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

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Date:	Thu, 11 Feb 2016 16:04:14 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Shannon Zhao <zhaoshenglong@...wei.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	<ard.biesheuvel@...aro.org>, <stefano.stabellini@...rix.com>,
	<david.vrabel@...rix.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Mark Rutland <mark.rutland@....com>, <julien.grall@...rix.com>,
	<xen-devel@...ts.xen.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	<linux-efi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<ian.campbell@...rix.com>, Shannon Zhao <shannon.zhao@...aro.org>,
	<peter.huangpeng@...wei.com>, Len Brown <lenb@...nel.org>,
	"open list:ACPI" <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH v4 01/17] Xen: ACPI: Hide UART used by Xen

On Wed, 10 Feb 2016, Rafael J. Wysocki wrote:
> On Tuesday, February 09, 2016 11:19:02 AM Stefano Stabellini wrote:
> > 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@...wei.com> wrote:
> > > > > > From: Shannon Zhao <shannon.zhao@...aro.org>
> > > > > >
> > > > > > 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@...aro.org>
> > > > > > Reviewed-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > > > > 
> > > > > 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:
> > 
> 
> I thought about doing that under ACPI_TYPE_DEVICE, because it shouldn't be
> applicable to the other types.  But generally, yes.

I was pondering about it myself. Maybe an ACPI_TYPE_PROCESSOR object
could theoretically be hidden with the STAO? I added the check before
the switch because I thought that there would be no harm in being
caution about it.


> Plus I'd move the table checks to acpi_scan_init(), so the UART address can
> be a static variable in scan.c.
>
> Also maybe rename acpi_check_device_is_ignored() to something like
> acpi_device_should_be_hidden().

Both make sense. Shannon, are you happy to make these changes?