arch/riscv: Pass the bootrom-provided FDT to the payload

The RISC-V boot protocol foresees that at every stage boundary (bootrom
to boot loader, boot loader -> OS), register a0 contains the Hart ID and
a1 contains the physical address of the Flattened Device Tree that the
stage shall use.

As a first step, pass the bootrom-provided FDT to the payload,
unmodified.

Change-Id: I468bc64a47153d564087235f1c7e2d10e3d7a658
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/23797
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/riscv/boot.c

@@ -15,20 +15,35 @@
 
 #include <program_loading.h>
 #include <vm.h>
+#include <arch/boot.h>
 #include <arch/encoding.h>
 #include <rules.h>
 #include <console/console.h>
 
+/*
+ * A pointer to the Flattened Device Tree passed to coreboot by the boot ROM.
+ * Presumably this FDT is also in ROM.
+ *
+ * This pointer is only used in ramstage!
+ */
+const void *rom_fdt;
+
 void arch_prog_run(struct prog *prog)
 {
 	void (*doit)(void *) = prog_entry(prog);
-	void riscvpayload(const char *configstring, void *payload);
-	const char *config = NULL;
+	void riscvpayload(const void *fdt, void *payload);
 
 	if (ENV_RAMSTAGE && prog_type(prog) == PROG_PAYLOAD) {
-		printk(BIOS_SPEW, "Config string: '%s'\n", config);
+		/*
+		 * FIXME: This is wrong and will crash. Linux can't (in early
+		 * boot) access memory that's before its own loading address.
+		 * We need to copy the FDT to a place where Linux can access it.
+		 */
+		const void *fdt = rom_fdt;
+
+		printk(BIOS_SPEW, "FDT is at %p\n", fdt);
 		printk(BIOS_SPEW, "OK, let's go\n");
-		riscvpayload(config, doit);
+		riscvpayload(fdt, doit);
 	}
 
 	doit(prog_entry_arg(prog));

src/arch/riscv/bootblock.S

@@ -25,7 +25,14 @@
 .globl _start
 _start:
 
-
+	# The boot ROM may pass the following arguments to coreboot:
+	#   a0: the value of mhartid
+	#   a1: a pointer to the flattened devicetree
+	#
+	# Preserve only the FDT pointer. We can query mhartid ourselves at any
+	# time.
+	#
+	csrw mscratch, a1
 
 	# N.B. This only works on low 4G of the address space
 	# and the stack must be page-aligned.

src/arch/riscv/include/arch/boot.h

@@ -0,0 +1,21 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright (C) 2018 Jonathan Neuschäfer
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#ifndef ARCH_RISCV_INCLUDE_ARCH_BOOT_H
+#define ARCH_RISCV_INCLUDE_ARCH_BOOT_H
+
+extern const void *rom_fdt;
+
+#endif

src/arch/riscv/payload.S

@@ -1,6 +1,9 @@
 /*
  * This file is part of the coreboot project.
  *
+ * Copyright (C) 2016 Google Inc
+ * Copyright (C) 2018 Jonathan Neuschäfer
+ *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation; version 2 of the License.
@@ -11,17 +14,22 @@
  * GNU General Public License for more details.
  */
 
-// "return" to a payload pointed to by a1 with
-// an M-mode pointer (or, to upper levels, physical address)
-// to the config string in a0.
+// "return" to a payload. a0: FDT, a1: entry point
 	.global riscvpayload
 riscvpayload:
-	mv t0,a1
+	/* Load the entry point */
+	mv	t0, a1
 	csrw	mepc, t0
 	csrr	t0, mstatus
+
+	/* Set mstatus.MPP (the previous privilege mode) to supervisor mode */
 	li	t1, ~(3<<11)
 	and	t0, t0, t1
 	li	t2,  (1<<11)
 	or	t0, t0, t2
 	csrw	mstatus, t0
+
+	/* Pass the right arguments and jump! */
+	mv	a1, a0
+	csrr	a0, mhartid
 	mret

src/arch/riscv/stages.c

@@ -24,9 +24,20 @@
  * linker script.
  */
 
+#include <arch/boot.h>
+#include <arch/encoding.h>
 #include <arch/stages.h>
+#include <rules.h>
 
 void stage_entry(void)
 {
+	/*
+	 * Save the FDT pointer before entering ramstage, because mscratch
+	 * might be overwritten in the trap handler, and there is code in
+	 * ramstage that generates misaligned access faults.
+	 */
+	if (ENV_RAMSTAGE)
+		rom_fdt = (const void *)read_csr(mscratch);
+
 	main();
 }