ARM: default machine descriptor for multiplatform

Since we now have default implementations for init_time and init_irq,
the init_machine callback is the only one that is not yet optional,
but since simple DT based platforms all have the same
of_platform_populate function call in there, we can consolidate them
as well, and then actually boot with a completely empty machine_desc.
Unofortunately we cannot just default to an empty init_machine: We
cannot call of_platform_populate before init_machine because that
does not work in case of auxdata, and we cannot call it after
init_machine either because the machine might need to run code
after adding the devices.

To take the final step, this adds support for booting without defining
any machine_desc whatsoever.

For the case that CONFIG_MULTIPLATFORM is enabled, it adds a
global machine descriptor that never matches any machine but is
used as a fallback if nothing else matches. We assume that without
CONFIG_MULTIPLATFORM, we only want to boot on the systems that the kernel
is built for, so we still retain the build-time warning for missing
machine descriptors and the run-time warning when the platform does not
match in that case.

In the case that we run on a multiplatform kernel and the machine
provides a fully populated device tree, we attempt to keep booting,
hoping that no machine specific callbacks are necessary.

Finally, this also removes the misguided "select ARCH_VEXPRESS" that
was only added to avoid a build error for allnoconfig kernels.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Cc: "Russell King - ARM Linux" <linux@arm.linux.org.uk>
Cc: Rob Herring <robherring2@gmail.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/arch/arm/kernel/devtree.c b/arch/arm/kernel/devtree.c
index 70f1bdeb241b..5af04f6daa33 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;
 
+#ifdef CONFIG_ARCH_MULTIPLATFORM
+	DT_MACHINE_START(GENERIC_DT, "Generic DT based system")
+	MACHINE_END
+
+	mdesc_best = (struct machine_desc *)&__mach_desc_GENERIC_DT;
+#endif
+
 	if (!dt_phys)
 		return NULL;