2.6.18 suspend regression on Intel Macs

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Well, I'm not sure it qualifies as a regression, because AFAIK no
official kernels can s2ram/resume Intel Macs correctly out of the box.

There has already been some discussion about the SCI_EN ACPI control bit
not being set when the Mactel boxes come out of suspend to ram. 

The symptom is:
    irq 9: nobody cared (try booting with the "irqpoll" option)
    Disabling IRQ #9
when the system comes out of sleep, making ACPI non-functional.

Two days after having released 2.6.17, Linus commited a fix for this
issue in his tree (commit 5603509137940f4cbc577281cee62110d4097b1b):

@@ -812,6 +812,9 @@ static int irqrouter_resume(struct sys_d


+ /* Make sure SCI is enabled again (Apple firmware bug?) */

I since then used lightly patched 2.6.17 kernels on my MacMini without a
problem. With the release of 2.6.18, I decided to switch to a vanilla
kernel but I realized that the above issue reappeared. 

I tracked it down to the ACPI merge that took place on July 1st. More
precisely the commit 967440e3be1af06ad4dc7bb18d2e3c16130fe067 (ACPI:
ACPICA 20060623) contains the following hunk:

@@ -635,6 +663,25 @@ acpi_status acpi_hw_register_write(u8 us
 	case ACPI_REGISTER_PM1_CONTROL:	/* 16-bit access */
+		/*
+		 * Perform a read first to preserve certain bits (per ACPI spec)
+		 *
+		 * Note: This includes SCI_EN, we never want to change this bit
+		 */
+		status = acpi_hw_register_read(ACPI_MTX_DO_NOT_LOCK,
+					       &read_value);
+		if (ACPI_FAILURE(status)) {
+			goto unlock_and_exit;
+		}
+		/* Insert the bits to be preserved */
+				 read_value);
+		/* Now we can write the data */
 		status =
 		    acpi_hw_low_level_write(16, value,

which makes Linus' fix a no-op, because it disallows setting the SCI_EN


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