powerpc/microwatt: Add SMP support

This adds support for Microwatt systems with more than one core, and
updates the device tree for a 2-core version.

The secondary CPUs are started and sent to spin in __secondary_hold
very early on, in the platform probe function. The reason for doing
this is so that they are there when smp_release_cpus() gets called,
which is before the platform init_smp function or even the platform
setup_arch function gets called.

Note that having two CPUs in the device tree doesn't preclude
operation with only one CPU. The SYSCON_CPU_CTRL register has a
read-only field which indicates the number of CPU cores, so
microwatt_init_smp() will only start as many CPU cores as are present
in the system, and any extra CPU device-tree nodes will just be
ignored.

Signed-off-by: Paul Mackerras <[e-mail]>
Signed-off-by: Madhavan Srinivasan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

arch/powerpc/boot/dts/microwatt.dts

···
 			ibm,mmu-lpid-bits = <12>;
 			ibm,mmu-pid-bits = <20>;
 		};
+
+		PowerPC,Microwatt@1 {
+			i-cache-sets = <2>;
+			ibm,dec-bits = <64>;
+			reservation-granule-size = <64>;
+			clock-frequency = <100000000>;
+			timebase-frequency = <100000000>;
+			i-tlb-sets = <1>;
+			ibm,ppc-interrupt-server#s = <1>;
+			i-cache-block-size = <64>;
+			d-cache-block-size = <64>;
+			d-cache-sets = <2>;
+			i-tlb-size = <64>;
+			cpu-version = <0x990000>;
+			status = "okay";
+			i-cache-size = <0x1000>;
+			ibm,processor-radix-AP-encodings = <0x0c 0xa0000010 0x20000015 0x4000001e>;
+			tlb-size = <0>;
+			tlb-sets = <0>;
+			device_type = "cpu";
+			d-tlb-size = <128>;
+			d-tlb-sets = <2>;
+			reg = <1>;
+			general-purpose;
+			64-bit;
+			d-cache-size = <0x1000>;
+			ibm,chip-id = <0>;
+			ibm,mmu-lpid-bits = <12>;
+			ibm,mmu-pid-bits = <20>;
+		};
 	};
 
 	soc@c0000000 {
···
 
 		interrupt-controller@4000 {
 			compatible = "openpower,xics-presentation", "ibm,ppc-xicp";
-			ibm,interrupt-server-ranges = <0x0 0x1>;
-			reg = <0x4000 0x100>;
+			ibm,interrupt-server-ranges = <0x0 0x2>;
+			reg = <0x4000 0x10 0x4010 0x10>;
 		};
 
 		ICS: interrupt-controller@5000 {

arch/powerpc/platforms/microwatt/Kconfig

···
 # SPDX-License-Identifier: GPL-2.0
 config PPC_MICROWATT
-	depends on PPC_BOOK3S_64 && !SMP
+	depends on PPC_BOOK3S_64
 	bool "Microwatt SoC platform"
 	select PPC_XICS
 	select PPC_ICS_NATIVE

arch/powerpc/platforms/microwatt/Makefile

···
 obj-y	+= setup.o rng.o
+obj-$(CONFIG_SMP) += smp.o

arch/powerpc/platforms/microwatt/microwatt.h

···
 #define _MICROWATT_H
 
 void microwatt_rng_init(void);
+void microwatt_init_smp(void);
 
 #endif /* _MICROWATT_H */

arch/powerpc/platforms/microwatt/setup.c

···
 }
 machine_arch_initcall(microwatt, microwatt_populate);
 
+static int __init microwatt_probe(void)
+{
+	/* Main reason for having this is to start the other CPU(s) */
+	if (IS_ENABLED(CONFIG_SMP))
+		microwatt_init_smp();
+	return 1;
+}
+
 static void __init microwatt_setup_arch(void)
 {
 	microwatt_rng_init();
···
 define_machine(microwatt) {
 	.name			= "microwatt",
 	.compatible		= "microwatt-soc",
+	.probe			= microwatt_probe,
 	.init_IRQ		= microwatt_init_IRQ,
 	.setup_arch		= microwatt_setup_arch,
 	.progress		= udbg_progress,

arch/powerpc/platforms/microwatt/smp.c

···
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/*
+ * SMP support functions for Microwatt
+ * Copyright 2025 Paul Mackerras <[e-mail]>
+ */
+
+#include <linux/kernel.h>
+#include <linux/smp.h>
+#include <linux/io.h>
+#include <asm/early_ioremap.h>
+#include <asm/ppc-opcode.h>
+#include <asm/reg.h>
+#include <asm/smp.h>
+#include <asm/xics.h>
+
+#include "microwatt.h"
+
+static void __init microwatt_smp_probe(void)
+{
+	xics_smp_probe();
+}
+
+static void microwatt_smp_setup_cpu(int cpu)
+{
+	if (cpu != 0)
+		xics_setup_cpu();
+}
+
+static struct smp_ops_t microwatt_smp_ops = {
+	.probe		= microwatt_smp_probe,
+	.message_pass	= NULL,		/* Use smp_muxed_ipi_message_pass */
+	.kick_cpu	= smp_generic_kick_cpu,
+	.setup_cpu	= microwatt_smp_setup_cpu,
+};
+
+/* XXX get from device tree */
+#define SYSCON_BASE	0xc0000000
+#define SYSCON_LENGTH	0x100
+
+#define SYSCON_CPU_CTRL	0x58
+
+void __init microwatt_init_smp(void)
+{
+	volatile unsigned char __iomem *syscon;
+	int ncpus;
+	int timeout;
+
+	syscon = early_ioremap(SYSCON_BASE, SYSCON_LENGTH);
+	if (syscon == NULL) {
+		pr_err("Failed to map SYSCON\n");
+		return;
+	}
+	ncpus = (readl(syscon + SYSCON_CPU_CTRL) >> 8) & 0xff;
+	if (ncpus < 2)
+		goto out;
+
+	smp_ops = &microwatt_smp_ops;
+
+	/*
+	 * Write two instructions at location 0:
+	 * mfspr r3, PIR
+	 * b __secondary_hold
+	 */
+	*(unsigned int *)KERNELBASE = PPC_RAW_MFSPR(3, SPRN_PIR);
+	*(unsigned int *)(KERNELBASE+4) = PPC_RAW_BRANCH(&__secondary_hold - (char *)(KERNELBASE+4));
+
+	/* enable the other CPUs, they start at location 0 */
+	writel((1ul << ncpus) - 1, syscon + SYSCON_CPU_CTRL);
+
+	timeout = 10000;
+	while (!__secondary_hold_acknowledge) {
+		if (--timeout == 0)
+			break;
+		barrier();
+	}
+
+ out:
+	early_iounmap((void *)syscon, SYSCON_LENGTH);
+}