Xilinx PIC and kernel interrupt handler

[Stig Telfer <linuxppc@lizardlogic.co.uk>]

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Hi -

There appears to be a kernel bug in the 2.4 and 2.6.17.7 kernel trees 
relating to reading the interrupt vector from the Xilinx PIC 
(xilinx_pic_get_irq() in xilinx_pic.c).  As I see it, here's the 
problem: If no interrupt is pending, the register should read all ones. 
  However, the IVR is only as wide as the number of interrupt sources.  
The routine mistakenly assumes sign extension and checks for a 32-bit 
read of -1 instead of a read of w bits where w is the number of 
connected interrupt sources.

The 2.6 version also has a search-and-replace glitch relating to 
removal of the reversal of bit numbering.  I have attached a two line 
patch (for 2.6.17.7) that makes the IVR comparison against the right 
bit pattern and removes the remnants of the former bit-reversal code.

Share and enjoy,
Stig Telfer


[-- Attachment #2: xilinx_pic.patch --]
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diff -Naurb linux-2.6.17.7/arch/ppc/syslib/xilinx_pic.c linux-2.6.17.7-stig/arch/ppc/syslib/xilinx_pic.c
--- linux-2.6.17.7/arch/ppc/syslib/xilinx_pic.c	2006-07-25 04:36:01.000000000 +0100
+++ linux-2.6.17.7-stig/arch/ppc/syslib/xilinx_pic.c	2006-07-31 14:20:04.000000000 +0100
@@ -97,8 +97,8 @@
 	 */
 
 	irq = intc_in_be32(intc + IVR);
-	if (irq != -1)
-		irq = irq;
+	if (irq == (1 << XPAR_INTC_MAX_NUM_INTR_INPUTS) - 1)
+		irq = -1;
 
 	pr_debug("get_irq: %d\n", irq);