[PATCH] powerpc/mpic: Add DT option to skip readback after EOI

[Bogdan Purcareata]

The readback is necessary in order to handle PCI posted
writes, or when the MPIC is handling interrupts in a loop
(ppc_md.get_irq). Newer MPIC versions don't require this
readback. Leave the option configurable using a device
tree entry.

This saves a MMIO trap per interrupt.

Signed-off-by: Scott Wood <scottwood@xxxxxxxxxxxxx>
Signed-off-by: Bogdan Purcareata <bogdan.purcareata@xxxxxxxxxxxxx>
---
 Documentation/devicetree/bindings/powerpc/fsl/mpic.txt | 13 +++++++++++++
 arch/powerpc/include/asm/mpic.h                        |  2 ++
 arch/powerpc/sysdev/mpic.c                             |  8 +++++++-
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/powerpc/fsl/mpic.txt b/Documentation/devicetree/bindings/powerpc/fsl/mpic.txt
index dc57446..9789094 100644
--- a/Documentation/devicetree/bindings/powerpc/fsl/mpic.txt
+++ b/Documentation/devicetree/bindings/powerpc/fsl/mpic.txt
@@ -77,6 +77,19 @@ PROPERTIES
           in the global feature registers.  If specified, this field will
           override the value read from MPIC_GREG_FEATURE_LAST_SRC.
 
+  - mpic-eoi-no-readback
+      Usage: optional
+      Value type: <empty>
+      Definition: The presence of this property specifies that the
+          MPIC will not issue a readback when delivering the EOI for an
+          external interrupt. The readback operation is done by reading
+          the CPU WHOAMI register after writing to the CPU EOI register.
+          Originally, this was required due to the fact that the MPIC
+          operates at lower frequencies, or in scenarios where the MPIC
+          is connected through PCI with write posting. This is not the
+          case in an emulated environment (e.g. KVM guest), or in scenarios
+          where interrupts are not handled in a loop of get_irq() calls.
+
 INTERRUPT SPECIFIER DEFINITION
 
   Interrupt specifiers consists of 4 cells encoded as
diff --git a/arch/powerpc/include/asm/mpic.h b/arch/powerpc/include/asm/mpic.h
index 754f93d..e2a4146 100644
--- a/arch/powerpc/include/asm/mpic.h
+++ b/arch/powerpc/include/asm/mpic.h
@@ -386,6 +386,8 @@ extern struct bus_type mpic_subsys;
  * from the BRR1 register).
 */
 #define MPIC_FSL_HAS_EIMR		0x00010000
+/* Dont bother with readback after MPIC EOI */
+#define MPIC_EOI_NO_READBACK	0x00020000
 
 /* MPIC HW modification ID */
 #define MPIC_REGSET_MASK		0xf0000000
diff --git a/arch/powerpc/sysdev/mpic.c b/arch/powerpc/sysdev/mpic.c
index f3e8624..431f68e 100644
--- a/arch/powerpc/sysdev/mpic.c
+++ b/arch/powerpc/sysdev/mpic.c
@@ -656,7 +656,9 @@ static inline struct mpic * mpic_from_irq_data(struct irq_data *d)
 static inline void mpic_eoi(struct mpic *mpic)
 {
 	mpic_cpu_write(MPIC_INFO(CPU_EOI), 0);
-	(void)mpic_cpu_read(MPIC_INFO(CPU_WHOAMI));
+
+	if (!(mpic->flags & MPIC_EOI_NO_READBACK))
+		(void)mpic_cpu_read(MPIC_INFO(CPU_WHOAMI));
 }
 
 /*
@@ -1290,6 +1292,10 @@ struct mpic * __init mpic_alloc(struct device_node *node,
 		flags |= MPIC_SINGLE_DEST_CPU;
 	if (of_device_is_compatible(node, "fsl,mpic"))
 		flags |= MPIC_FSL | MPIC_LARGE_VECTORS;
+	if (of_get_property(node, "mpic-eoi-no-readback", NULL)) {
+		pr_debug("mpic: no readback activated");
+		flags |= MPIC_EOI_NO_READBACK;
+	}
 
 	mpic = kzalloc(sizeof(struct mpic), GFP_KERNEL);
 	if (mpic == NULL)
-- 
2.1.3

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