From: Rafael J. Wysocki <[email protected]>
The underlying problem is the handling of the enabled bit in device
status (bit 1 of _STA return value) which is required by the ACPI
specification to be observed in addition to the present bit (bit 0
of _STA return value) [1], but Linux does not observe it.
Since Linux has not looked at that bit for a long time, it is generally
risky to start obseving it in all device enumeration cases, especially
at the system initialization time, but it can be observed when the
kernel receives a Bus Check or Device Check notification indicating a
change in device configuration. In those cases, seeing the enabled bit
clear may be regarded as an indication that the device at hand should
not be used any more.
For this reason, rework the handling of Device Check and Bus Check
notifications in the ACPI core device enumeration code in the
following way:
1. Make acpi_bus_trim_one() check device status if its second argument
is not NULL, in which case it will only detach scan handlers or ACPI
drivers from devices whose _STA returns the enabled bit clear.
2. Make acpi_scan_device_check() and acpi_scan_bus_check() invoke
acpi_bus_trim_one() with a non-NULL second argument unconditionally,
so scan handlers and ACPI drivers are detached from the target
device and its ancestors if their _STA returns the enabled bit
clear.
3. Make acpi_scan_device_check() skip the bus rescan if _STA for the
target device return the enabled bit clear, which is allowed by the
ACPI specification in the Device Check case. [2]
In addition to the above:
4. Make sure that the bus rescan that needs to be triggered in the case
when a new device has appeared is carried out in the same way in
both the Device Check and Bus Check cases.
5. In the Device Check case, start the bus rescan mentioned above from
the target device's parent, as per the specification. [2]
Link: https://uefi.org/specs/ACPI/6.5/06_Device_Configuration.html#sta-device-status # [1]
Link: https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#device-object-notification-values # [2]
Signed-off-by: Rafael J. Wysocki <[email protected]>
---
drivers/acpi/internal.h | 1
drivers/acpi/scan.c | 109 +++++++++++++++++++++++++++---------------------
2 files changed, 64 insertions(+), 46 deletions(-)
Index: linux-pm/drivers/acpi/scan.c
===================================================================
--- linux-pm.orig/drivers/acpi/scan.c
+++ linux-pm/drivers/acpi/scan.c
@@ -244,11 +244,27 @@ static int acpi_scan_try_to_offline(stru
return 0;
}
-static int acpi_bus_trim_one(struct acpi_device *adev, void *not_used)
+static int acpi_bus_trim_one(struct acpi_device *adev, void *check_status)
{
struct acpi_scan_handler *handler = adev->handler;
- acpi_dev_for_each_child_reverse(adev, acpi_bus_trim_one, NULL);
+ acpi_dev_for_each_child_reverse(adev, acpi_bus_trim_one, check_status);
+
+ if (check_status) {
+ acpi_bus_get_status(adev);
+ /*
+ * Skip devices that are still there and take the enabled
+ * flag into account.
+ */
+ if (acpi_device_is_enabled(adev))
+ return 0;
+
+ /* Skip device that have not been enumerated. */
+ if (!acpi_device_enumerated(adev)) {
+ dev_dbg(&adev->dev, "Still not enumerated\n");
+ return 0;
+ }
+ }
adev->flags.match_driver = false;
if (handler) {
@@ -315,71 +331,67 @@ static int acpi_scan_hot_remove(struct a
return 0;
}
-static int acpi_scan_device_not_enumerated(struct acpi_device *adev)
+static void acpi_scan_check_gone(struct acpi_device *adev)
{
- if (!acpi_device_enumerated(adev)) {
- dev_warn(&adev->dev, "Still not enumerated\n");
- return -EALREADY;
- }
- acpi_bus_trim(adev);
- return 0;
+ acpi_bus_trim_one(adev, (void *)true);
}
-static int acpi_scan_device_check(struct acpi_device *adev)
+static int acpi_scan_rescan_bus(struct acpi_device *adev)
{
+ struct acpi_scan_handler *handler = adev->handler;
int error;
- acpi_bus_get_status(adev);
- if (acpi_device_is_present(adev)) {
- /*
- * This function is only called for device objects for which
- * matching scan handlers exist. The only situation in which
- * the scan handler is not attached to this device object yet
- * is when the device has just appeared (either it wasn't
- * present at all before or it was removed and then added
- * again).
