[PATCH, RFC] default machine descriptor for multiplatform

[Arnd Bergmann <arnd@arndb.de>]

This is what I think it would look like to do a default platform
with an empty machine descriptor on ARM. It makes the few required
entries in the descriptor optional by using the new irqchip_init()
and clocksource_of_init() functions as defaults, and adds
a fallback for the DT case to customize_machine to probe all
the default devices.

For the case that CONFIG_MULTIPLATFORM is enabled, it then
adds a machine descriptor that never matches any machine but
is used as a fallback if nothing else matches.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 3e3444e..8ff1d38 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -979,7 +979,6 @@ config ARCH_MULTI_V7
 	bool "ARMv7 based platforms (Cortex-A, PJ4, Krait)"
 	default y
 	select ARCH_MULTI_V6_V7
-	select ARCH_VEXPRESS
 	select CPU_V7
 
 config ARCH_MULTI_V6_V7
diff --git a/arch/arm/kernel/devtree.c b/arch/arm/kernel/devtree.c
index 70f1bde..e6e34ba 100644
--- a/arch/arm/kernel/devtree.c
+++ b/arch/arm/kernel/devtree.c
@@ -180,6 +180,13 @@ struct machine_desc * __init setup_machine_fdt(unsigned int dt_phys)
 	unsigned long dt_root;
 	const char *model;
 
+	if (IS_ENABLED(CONFIG_ARCH_MULTIPLATFORM)) {
+		DT_MACHINE_START(GENERIC_DT, "Generic DT based system")
+		MACHINE_END
+
+		mdesc_best = (struct machine_desc *)&__mach_desc_GENERIC_DT;
+	}
+
 	if (!dt_phys)
 		return NULL;
 
@@ -199,7 +206,7 @@ struct machine_desc * __init setup_machine_fdt(unsigned int dt_phys)
 			mdesc_score = score;
 		}
 	}
-	if (!mdesc_best) {
+	if (!mdesc_best && !IS_ENABLED(CONFIG_ARCH_MULTIPLATFORM)) {
 		const char *prop;
 		long size;
 
diff --git a/arch/arm/kernel/irq.c b/arch/arm/kernel/irq.c
index 8e4ef4c..df6f9a1 100644
--- a/arch/arm/kernel/irq.c
+++ b/arch/arm/kernel/irq.c
@@ -26,6 +26,7 @@
 #include <linux/ioport.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>
+#include <linux/irqchip.h>
 #include <linux/random.h>
 #include <linux/smp.h>
 #include <linux/init.h>
@@ -114,7 +115,10 @@ EXPORT_SYMBOL_GPL(set_irq_flags);
 
 void __init init_IRQ(void)
 {
-	machine_desc->init_irq();
+	if (machine_desc->init_irq)
+		machine_desc->init_irq();
+	else
+		irqchip_init();
 }
 
 #ifdef CONFIG_MULTI_IRQ_HANDLER
diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index 3f6cbb2..1d40c9d 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -18,6 +18,7 @@
 #include <linux/bootmem.h>
 #include <linux/seq_file.h>
 #include <linux/screen_info.h>
+#include <linux/of_platform.h>
 #include <linux/init.h>
 #include <linux/kexec.h>
 #include <linux/of_fdt.h>
@@ -640,9 +641,17 @@ struct screen_info screen_info = {
 
 static int __init customize_machine(void)
 {
-	/* customizes platform devices, or adds new ones */
+	/*
+	 * customizes platform devices, or adds new ones
+	 * On DT based machines, we fall back to populating the
+	 * machine from the device tree, if no callback is provided,
+	 * otherwise we would always need an init_machine callback.
+	 */
 	if (machine_desc->init_machine)
 		machine_desc->init_machine();
+	else
+		of_platform_populate(NULL, of_default_bus_match_table,
+					NULL, NULL);
 	return 0;
 }
 arch_initcall(customize_machine);
@@ -732,7 +741,7 @@ void __init setup_arch(char **cmdline_p)
 
 	setup_processor();
 	mdesc = setup_machine_fdt(__atags_pointer);
-	if (!mdesc)
+	if (!mdesc && __machine_arch_type != ~0)
 		mdesc = setup_machine_tags(__atags_pointer, __machine_arch_type);
 	machine_desc = mdesc;
 	machine_name = mdesc->name;
diff --git a/arch/arm/kernel/time.c b/arch/arm/kernel/time.c
index 955d92d..abff4e9 100644
--- a/arch/arm/kernel/time.c
+++ b/arch/arm/kernel/time.c
@@ -22,6 +22,7 @@
 #include <linux/errno.h>
 #include <linux/profile.h>
 #include <linux/timer.h>
+#include <linux/clocksource.h>
 #include <linux/irq.h>
 
 #include <asm/thread_info.h>
@@ -115,6 +116,10 @@ int __init register_persistent_clock(clock_access_fn read_boot,
 
 void __init time_init(void)
 {
-	machine_desc->init_time();
+	if (machine_desc->init_time)
+		machine_desc->init_time();
+	else
+		clocksource_of_init();
+
 	sched_clock_postinit();
 }
diff --git a/drivers/irqchip/irqchip.c b/drivers/irqchip/irqchip.c
index f496afc..c5e7a45 100644
--- a/drivers/irqchip/irqchip.c
+++ b/drivers/irqchip/irqchip.c
@@ -24,7 +24,13 @@ irqchip_of_match_end __used __section(__irqchip_of_end);
 
 extern struct of_device_id __irqchip_begin[];
 
+#ifdef CONFIG_IRQCHIP
 void __init irqchip_init(void)
 {
 	of_irq_init(__irqchip_begin);
 }
+#else
+static inline void irqchip_init(void)
+{
+}
+#endif
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 7944f14..b14d224 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -339,6 +339,10 @@ extern void clocksource_of_init(void);
 	static const struct of_device_id __clksrc_of_table_##name	\
 		__used __section(__clksrc_of_table)			\
 		 = { .compatible = compat, .data = fn };
+#else
+static inline void clocksource_of_init(void)
+{
+}
 #endif
 
 #endif /* _LINUX_CLOCKSOURCE_H */