1 files changed, 30 insertions, 8 deletions

diff --git a/arch/x86/kernel/i8259.c b/arch/x86/kernel/i8259.c
index 8821d0ab0..82753622f 100644
--- a/arch/x86/kernel/i8259.c
+++ b/arch/x86/kernel/i8259.c
@@ -32,6 +32,7 @@
  */
 static void init_8259A(int auto_eoi);
 
+static bool pcat_compat __ro_after_init;
 static int i8259A_auto_eoi;
 DEFINE_RAW_SPINLOCK(i8259A_lock);
 
@@ -301,15 +302,32 @@ static void unmask_8259A(void)
 
 static int probe_8259A(void)
 {
+	unsigned char new_val, probe_val = ~(1 << PIC_CASCADE_IR);
 	unsigned long flags;
-	unsigned char probe_val = ~(1 << PIC_CASCADE_IR);
-	unsigned char new_val;
+
+	/*
+	 * If MADT has the PCAT_COMPAT flag set, then do not bother probing
+	 * for the PIC. Some BIOSes leave the PIC uninitialized and probing
+	 * fails.
+	 *
+	 * Right now this causes problems as quite some code depends on
+	 * nr_legacy_irqs() > 0 or has_legacy_pic() == true. This is silly
+	 * when the system has an IO/APIC because then PIC is not required
+	 * at all, except for really old machines where the timer interrupt
+	 * must be routed through the PIC. So just pretend that the PIC is
+	 * there and let legacy_pic->init() initialize it for nothing.
+	 *
+	 * Alternatively this could just try to initialize the PIC and
+	 * repeat the probe, but for cases where there is no PIC that's
+	 * just pointless.
+	 */
+	if (pcat_compat)
+		return nr_legacy_irqs();
+
 	/*
-	 * Check to see if we have a PIC.
-	 * Mask all except the cascade and read
-	 * back the value we just wrote. If we don't
-	 * have a PIC, we will read 0xff as opposed to the
-	 * value we wrote.
+	 * Check to see if we have a PIC.  Mask all except the cascade and
+	 * read back the value we just wrote. If we don't have a PIC, we
+	 * will read 0xff as opposed to the value we wrote.
 	 */
 	raw_spin_lock_irqsave(&i8259A_lock, flags);
 
@@ -431,5 +449,9 @@ static int __init i8259A_init_ops(void)
 
 	return 0;
 }
-
 device_initcall(i8259A_init_ops);
+
+void __init legacy_pic_pcat_compat(void)
+{
+	pcat_compat = true;
+}