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);