[PATCH] ARC: Troubleshoot execution of UP Linux on SMP HW and vice versa

[Alexey Brodkin]

ARC SMP hardware heavily relies on Interrupt Distribution Unit (IDU)
for all interrupts serving. And UP ARC hardware lacks this block.

That leads to incompatibility between UP and SMP Linux builds.

Even though UP build of Linux will run on SMP hardware at some
point strange behavior will appear. Very good example is serial port
will stop functioning once it switches from earlycon driver (which
doesn't use interrupts) to full-scale serial driver (that will rely
on interrupts).

The same is applicable to reverse combination: SMP build won't
work on UP hardware and symptoms will be pretty much the same.

And so to save [especially  newcomers] from spending hours in
frustration we're doing a check very early on boot if the kernel was
configured with CONFIG_ARC_MCIP (which is automatically selected as
a dependency of CONFIG_SMP) and in run-time we're seeing SMP-specific
register that holds a number of SMP cores.

Signed-off-by: Alexey Brodkin <abrodkin@xxxxxxxxxxxx>
---
 arch/arc/kernel/setup.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/arc/kernel/setup.c b/arch/arc/kernel/setup.c
index f63b8bf..78a1476 100644
--- a/arch/arc/kernel/setup.c
+++ b/arch/arc/kernel/setup.c
@@ -24,6 +24,7 @@
 #include <asm/irq.h>
 #include <asm/unwind.h>
 #include <asm/mach_desc.h>
+#include <asm/mcip.h>
 #include <asm/smp.h>
 
 #define FIX_PTR(x)  __asm__ __volatile__(";" : "+r"(x))
@@ -374,6 +375,8 @@ static inline int is_kernel(unsigned long addr)
 
 void __init setup_arch(char **cmdline_p)
 {
+	unsigned int num_cores;
+
 #ifdef CONFIG_ARC_UBOOT_SUPPORT
 	/* make sure that uboot passed pointer to cmdline/dtb is valid */
 	if (uboot_tag && is_kernel((unsigned long)uboot_arg))
@@ -413,6 +416,15 @@ void __init setup_arch(char **cmdline_p)
 	if (machine_desc->init_early)
 		machine_desc->init_early();
 
+	num_cores = (read_aux_reg(ARC_REG_MCIP_BCR) >> 16) & 0x3F;
+#ifdef CONFIG_ARC_MCIP
+	if (!num_cores)
+		panic("SMP kernel is run on a UP hardware!\n");
+#else
+	if (num_cores)
+		panic("UP kernel is run on a SMP hardware!\n");
+#endif
+
 	smp_init_cpus();
 
 	setup_processor();
-- 
2.5.5