ACPI: Avoid bogus EC timeout when EC is in Polling mode

In http://bugzilla.kernel.org/show_bug.cgi?id=11594, Robert points out that this fix should be backported into 2.6.26.x.

Begin forwarded message:

Date: Fri, 15 Aug 2008 17:00:31 GMT
From: Linux Kernel Mailing List <[email protected]>
To: [email protected]
Subject: ACPI: Avoid bogus EC timeout when EC is in Polling mode

Gitweb:     http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=9d699ed92a459cb408e2577e8bbeabc8ec3989e1
Commit:     9d699ed92a459cb408e2577e8bbeabc8ec3989e1
Parent:     2500822bf4eb0179ef80e5b072c1e0fa83037381
Author:     Zhao Yakui <[email protected]>
AuthorDate: Mon Aug 11 10:33:31 2008 +0800
Committer:  Andi Kleen <[email protected]>
CommitDate: Fri Aug 15 03:13:06 2008 +0200

ACPI: Avoid bogus EC timeout when EC is in Polling mode

When EC is in Polling mode, OS will check the EC status continually by using the following source code:
clear_bit(EC_FLAGS_WAIT_GPE, &ec->flags);
while (time_before(jiffies, delay)) {
if (acpi_ec_check_status(ec, event))
return 0;
msleep(1);
}
But msleep is realized by the function of schedule_timeout. At the same time although one process is already waken up by some events, it won't be scheduled immediately. So maybe there exists the following phenomena:
a. The current jiffies is already after the predefined jiffies.
But before timeout happens, OS has no chance to check the EC status again.
b. If preemptible schedule is enabled, maybe preempt schedule will happen
before checking loop. When the process is resumed again, maybe timeout already happens, which means that OS has no chance to check the EC status.

In such case maybe EC status is already what OS expects when timeout happens. But OS has no chance to check the EC status and regards it as AE_TIME.

So it will be more appropriate that OS will try to check the EC status again when timeout happens. If the EC status is what we expect, it won't be regarded as timeout. Only when the EC status is not what we expect, it will be regarded as timeout, which means that EC controller can't give a response in time.

http://bugzilla.kernel.org/show_bug.cgi?id=9823
http://bugzilla.kernel.org/show_bug.cgi?id=11141

---
 drivers/acpi/ec.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

index 76784ae..13593f9 100644
@@ -221,6 +221,8 @@ static int acpi_ec_wait(struct acpi_ec *ec, enum ec_event event, int force_poll)
 				return 0;
 			msleep(1);
 		}
+		if (acpi_ec_check_status(ec,event))
+			return 0;
 	}
 	pr_err(PREFIX "acpi_ec_wait timeout, status = 0x%2.2x, event = %s\n",
 		acpi_ec_read_status(ec),

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EC changes are typically fragile. I think it would be better to be conservative here and avoid early backports if possible. Of course it's Len's final call.

-Andi