stage second_stage;	/* PBL_STAGE_DRAM */
};

A board can now fill this out, define its own callbacks. Common
functionality like the soc_chainload can be shared between boards of the
same SoC. The hairy parts that can not be robustly done in C can be
defined declaratively as data that will be acted upon by the assembly
code. Code that must be written in assembly or run pre-relocation is
explicitly marked as such.

This can get quite verbose, so the idea is that SoC support will define
a macro with DEFAULTS that fills out the common bits that can be
extended (and overridden if needed) by the board code. See the last few
patches for how this looks like.

To make discussion easier, I am switching over all of RISC-V first in
this series. If we reach consensus and this is merged, I will follow-up
by doing the same on ARM.

This applies on top of next (PR #93 is prerequisite)

Ahmad Fatoum (17):
  asm-generic: add registers.h with a pc_inside() helper
  RISC-V: add call_with_stack() helper
  RISC-V: implement relocate_to_adr_full()
  PBL: add SoC type declaration and handoff support
  PBL: introduce board info based entry point framework
  PBL: describe the console in the board info and report the stage
  RISC-V: make barebox_riscv_entry() non-naked
  RISC-V: add PBL heads for the board info stage dispatch
  RISC-V: board-dt-2nd: migrate to board info entry point
  RISC-V: hifive: migrate to board info entry point
  RISC-V: allow relocating the PBL more than once
  RISC-V: nmon: preserve s1-s3 for the caller
  RISC-V: beaglev: migrate to board info entry point
  RISC-V: allwinner-d1: migrate to board info entry point
  RISC-V: erizo: migrate to board info entry point
  RISC-V: litex-linux: migrate to board info entry point
  Documentation: porting: describe the board info entry point

 Documentation/devel/porting.rst           | 124 +++++++++++-
 Documentation/devel/troubleshooting.rst   |   6 +
 arch/Kconfig                              |  28 +++
 arch/riscv/Kconfig                        |   2 +
 arch/riscv/boards/allwinner-d1/lowlevel.c |  18 +-
 arch/riscv/boards/beaglev/lowlevel.c      |  30 ++-
 arch/riscv/boards/erizo/lowlevel.c        |  32 +--
 arch/riscv/boards/hifive/lowlevel.c       |  30 +--
 arch/riscv/boards/litex-linux/lowlevel.c  |  30 +--
 arch/riscv/boot/Makefile                  |   4 +-
 arch/riscv/boot/board-dt-2nd-head.S       |  36 ++++
 arch/riscv/boot/board-dt-2nd.c            |  89 ++++-----
 arch/riscv/boot/entry.c                   |  33 +---
 arch/riscv/boot/entry_ll.S                |  15 ++
 arch/riscv/boot/head.S                    | 198 +++++++++++++++++++
 arch/riscv/boot/nmon_entry.S              |   7 +
 arch/riscv/include/asm/barebox-riscv.h    |  36 +++-
 arch/riscv/include/asm/reloc.h            |   1 +
 arch/riscv/lib/Makefile                   |   1 +
 arch/riscv/lib/asm-offsets.c              |  27 +++
 arch/riscv/lib/call_with_stack.S          |  12 ++
 arch/riscv/lib/pbl.lds.S                  |  13 +-
 arch/riscv/lib/reloc.c                    |   6 +-
 arch/riscv/lib/setupc.S                   |  16 +-
 include/asm-generic/registers.h           |  33 ++++
 include/linux/compiler-clang.h            |  21 ++
 include/linux/compiler-gcc.h              |   3 +
 include/linux/compiler_types.h            |  10 +
 include/mach/sifive/entry.h               |  32 +++
 include/pbl.h                             |   6 +
 include/pbl/entry.h                       | 175 ++++++++++++++++
 include/pbl/handoff-data.h                |  15 ++
 include/pbl/soc-type.h                    | 170 ++++++++++++++++
 pbl/Makefile                              |   2 +
 pbl/entry.c                               | 230 ++++++++++++++++++++++
 pbl/handoff-data.c                        |   2 +
 pbl/soc-type.c                            |  71 +++++++
 37 files changed, 1418 insertions(+), 146 deletions(-)
 create mode 100644 arch/riscv/boot/board-dt-2nd-head.S
 create mode 100644 arch/riscv/boot/entry_ll.S
 create mode 100644 arch/riscv/boot/head.S
 create mode 100644 arch/riscv/lib/call_with_stack.S
 create mode 100644 include/asm-generic/registers.h
 create mode 100644 include/mach/sifive/entry.h
 create mode 100644 include/pbl/entry.h
 create mode 100644 include/pbl/soc-type.h
 create mode 100644 pbl/entry.c
 create mode 100644 pbl/soc-type.c

-- 
2.47.3