- */
- if (adev->handler) {
- dev_dbg(&adev->dev, "Already enumerated\n");
- return 0;
- }
+ if (handler && handler->hotplug.scan_dependent)
+ error = handler->hotplug.scan_dependent(adev);
+ else
error = acpi_bus_scan(adev->handle);
- if (error) {
- dev_warn(&adev->dev, "Namespace scan failure\n");
- return error;
- }
- } else {
- error = acpi_scan_device_not_enumerated(adev);
- }
+
+ if (error)
+ dev_info(&adev->dev, "Namespace scan failure\n");
+
return error;
}
-static int acpi_scan_bus_check(struct acpi_device *adev, void *not_used)
+static int acpi_scan_device_check(struct acpi_device *adev)
{
- struct acpi_scan_handler *handler = adev->handler;
- int error;
+ struct acpi_device *parent;
- acpi_bus_get_status(adev);
- if (!acpi_device_is_present(adev)) {
- acpi_scan_device_not_enumerated(adev);
+ acpi_scan_check_gone(adev);
+
+ if (!acpi_device_is_enabled(adev))
return 0;
- }
- if (handler && handler->hotplug.scan_dependent)
- return handler->hotplug.scan_dependent(adev);
- error = acpi_bus_scan(adev->handle);
- if (error) {
- dev_warn(&adev->dev, "Namespace scan failure\n");
- return error;
+ /*
+ * This function is only called for device objects for which matching
+ * scan handlers exist. The only situation in which the scan handler
+ * is not attached to this device object yet is when the device has
+ * just appeared (either it wasn't present or enabled at all before or
+ * it was removed and then added again).
+ */
+ if (adev->handler) {
+ dev_dbg(&adev->dev, "Already enumerated\n");
+ return 0;
}
- return acpi_dev_for_each_child(adev, acpi_scan_bus_check, NULL);
+
+ parent = acpi_dev_parent(adev);
+ if (!parent)
+ parent = adev;
+
+ return acpi_scan_rescan_bus(parent);
+}
+
+static int acpi_scan_bus_check(struct acpi_device *adev)
+{
+ acpi_scan_check_gone(adev);
+
+ return acpi_scan_rescan_bus(adev);
}
static int acpi_generic_hotplug_event(struct acpi_device *adev, u32 type)
{
switch (type) {
case ACPI_NOTIFY_BUS_CHECK:
- return acpi_scan_bus_check(adev, NULL);
+ return acpi_scan_bus_check(adev);
case ACPI_NOTIFY_DEVICE_CHECK:
return acpi_scan_device_check(adev);
case ACPI_NOTIFY_EJECT_REQUEST:
@@ -1945,6 +1957,11 @@ bool acpi_device_is_present(const struct
return adev->status.present || adev->status.functional;
}
+bool acpi_device_is_enabled(const struct acpi_device *adev)
+{
+ return acpi_device_is_present(adev) && adev->status.enabled;
+}
+
static bool acpi_scan_handler_matching(struct acpi_scan_handler *handler,
const char *idstr,
const struct acpi_device_id **matchid)
Index: linux-pm/drivers/acpi/internal.h
===================================================================
--- linux-pm.orig/drivers/acpi/internal.h
+++ linux-pm/drivers/acpi/internal.h
@@ -121,6 +121,7 @@ int acpi_device_setup_files(struct acpi_
void acpi_device_remove_files(struct acpi_device *dev);
void acpi_device_add_finalize(struct acpi_device *device);
void acpi_free_pnp_ids(struct acpi_device_pnp *pnp);
+bool acpi_device_is_enabled(const struct acpi_device *adev);
bool acpi_device_is_present(const struct acpi_device *adev);
bool acpi_device_is_battery(struct acpi_device *adev);
bool acpi_device_is_first_physical_node(struct acpi_device *adev,
On Wed, 21 Feb 2024 21:02:33 +0100
"Rafael J. Wysocki" <[email protected]> wrote:
> From: Rafael J. Wysocki <[email protected]>
>
> The underlying problem is the handling of the enabled bit in device
> status (bit 1 of _STA return value) which is required by the ACPI
> specification to be observed in addition to the present bit (bit 0
> of _STA return value) [1], but Linux does not observe it.
