Re: [PATCH 11/11] powerpc/smp: Add a doorbell=off kernel parameter

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 arch/powerpc/include/asm/dbell.h                |  1 +
 arch/powerpc/kernel/dbell.c                     | 17 +++++++++++++++++
 arch/powerpc/platforms/powernv/smp.c            |  7 +++++--
 arch/powerpc/platforms/pseries/smp.c            |  3 +++
 Documentation/admin-guide/kernel-parameters.txt | 10 ++++++++++
 5 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/dbell.h b/arch/powerpc/include/asm/dbell.h
index 3e9da22a2779..07775aa3e81b 100644
--- a/arch/powerpc/include/asm/dbell.h
+++ b/arch/powerpc/include/asm/dbell.h

@@ -90,6 +90,7 @@ static inline void ppc_msgsync(void)
 #endif /* CONFIG_PPC_BOOK3S */
 
 extern void doorbell_exception(struct pt_regs *regs);
+extern bool doorbell_disabled;
 
 static inline void ppc_msgsnd(enum ppc_dbell type, u32 flags, u32 tag)
 {

diff --git a/arch/powerpc/kernel/dbell.c b/arch/powerpc/kernel/dbell.c
index 5545c9cd17c1..681ee4775629 100644
--- a/arch/powerpc/kernel/dbell.c
+++ b/arch/powerpc/kernel/dbell.c

@@ -38,6 +38,23 @@ DEFINE_INTERRUPT_HANDLER_ASYNC(doorbell_exception)
 
 	set_irq_regs(old_regs);
 }
+
+bool doorbell_disabled;
+
+static int __init doorbell_cmdline(char *arg)
+{
+	if (!arg)
+		return -EINVAL;
+
+	if (strncmp(arg, "off", 3) == 0) {
+		pr_info("Doorbell disabled on kernel command line\n");
+		doorbell_disabled = true;
+	}
+
+	return 0;
+}
+__setup("doorbell=", doorbell_cmdline);
+
 #else /* CONFIG_SMP */
 DEFINE_INTERRUPT_HANDLER_ASYNC(doorbell_exception)
 {

diff --git a/arch/powerpc/platforms/powernv/smp.c b/arch/powerpc/platforms/powernv/smp.c
index cbb67813cd5d..1311bda9446a 100644
--- a/arch/powerpc/platforms/powernv/smp.c
+++ b/arch/powerpc/platforms/powernv/smp.c

@@ -338,10 +338,13 @@ static void __init pnv_smp_probe(void)
 		ic_cause_ipi = smp_ops->cause_ipi;
 		WARN_ON(!ic_cause_ipi);
 
-		if (cpu_has_feature(CPU_FTR_ARCH_300))
+		if (cpu_has_feature(CPU_FTR_ARCH_300)) {
+			if (doorbell_disabled)
+				return;
 			smp_ops->cause_ipi = doorbell_global_ipi;
-		else
+		} else {
 			smp_ops->cause_ipi = pnv_cause_ipi;
+		}
 	}
 }
 

diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
index f47429323eee..3bc9e6aaf645 100644
--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c

@@ -229,6 +229,9 @@ static __init void pSeries_smp_probe(void)
 			return;
 	}
 
+	if (doorbell_disabled)
+		return;
+
 	/*
 	 * Under PowerVM, FSCR[MSGP] is enabled as guest vCPU siblings are
 	 * gang scheduled on the same physical core, so doorbells are always

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 10fa093251e8..2e1284febe39 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -1041,6 +1041,16 @@
 			The filter can be disabled or changed to another
 			driver later using sysfs.
 
+	doorbell=off	[PPC]
+			On processors with a XIVE interrupt controller
+			(POWER9 and above), the kernel can use either
+			doorbells or XIVE to generate CPU IPIs.	Sending
+			doorbell is generally preferred to using the XIVE
+			IC because it is faster. There are cases where
+			we want to avoid doorbells and use XIVE only,
+			for debug or performance. Only useful on
+			POWER9 and above.
+
 	driver_async_probe=  [KNL]
 			List of driver names to be probed asynchronously.
 			Format: <driver_name1>,<driver_name2>...

-- 
2.31.1