powerpc/64: Set DSCR default initially from SPR

Take the DSCR value set by firmware as the dscr_default value,
rather than zero.

POWER9 recommends DSCR default to a non-zero value.

Signed-off-by: From: Nicholas Piggin <npiggin@gmail.com>
[mpe: Make record_spr_defaults() __init]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/setup.h

@@ -45,6 +45,12 @@ void emergency_stack_init(void);
 static inline void emergency_stack_init(void) { };
 #endif
 
+#ifdef CONFIG_PPC64
+void record_spr_defaults(void);
+#else
+static inline void record_spr_defaults(void) { };
+#endif
+
 /*
  * Having this in kvm_ppc.h makes include dependencies too
  * tricky to solve for setup-common.c so have it here.

arch/powerpc/kernel/setup_64.c

@@ -69,6 +69,8 @@
 #include <asm/opal.h>
 #include <asm/cputhreads.h>
 
+#include "setup.h"
+
 #ifdef DEBUG
 #define DBG(fmt...) udbg_printf(fmt)
 #else
@@ -316,6 +318,13 @@ void __init early_setup(unsigned long dt_ptr)
 	/* Initialize the hash table or TLB handling */
 	early_init_mmu();
 
+	/*
+	 * After firmware and early platform setup code has set things up,
+	 * we note the SPR values for configurable control/performance
+	 * registers, and use those as initial defaults.
+	 */
+	record_spr_defaults();
+
 	/*
 	 * At this point, we can let interrupts switch to virtual mode
 	 * (the MMU has been setup), so adjust the MSR in the PACA to

arch/powerpc/kernel/sysfs.c

@@ -590,6 +590,17 @@ static void sysfs_create_dscr_default(void)
 	if (cpu_has_feature(CPU_FTR_DSCR))
 		err = device_create_file(cpu_subsys.dev_root, &dev_attr_dscr_default);
 }
+
+void __init record_spr_defaults(void)
+{
+	int cpu;
+
+	if (cpu_has_feature(CPU_FTR_DSCR)) {
+		dscr_default = mfspr(SPRN_DSCR);
+		for (cpu = 0; cpu < nr_cpu_ids; cpu++)
+			paca[cpu].dscr_default = dscr_default;
+	}
+}
 #endif /* CONFIG_PPC64 */
 
 #ifdef HAS_PPC_PMC_PA6T