>
> Since Linux has not looked at that bit for a long time, it is generally
> risky to start obseving it in all device enumeration cases, especially
> at the system initialization time, but it can be observed when the
> kernel receives a Bus Check or Device Check notification indicating a
> change in device configuration. In those cases, seeing the enabled bit
> clear may be regarded as an indication that the device at hand should
> not be used any more.
Hi Rafael,
I rebased the vCPU HP series Russell was working to go on top of this
and give me a basis to check the flows through your new conditions.
It may have it's own issues, but at least it makes use of some of these
bits and related checks.
For now the only key thing is make sure we don't check enabled()
in the hot remove path (until after _EJ0). That happens correctly
because acpi_device_trim() is called and that doesn't have check_status
set. The naming however doesn't make it obvious that path elides the
check, so I wonder if that call in acpi_scan_hotremove()
should be replaced with acpi_bus_trim_one(, NULL);
>
> For this reason, rework the handling of Device Check and Bus Check
> notifications in the ACPI core device enumeration code in the
> following way:
>
> 1. Make acpi_bus_trim_one() check device status if its second argument
> is not NULL, in which case it will only detach scan handlers or ACPI
> drivers from devices whose _STA returns the enabled bit clear.
>
> 2. Make acpi_scan_device_check() and acpi_scan_bus_check() invoke
> acpi_bus_trim_one() with a non-NULL second argument unconditionally,
> so scan handlers and ACPI drivers are detached from the target
> device and its ancestors if their _STA returns the enabled bit
> clear.
>
> 3. Make acpi_scan_device_check() skip the bus rescan if _STA for the
> target device return the enabled bit clear, which is allowed by the
> ACPI specification in the Device Check case. [2]
>
> In addition to the above:
>
> 4. Make sure that the bus rescan that needs to be triggered in the case
> when a new device has appeared is carried out in the same way in
> both the Device Check and Bus Check cases.
>
> 5. In the Device Check case, start the bus rescan mentioned above from
> the target device's parent, as per the specification. [2]
This feels like an 'and' kind of a patch. Can we split it up so refactors
are done first and the _STA check stuff in a follow up patch?
End result is good, just could possibly be easier to review in two hops.
>
> Link: https://uefi.org/specs/ACPI/6.5/06_Device_Configuration.html#sta-device-status # [1]
> Link: https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#device-object-notification-values # [2]
> Signed-off-by: Rafael J. Wysocki <[email protected]>
Diff had a field day here and generated an somewhat unreadable patch.
A few other comments inline, but superficial stuff. The handling looks
correct to me.
> ---
> drivers/acpi/internal.h | 1
> drivers/acpi/scan.c | 109 +++++++++++++++++++++++++++---------------------
> 2 files changed, 64 insertions(+), 46 deletions(-)
>
> Index: linux-pm/drivers/acpi/scan.c
> ===================================================================
> --- linux-pm.orig/drivers/acpi/scan.c
> +++ linux-pm/drivers/acpi/scan.c
> @@ -244,11 +244,27 @@ static int acpi_scan_try_to_offline(stru
> return 0;
> }
>
> -static int acpi_bus_trim_one(struct acpi_device *adev, void *not_used)
> +static int acpi_bus_trim_one(struct acpi_device *adev, void *check_status)
Bool as pointer is a bit hard to read particularly as you use (void *)true
for true, but not (void *)false for false.
However it's not too bad. My current version of the vCPU patches needs
to pass more data in here anyway (as there a few things we need to
not do on eject, that don't correspond to !check_status)
so I have this as a struct anyway after a rebase.
> {
> struct acpi_scan_handler *handler = adev->handler;
>
> - acpi_dev_for_each_child_reverse(adev, acpi_bus_trim_one, NULL);
> + acpi_dev_for_each_child_reverse(adev, acpi_bus_trim_one, check_status);
> +
> + if (check_status) {
> + acpi_bus_get_status(adev);
> + /*
> + * Skip devices that are still there and take the enabled
> + * flag into account.
