Re: [Xen-devel] [PATCH v4 01/17] Xen: ACPI: Hide UART used by Xen

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 8 Feb 2016, Rafael J. Wysocki wrote:
> On Monday, February 08, 2016 10:57:01 AM Stefano Stabellini wrote:
> > On Sat, 6 Feb 2016, Rafael J. Wysocki wrote:
> > > On Fri, Feb 5, 2016 at 4:05 AM, Shannon Zhao <zhaoshenglong@xxxxxxxxxx> 
> > > wrote:
> > > > From: Shannon Zhao <shannon.zhao@xxxxxxxxxx>
> > > >
> > > > ACPI 6.0 introduces a new table STAO to list the devices which are used
> > > > by Xen and can't be used by Dom0. On Xen virtual platforms, the physical
> > > > UART is used by Xen. So here it hides UART from Dom0.
> > > >
> > > > Signed-off-by: Shannon Zhao <shannon.zhao@xxxxxxxxxx>
> > > > Reviewed-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > > 
> > > Well, this doesn't look right to me.
> > > 
> > > We need to find a nicer way to achieve what you want.
> > 
> > I take that you are talking about how to honor the STAO table in Linux.
> > Do you have any concrete suggestions?
> 
> I do.
> 
> The last hunk of the patch is likely what it needs to be, although I'm
> not sure if the place it is added to is the right one.  That's a minor thing,
> though.
> 
> The other part is problematic.  Not that as it doesn't work, but because of
> how it works.  With these changes the device will be visible to the OS (in
> fact to user space even), but will never be "present".  I'm not sure if
> that's what you want?
> 
> It might be better to add a check to acpi_bus_type_and_status() that will
> evaluate the "should ignore?" thing and return -ENODEV if this is true.  This
> way the device won't be visible at all.

Something like below?  Actually your suggestion is better, thank you!

diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
index 78d5f02..4778c51 100644
--- a/drivers/acpi/scan.c
+++ b/drivers/acpi/scan.c
@@ -1455,6 +1455,9 @@ static int acpi_bus_type_and_status(acpi_handle handle, 
int *type,
        if (ACPI_FAILURE(status))
                return -ENODEV;
 
+       if (acpi_check_device_is_ignored(handle))
+               return -ENODEV;
+
        switch (acpi_type) {
        case ACPI_TYPE_ANY:             /* for ACPI_ROOT_OBJECT */
        case ACPI_TYPE_DEVICE:


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