riscv: Allow device trees to be built into the kernel

Some systems don't provide a useful device tree to the kernel on boot.
Chasing around bootloaders for these systems is a headache, so instead
le't's just keep a device tree table in the kernel, keyed by the SOC's
unique identifier, that contains the relevant DTB.

This is only implemented for M mode right now. While we could implement
this via the SBI calls that allow access to these identifiers, we don't
have any systems that need this right now.
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/Kbuild

 # SPDX-License-Identifier: GPL-2.0-only
 
 obj-y += kernel/ mm/ net/
+obj-$(CONFIG_BUILTIN_DTB) += boot/dts/

arch/riscv/Kconfig

@@ -381,6 +381,11 @@ endchoice
 
 endmenu
 
+config BUILTIN_DTB
+	def_bool n
+	depends on RISCV_M_MODE
+	depends on OF
+
 menu "Power management options"
 
 source "kernel/power/Kconfig"

arch/riscv/include/asm/soc.h

 /* SPDX-License-Identifier: GPL-2.0-or-later */
 /*
  * Copyright (C) 2020 Western Digital Corporation or its affiliates.
+ * Copyright (C) 2020 Google, Inc
  */
 
 #ifndef _ASM_RISCV_SOC_H
@@ -20,4 +21,42 @@ void soc_early_init(void);
 extern unsigned long __soc_early_init_table_start;
 extern unsigned long __soc_early_init_table_end;
 
+/*
+ * Allows Linux to provide a device tree, which is necessary for SOCs that
+ * don't provide a useful one on their own.
+ */
+struct soc_builtin_dtb {
+	unsigned long vendor_id;
+	unsigned long arch_id;
+	unsigned long imp_id;
+	void *(*dtb_func)(void);
+};
+
+/*
+ * The argument name must specify a valid DTS file name without the dts
+ * extension.
+ */
+#define SOC_BUILTIN_DTB_DECLARE(name, vendor, arch, impl)		\
+	extern void *__dtb_##name##_begin;				\
+									\
+	static __init __used						\
+	void *__soc_builtin_dtb_f__##name(void)				\
+	{								\
+		return (void *)&__dtb_##name##_begin;			\
+	}								\
+									\
+	static const struct soc_builtin_dtb __soc_builtin_dtb__##name	\
+		__used __section(__soc_builtin_dtb_table) =		\
+	{								\
+		.vendor_id = vendor,					\
+		.arch_id   = arch,					\
+		.imp_id    = impl,					\
+		.dtb_func  = __soc_builtin_dtb_f__##name,		\
+	}
+
+extern unsigned long __soc_builtin_dtb_table_start;
+extern unsigned long __soc_builtin_dtb_table_end;
+
+void *soc_lookup_builtin_dtb(void);
+
 #endif

arch/riscv/kernel/setup.c

@@ -75,7 +75,11 @@ void __init setup_arch(char **cmdline_p)
 
 	setup_bootmem();
 	paging_init();
+#if IS_ENABLED(CONFIG_BUILTIN_DTB)
+	unflatten_and_copy_device_tree();
+#else
 	unflatten_device_tree();
+#endif
 	clint_init_boot_cpu();
 
 #ifdef CONFIG_SWIOTLB

arch/riscv/kernel/soc.c

@@ -26,3 +26,30 @@ void __init soc_early_init(void)
 		}
 	}
 }
+
+static bool soc_builtin_dtb_match(unsigned long vendor_id,
+				unsigned long arch_id, unsigned long imp_id,
+				const struct soc_builtin_dtb *entry)
+{
+	return entry->vendor_id == vendor_id &&
+	       entry->arch_id == arch_id &&
+	       entry->imp_id == imp_id;
+}
+
+void * __init soc_lookup_builtin_dtb(void)
+{
+	unsigned long vendor_id, arch_id, imp_id;
+	const struct soc_builtin_dtb *s;
+
+	__asm__ ("csrr %0, mvendorid" : "=r"(vendor_id));
+	__asm__ ("csrr %0, marchid" : "=r"(arch_id));
+	__asm__ ("csrr %0, mimpid" : "=r"(imp_id));
+
+	for (s = (void *)&__soc_builtin_dtb_table_start;
+	     (void *)s < (void *)&__soc_builtin_dtb_table_end; s++) {
+		if (soc_builtin_dtb_match(vendor_id, arch_id, imp_id, s))
+			return s->dtb_func();
+	}
+
+	return NULL;
+}

arch/riscv/kernel/vmlinux.lds.S

@@ -34,6 +34,11 @@ SECTIONS
 		KEEP(*(__soc_early_init_table))
 		__soc_early_init_table_end = .;
 	}
+	__soc_builtin_dtb_table : {
+		__soc_builtin_dtb_table_start = .;
+		KEEP(*(__soc_builtin_dtb_table))
+		__soc_builtin_dtb_table_end = .;
+	}
 	/* we have to discard exit text and such at runtime, not link time */
 	.exit.text :
 	{

arch/riscv/mm/init.c

@@ -17,6 +17,7 @@
 #include <asm/fixmap.h>
 #include <asm/tlbflush.h>
 #include <asm/sections.h>
+#include <asm/soc.h>
 #include <asm/pgtable.h>
 #include <asm/io.h>
 
@@ -493,7 +494,15 @@ void free_initmem(void)
 #else
 asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 {
+#ifdef CONFIG_BUILTIN_DTB
+	dtb_early_va = soc_lookup_builtin_dtb();
+	if (!dtb_early_va) {
+		/* Fallback to first available DTS */
+		dtb_early_va = (void *) __dtb_start;
+	}
+#else
 	dtb_early_va = (void *)dtb_pa;
+#endif
 }
 
 static inline void setup_vm_final(void)