I think this should also improve performance on the spurious interrupt code path
(ISR reads), without hurting the common case (IRR reads).
Comments/flames are very welcome
Cheers,
Zwane Mwaikambo
--- linux-2.5.2-pre1.orig/arch/i386/kernel/i8259.c Tue Sep 18 08:03:09 2001
+++ linux/arch/i386/kernel/i8259.c Thu Jan 1 05:30:49 1998
@@ -210,14 +210,21 @@
spin_unlock_irqrestore(&i8259A_lock, flags);
}
+static unsigned char cached_reg[2] = {IRR_REG, IRR_REG};
+
int i8259A_irq_pending(unsigned int irq)
{
unsigned int mask = 1<<irq;
unsigned long flags;
- int ret;
-
+ int ret, pic = (irq < 8) ? 0 : 1;
+
spin_lock_irqsave(&i8259A_lock, flags);
- if (irq < 8)
+ if (unlikely (cached_reg[pic] != IRR_REG)) {
+ outb(IRR_REG, (pic) ? 0xA0 : 0x20);
+ cached_reg[pic] = IRR_REG;
+ }
+
+ if (!pic)
ret = inb(0x20) & mask;
else
ret = inb(0xA0) & (mask >> 8);
@@ -236,7 +243,8 @@
/*
* This function assumes to be called rarely. Switching between
- * 8259A registers is slow.
+ * 8259A registers is slow. The default cached_reg is set to IRR
+ * since that is the default when the PIC is setup.
* This has to be protected by the irq controller spinlock
* before being called.
*/
@@ -244,16 +252,18 @@
{
int value;
int irqmask = 1<<irq;
+ int pic = (irq < 8) ? 0 : 1;
- if (irq < 8) {
- outb(0x0B,0x20); /* ISR register */
- value = inb(0x20) & irqmask;
- outb(0x0A,0x20); /* back to the IRR register */
- return value;
+ if (cached_reg[pic] != ISR_REG) {
+ outb(ISR_REG, (pic) ? 0xA0 : 0x20);
+ cached_reg[pic] = ISR_REG;
}
- outb(0x0B,0xA0); /* ISR register */
- value = inb(0xA0) & (irqmask >> 8);
- outb(0x0A,0xA0); /* back to the IRR register */
+
+ if (!pic) {
+ value = inb(0x20) & irqmask;
+ } else
+ value = inb(0xA0) & (irqmask >> 8);
+
return value;
}
@@ -348,6 +358,8 @@
outb_p(0x11, 0x20); /* ICW1: select 8259A-1 init */
outb_p(0x20 + 0, 0x21); /* ICW2: 8259A-1 IR0-7 mapped to 0x20-0x27 */
outb_p(0x04, 0x21); /* 8259A-1 (the master) has a slave on IR2 */
+ outb_p(IRR_REG, 0x21); /* Default to IRR */
+
if (auto_eoi)
outb_p(0x03, 0x21); /* master does Auto EOI */
else
@@ -358,6 +370,7 @@
outb_p(0x02, 0xA1); /* 8259A-2 is a slave on master's IR2 */
outb_p(0x01, 0xA1); /* (slave's support for AEOI in flat mode
is to be investigated) */
+ outb_p(IRR_REG, 0xA1); /* Default to IRR */
if (auto_eoi)
/*