[Updated changelog][PATCH 5/10] ACPI / PCI: Do not preserve _OSC control bits returned by a query (v4)

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Saturday, August 21, 2010, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rjw@sisk.pl>
> 
> There is the assumption in acpi_pci_osc_control_set() that it is
> always sufficient to compare the mask of _OSC control bits to be
> requested with the result of an _OSC query where all of the known
> control bits have been checked.  However, in general, that need not
> be the case.  For example, if an _OSC feature A depends on an _OSC
> feature B and control of A, B plus another _OSC feature C is
> requested simultaneously, the BIOS may return A, B, C, while it would
> only return C if A and C were requested without B.
> 
> That may result in passing a wrong mask of _OSC control bits to an
> _OSC control request, in which case the BIOS may only grant control
> of a subset of the requested features.  Moreover, acpi_pci_run_osc()
> will return error code if that happens and the caller of
> acpi_pci_osc_control_set() will not know that it's been granted
> control of some _OSC features.  Consequently, the system will
> generally not work as expected.
> 
> For this reason, make acpi_pci_osc_control_set() always check if
> control of the requested _OSC features will be granted before making
> the final control request.  As a result, the osc_control_qry and
> osc_queried members of struct acpi_pci_root are not necessary any
> more, so drop them and remove the remaining code referring to them.
> 
> Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

I have a new version of the changelog due to a discovery I made while testing
the patchset, but the patch is not too big, so appended is the entire thing.

Thanks,
Rafael

---
From: Rafael J. Wysocki <rjw@sisk.pl>
Subject: ACPI / PCI: Do not preserve _OSC control bits returned by a query (v4)

There is the assumption in acpi_pci_osc_control_set() that it is
always sufficient to compare the mask of _OSC control bits to be
requested with the result of an _OSC query where all of the known
control bits have been checked.  However, in general, that need not
be the case.  For example, if an _OSC feature A depends on an _OSC
feature B and control of A, B plus another _OSC feature C is
requested simultaneously, the BIOS may return A, B, C, while it would
only return C if A and C were requested without B.

That may result in passing a wrong mask of _OSC control bits to an
_OSC control request, in which case the BIOS may only grant control
of a subset of the requested features.  Moreover, acpi_pci_run_osc()
will return error code if that happens and the caller of
acpi_pci_osc_control_set() will not know that it's been granted
control of some _OSC features.  Consequently, the system will
generally not work as expected.

Apart from this acpi_pci_osc_control_set() always uses the mask
of _OSC control bits returned by the very first invocation of
acpi_pci_query_osc(), but that is done with the second argument
equal to OSC_PCI_SEGMENT_GROUPS_SUPPORT which generally happens
to affect the returned _OSC control bits.

For these reasons, make acpi_pci_osc_control_set() always check if
control of the requested _OSC features will be granted before making
the final control request.  As a result, the osc_control_qry and
osc_queried members of struct acpi_pci_root are not necessary any
more, so drop them and remove the remaining code referring to them.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
 drivers/acpi/pci_root.c |   20 +++++++-------------
 include/acpi/acpi_bus.h |    3 ---
 2 files changed, 7 insertions(+), 16 deletions(-)

Index: linux-2.6/drivers/acpi/pci_root.c
===================================================================
--- linux-2.6.orig/drivers/acpi/pci_root.c
+++ linux-2.6/drivers/acpi/pci_root.c
@@ -249,12 +249,8 @@ static acpi_status acpi_pci_query_osc(st
 	status = acpi_pci_run_osc(root->device->handle, capbuf, &result);
 	if (ACPI_SUCCESS(status)) {
 		root->osc_support_set = support;
-		if (control) {
+		if (control)
 			*control = result;
-		} else {
-			root->osc_control_qry = result;
-			root->osc_queried = 1;
-		}
 	}
 	return status;
 }
@@ -409,14 +405,12 @@ acpi_status acpi_pci_osc_control_set(acp
 		goto out;
 
 	/* Need to query controls first before requesting them */
-	if (!root->osc_queried) {
-		status = acpi_pci_query_osc(root, root->osc_support_set, NULL);
-		if (ACPI_FAILURE(status))
-			goto out;
-	}
-	if ((root->osc_control_qry & control_req) != control_req) {
-		printk(KERN_DEBUG
-		       "Firmware did not grant requested _OSC control\n");
+	flags = control_req;
+	status = acpi_pci_query_osc(root, root->osc_support_set, &flags);
+	if (ACPI_FAILURE(status))
+		goto out;
+
+	if (flags != control_req) {
 		status = AE_SUPPORT;
 		goto out;
 	}
Index: linux-2.6/include/acpi/acpi_bus.h
===================================================================
--- linux-2.6.orig/include/acpi/acpi_bus.h
+++ linux-2.6/include/acpi/acpi_bus.h
@@ -377,9 +377,6 @@ struct acpi_pci_root {
 
 	u32 osc_support_set;	/* _OSC state of support bits */
 	u32 osc_control_set;	/* _OSC state of control bits */
-	u32 osc_control_qry;	/* the latest _OSC query result */
-
-	u32 osc_queried:1;	/* has _OSC control been queried? */
 };
 
 /* helper */