> + */
> + if (acpi_device_is_enabled(adev))
> + return 0;
> +
> + /* Skip device that have not been enumerated. */
> + if (!acpi_device_enumerated(adev)) {
> + dev_dbg(&adev->dev, "Still not enumerated\n");
> + return 0;
> + }
> + }
>
> adev->flags.match_driver = false;
> if (handler) {
> @@ -315,71 +331,67 @@ static int acpi_scan_hot_remove(struct a
> return 0;
> }
>
> -static int acpi_scan_device_not_enumerated(struct acpi_device *adev)
> +static void acpi_scan_check_gone(struct acpi_device *adev)
The name of this had me initially a little confused. Maybe
acpi_bus_trim_if_gone()
Or maybe just drop this wrapper entirely as it doesn't save much
and naming it clearly is hard.
> {
> - if (!acpi_device_enumerated(adev)) {
> - dev_warn(&adev->dev, "Still not enumerated\n");
> - return -EALREADY;
> - }
> - acpi_bus_trim(adev);
> - return 0;
> + acpi_bus_trim_one(adev, (void *)true);
> }
> static int acpi_generic_hotplug_event(struct acpi_device *adev, u32 type)
> {
> switch (type) {
> case ACPI_NOTIFY_BUS_CHECK:
> - return acpi_scan_bus_check(adev, NULL);
> + return acpi_scan_bus_check(adev);
> case ACPI_NOTIFY_DEVICE_CHECK:
> return acpi_scan_device_check(adev);
> case ACPI_NOTIFY_EJECT_REQUEST:
> @@ -1945,6 +1957,11 @@ bool acpi_device_is_present(const struct
> return adev->status.present || adev->status.functional;
> }
>
> +bool acpi_device_is_enabled(const struct acpi_device *adev)
> +{
> + return acpi_device_is_present(adev) && adev->status.enabled;
This resolves as (present or functional) && enabled.
By my reading you are not allowed functional && enabled, but not present.
Line one of the description says.
"If bit [0] is cleared, then bit 1 must also be cleared (in other words, a device that is not present cannot be enabled)."
I don't much care about that though (I think we discussed this
in Russel's earlier series)
> +}
> +
> static bool acpi_scan_handler_matching(struct acpi_scan_handler *handler,
> const char *idstr,
> const struct acpi_device_id **matchid)
> Index: linux-pm/drivers/acpi/internal.h
> ===================================================================
> --- linux-pm.orig/drivers/acpi/internal.h
> +++ linux-pm/drivers/acpi/internal.h
> @@ -121,6 +121,7 @@ int acpi_device_setup_files(struct acpi_
> void acpi_device_remove_files(struct acpi_device *dev);
> void acpi_device_add_finalize(struct acpi_device *device);
> void acpi_free_pnp_ids(struct acpi_device_pnp *pnp);
> +bool acpi_device_is_enabled(const struct acpi_device *adev);
> bool acpi_device_is_present(const struct acpi_device *adev);
> bool acpi_device_is_battery(struct acpi_device *adev);
> bool acpi_device_is_first_physical_node(struct acpi_device *adev,
>
>
>
On Thu, Feb 22, 2024 at 7:28 PM Jonathan Cameron
<[email protected]> wrote:
>
> On Wed, 21 Feb 2024 21:02:33 +0100
> "Rafael J. Wysocki" <[email protected]> wrote:
>
> > From: Rafael J. Wysocki <[email protected]>
> >
> > The underlying problem is the handling of the enabled bit in device
> > status (bit 1 of _STA return value) which is required by the ACPI
> > specification to be observed in addition to the present bit (bit 0
> > of _STA return value) [1], but Linux does not observe it.
> >
> > Since Linux has not looked at that bit for a long time, it is generally
> > risky to start obseving it in all device enumeration cases, especially
> > at the system initialization time, but it can be observed when the
> > kernel receives a Bus Check or Device Check notification indicating a
> > change in device configuration. In those cases, seeing the enabled bit
> > clear may be regarded as an indication that the device at hand should
> > not be used any more.
>
> Hi Rafael,
>
> I rebased the vCPU HP series Russell was working to go on top of this
> and give me a basis to check the flows through your new conditions.
> It may have it's own issues, but at least it makes use of some of these
> bits and related checks.
>
> For now the only key thing is make sure we don't check enabled()
> in the hot remove path (until after _EJ0). That happens correctly
> because acpi_device_trim() is called and that doesn't have check_status
> set. The naming however doesn't make it obvious that path elides the
> check, so I wonder if that call in acpi_scan_hotremove()
> should be replaced with acpi_bus_trim_one(, NULL);
Well, that's how acpi_bus_trim() is supposed to work: Detach
everything under the target device (and including that device itself)
unconditionally.
