[PATCH v2 2/3] ARM: bcm: Add CFE SMP support for the Genexis XG6846B

[Linus Walleij]

The XG6846B CFE bootloader powers up CPU1 and leaves it in a holding pen
waiting for an interrupt. After waking, the stub reads the GPIO
GeneralPurpose register at 0xff800568 and branches to the stored address
in ARM state. The vendor kernel uses this mailbox and a wakeup IPI.

Add a machine SMP fallback for the XG6846B when PSCI is unavailable.
Read the mailbox address from the secondary CPU's cpu-release-addr
property. Map it as device memory, write secondary_startup_arm and
complete the write before sending the wakeup IPI. CPU1 is already
running, so no power or reset sequence is needed. Using the ARM entry
also supports kernels built in Thumb-2 mode. Clear any Thumb function
pointer tag before programming this ARM entry address.

Build the BCMBCA machine description independently of DEBUG_BCMBCA,
keeping the debug UART mapping conditional. Restrict the fallback to
the XG6846B and retain the generic PSCI path when firmware provides it.

Suggested-by: William Zhang <william.zhang at broadcom.com>
Assisted-by: LLM
Signed-off-by: Linus Walleij <linusw at kernel.org>
---
 arch/arm/mach-bcm/Makefile       |  2 +-
 arch/arm/mach-bcm/board_bcmbca.c | 65 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mach-bcm/Makefile b/arch/arm/mach-bcm/Makefile
index 6baedf4c6304..fcc4e0174adc 100644
--- a/arch/arm/mach-bcm/Makefile
+++ b/arch/arm/mach-bcm/Makefile
@@ -58,6 +58,6 @@ endif
 
 # BCMBCA
 ifeq ($(CONFIG_ARCH_BCMBCA),y)
-obj-$(CONFIG_DEBUG_BCMBCA)	+= board_bcmbca.o
+obj-y				+= board_bcmbca.o
 obj-$(CONFIG_SMP)		+= bcm63xx_smp.o bcm63xx_pmb.o
 endif
diff --git a/arch/arm/mach-bcm/board_bcmbca.c b/arch/arm/mach-bcm/board_bcmbca.c
index b510d959870a..1c2e3ce71a82 100644
--- a/arch/arm/mach-bcm/board_bcmbca.c
+++ b/arch/arm/mach-bcm/board_bcmbca.c
@@ -1,9 +1,16 @@
 // SPDX-License-Identifier: GPL-2.0-only
 // Copyright (C) 2024 Linus Walleij <linus.walleij at linaro.org>
 
+#include <linux/io.h>
+#include <linux/of.h>
+#include <linux/smp.h>
+
 #include <asm/mach/arch.h>
 #include <asm/mach/map.h>
+#include <asm/psci.h>
+#include <asm/smp_plat.h>
 
+#ifdef CONFIG_DEBUG_BCMBCA
 /* This is needed for LL-debug/earlyprintk/debug-macro.S */
 static struct map_desc bcmbca_io_desc[] __initdata = {
 	{
@@ -13,11 +20,68 @@ static struct map_desc bcmbca_io_desc[] __initdata = {
 		.type = MT_DEVICE,
 	},
 };
+#endif
 
 static void __init bcmbca_map_io(void)
 {
+#ifdef CONFIG_DEBUG_BCMBCA
 	iotable_init(bcmbca_io_desc, ARRAY_SIZE(bcmbca_io_desc));
+#endif
+}
+
+#ifdef CONFIG_SMP
+static int bcm6846_boot_secondary(unsigned int cpu, struct task_struct *idle)
+{
+	struct device_node *np;
+	void __iomem *mailbox;
+	u32 release_addr;
+	int ret;
+
+	/* CFE has already powered up CPU1 and left it waiting in WFI. */
+	if (cpu_logical_map(cpu) != 1)
+		return -EINVAL;
+
+	np = of_get_cpu_node(cpu, NULL);
+	if (!np)
+		return -ENODEV;
+
+	ret = of_property_read_u32(np, "cpu-release-addr", &release_addr);
+	of_node_put(np);
+	if (ret) {
+		pr_err("CPU%u: missing or invalid cpu-release-addr property\n", cpu);
+		return ret;
+	}
+
+	mailbox = ioremap(release_addr, sizeof(u32));
+	if (!mailbox)
+		return -ENOMEM;
+
+	/*
+	 * The CFE stub reads GPIO GeneralPurpose after an interrupt and
+	 * branches to it in ARM state. Clear any Thumb function pointer
+	 * tag and complete the MMIO write before sending the wakeup IPI.
+	 */
+	writel(__pa_symbol(secondary_startup_arm) & ~1, mailbox);
+	dsb(sy);
+	arch_send_wakeup_ipi_mask(cpumask_of(cpu));
+	iounmap(mailbox);
+
+	return 0;
+}
+
+static const struct smp_operations bcm6846_smp_ops __initconst = {
+	.smp_boot_secondary = bcm6846_boot_secondary,
+};
+
+static bool __init bcm6846_smp_init(void)
+{
+	if (!of_machine_is_compatible("genexis,xg6846b") || psci_smp_available())
+		return false;
+
+	smp_set_ops(&bcm6846_smp_ops);
+	return true;
 }
+#endif
 
 static const char * const bcmbca_dt_compat[] = {
 	/* TODO: Add other BCMBCA SoCs here to get debug UART support */
@@ -27,5 +91,6 @@ static const char * const bcmbca_dt_compat[] = {
 
 DT_MACHINE_START(BCMBCA_DT, "BCMBCA Broadband Access Processors")
 	.map_io = bcmbca_map_io,
+	.smp_init = smp_init_ops(bcm6846_smp_init),
 	.dt_compat = bcmbca_dt_compat,
 MACHINE_END

-- 
2.56.0