I would prefer to rename acpi_bus_trim_one() to something closer
reflecting its purpose.
> >
> > For this reason, rework the handling of Device Check and Bus Check
> > notifications in the ACPI core device enumeration code in the
> > following way:
> >
> > 1. Make acpi_bus_trim_one() check device status if its second argument
> > is not NULL, in which case it will only detach scan handlers or ACPI
> > drivers from devices whose _STA returns the enabled bit clear.
> >
> > 2. Make acpi_scan_device_check() and acpi_scan_bus_check() invoke
> > acpi_bus_trim_one() with a non-NULL second argument unconditionally,
> > so scan handlers and ACPI drivers are detached from the target
> > device and its ancestors if their _STA returns the enabled bit
> > clear.
> >
> > 3. Make acpi_scan_device_check() skip the bus rescan if _STA for the
> > target device return the enabled bit clear, which is allowed by the
> > ACPI specification in the Device Check case. [2]
> >
> > In addition to the above:
> >
> > 4. Make sure that the bus rescan that needs to be triggered in the case
> > when a new device has appeared is carried out in the same way in
> > both the Device Check and Bus Check cases.
> >
> > 5. In the Device Check case, start the bus rescan mentioned above from
> > the target device's parent, as per the specification. [2]
>
> This feels like an 'and' kind of a patch. Can we split it up so refactors
> are done first and the _STA check stuff in a follow up patch?
Sure, that will produce more readable patches.
> End result is good, just could possibly be easier to review in two hops.
Sure.
> >
> > Link: https://uefi.org/specs/ACPI/6.5/06_Device_Configuration.html#sta-device-status # [1]
> > Link: https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#device-object-notification-values # [2]
> > Signed-off-by: Rafael J. Wysocki <[email protected]>
>
> Diff had a field day here and generated an somewhat unreadable patch.
Well, agreed.
> A few other comments inline, but superficial stuff. The handling looks
> correct to me.
>
> > ---
> > drivers/acpi/internal.h | 1
> > drivers/acpi/scan.c | 109 +++++++++++++++++++++++++++---------------------
> > 2 files changed, 64 insertions(+), 46 deletions(-)
> >
> > Index: linux-pm/drivers/acpi/scan.c
> > ===================================================================
> > --- linux-pm.orig/drivers/acpi/scan.c
> > +++ linux-pm/drivers/acpi/scan.c
> > @@ -244,11 +244,27 @@ static int acpi_scan_try_to_offline(stru
> > return 0;
> > }
> >
> > -static int acpi_bus_trim_one(struct acpi_device *adev, void *not_used)
> > +static int acpi_bus_trim_one(struct acpi_device *adev, void *check_status)
>
> Bool as pointer is a bit hard to read particularly as you use (void *)true
> for true, but not (void *)false for false.
>
> However it's not too bad. My current version of the vCPU patches needs
> to pass more data in here anyway (as there a few things we need to
> not do on eject, that don't correspond to !check_status)
> so I have this as a struct anyway after a rebase.
The reason for using void * here is that this function is called
recursively via acpi_dev_for_each_child_reverse() which requires a
pointer as the second arg.
I guess I could define a wrapper around it for this, but that would be
more code without any functional difference.
> > {
> > struct acpi_scan_handler *handler = adev->handler;
> >
> > - acpi_dev_for_each_child_reverse(adev, acpi_bus_trim_one, NULL);
> > + acpi_dev_for_each_child_reverse(adev, acpi_bus_trim_one, check_status);
> > +
> > + if (check_status) {
> > + acpi_bus_get_status(adev);
> > + /*
> > + * Skip devices that are still there and take the enabled
> > + * flag into account.
> > + */
> > + if (acpi_device_is_enabled(adev))
> > + return 0;
> > +
> > + /* Skip device that have not been enumerated. */
> > + if (!acpi_device_enumerated(adev)) {
> > + dev_dbg(&adev->dev, "Still not enumerated\n");
> > + return 0;
> > + }
> > + }
> >
> > adev->flags.match_driver = false;
> > if (handler) {
> > @@ -315,71 +331,67 @@ static int acpi_scan_hot_remove(struct a
> > return 0;
> > }
> >
> > -static int acpi_scan_device_not_enumerated(struct acpi_device *adev)
> > +static void acpi_scan_check_gone(struct acpi_device *adev)
>
> The name of this had me initially a little confused. Maybe
> acpi_bus_trim_if_gone()
>
> Or maybe just drop this wrapper entirely as it doesn't save much
> and naming it clearly is hard.
I'll try to make it somewhat better.
>
> > {
> > - if (!acpi_device_enumerated(adev)) {
> > - dev_warn(&adev->dev, "Still not enumerated\n");
> > - return -EALREADY;
> > - }
> > - acpi_bus_trim(adev);
> > - return 0;
> > + acpi_bus_trim_one(adev, (void *)true);
> > }
>
>
> > static int acpi_generic_hotplug_event(struct acpi_device *adev, u32 type)
> > {
> > switch (type) {
> > case ACPI_NOTIFY_BUS_CHECK:
> > - return acpi_scan_bus_check(adev, NULL);
> > + return acpi_scan_bus_check(adev);
> > case ACPI_NOTIFY_DEVICE_CHECK:
> > return acpi_scan_device_check(adev);
> > case ACPI_NOTIFY_EJECT_REQUEST:
> > @@ -1945,6 +1957,11 @@ bool acpi_device_is_present(const struct
> > return adev->status.present || adev->status.functional;
> > }
> >
> > +bool acpi_device_is_enabled(const struct acpi_device *adev)
> > +{
> > + return acpi_device_is_present(adev) && adev->status.enabled;
>
> This resolves as (present or functional) && enabled.
>
> By my reading you are not allowed functional && enabled, but not present.
> Line one of the description says.
>
> "If bit [0] is cleared, then bit 1 must also be cleared (in other words, a device that is not present cannot be enabled)."
>
> I don't much care about that though (I think we discussed this
> in Russel's earlier series)
Functional and enabled, but not present would go against the spec. I
guess the kernel could protect itself against this, but then whatever
it chooses to do has not been defined anyway.
The spec doesn't actually say what the OSPM is supposed to do when it
sees that combination of bits. I'm inclined to clarify it to say "if
bit [0] is cleared, bit [1] has no defined meaning and it should be
ignored by the OSPM". To be consistent with this interpretation,
acpi_device_is_enabled() should return "(present and enabled) or
functional".
I'll change it along these lines.
On Mon, Feb 26, 2024 at 4:37 PM Rafael J. Wysocki <[email protected]> wrote:
>
> On Thu, Feb 22, 2024 at 7:28 PM Jonathan Cameron
> <[email protected]> wrote:
> >
> > On Wed, 21 Feb 2024 21:02:33 +0100
> > "Rafael J. Wysocki" <[email protected]> wrote:
[...]
> > > +bool acpi_device_is_enabled(const struct acpi_device *adev)
> > > +{
> > > + return acpi_device_is_present(adev) && adev->status.enabled;
> >
> > This resolves as (present or functional) && enabled.
> >
> > By my reading you are not allowed functional && enabled, but not present.
> > Line one of the description says.
> >
> > "If bit [0] is cleared, then bit 1 must also be cleared (in other words, a device that is not present cannot be enabled)."
> >
> > I don't much care about that though (I think we discussed this
> > in Russel's earlier series)
>
> Functional and enabled, but not present would go against the spec. I
> guess the kernel could protect itself against this, but then whatever
> it chooses to do has not been defined anyway.
>
> The spec doesn't actually say what the OSPM is supposed to do when it
> sees that combination of bits. I'm inclined to clarify it to say "if
> bit [0] is cleared, bit [1] has no defined meaning and it should be
> ignored by the OSPM". To be consistent with this interpretation,
> acpi_device_is_enabled() should return "(present and enabled) or
> functional".
>
> I'll change it along these lines.
Actually, I don't think that the "functional" bit has any bearing on
this. It only means that the OSPM should continue the enumeration
below the device regardless of the present bit value.
In the acpi_processor_add() case it is clearly irrelevant.
acpi_scan_bus_check() needs to walk the entire subtree below the
target device anyway.
As for acpi_scan_device_check(), IMO it's better to make it walk the
subtree below the device if it is present, but not enabled, for
backwards compatibility.