linux-kernel - Re: [PATCH v2 01/16] Xen: ACPI: Hide UART used by Xen

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Date:	Mon, 18 Jan 2016 11:38:08 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Mark Rutland <mark.rutland@....com>
CC:	Shannon Zhao <zhaoshenglong@...wei.com>,
	<leif.lindholm@...aro.org>, <linux-arm-kernel@...ts.infradead.org>,
	<ard.biesheuvel@...aro.org>, <stefano.stabellini@...rix.com>,
	<david.vrabel@...rix.com>, <catalin.marinas@....com>,
	<will.deacon@....com>, <julien.grall@...rix.com>,
	<xen-devel@...ts.xen.org>, <devicetree@...r.kernel.org>,
	<linux-efi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<shannon.zhao@...aro.org>, <peter.huangpeng@...wei.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	"open list:ACPI" <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH v2 01/16] Xen: ACPI: Hide UART used by Xen

On Mon, 18 Jan 2016, Mark Rutland wrote:
> On Fri, Jan 15, 2016 at 02:55:14PM +0800, Shannon Zhao 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>
> > ---
> > CC: "Rafael J. Wysocki" <rjw@...ysocki.net> (supporter:ACPI)
> > CC: Len Brown <lenb@...nel.org> (supporter:ACPI)
> > CC: linux-acpi@...r.kernel.org (open list:ACPI)
> > ---
> >  drivers/acpi/bus.c | 30 ++++++++++++++++++++++++++++++
> >  1 file changed, 30 insertions(+)
> > 
> > diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
> > index a212cef..d7a559f 100644
> > --- a/drivers/acpi/bus.c
> > +++ b/drivers/acpi/bus.c
> > @@ -46,6 +46,7 @@ ACPI_MODULE_NAME("bus");
> >  struct acpi_device *acpi_root;
> >  struct proc_dir_entry *acpi_root_dir;
> >  EXPORT_SYMBOL(acpi_root_dir);
> > +static u64 spcr_uart_addr;
> >  
> >  #ifdef CONFIG_X86
> >  #ifdef CONFIG_ACPI_CUSTOM_DSDT
> > @@ -93,6 +94,17 @@ acpi_status acpi_bus_get_status_handle(acpi_handle handle,
> >  {
> >  	acpi_status status;
> >  
> > +	if (spcr_uart_addr != 0xffffffffffffffff) {
> 
> The SPCR spec says that the Base Address fields being zero means that
> console redirection is disabled (though I'm not clear on whether or not
> that requires the whole acpi_generic_address to be zero).
> 
> Can we not use that here?

Shannon changed the code from 0 to 0xffffffffffffffff following my
suggestion to his previous series, but I didn't realize that the SPCR
spec says 0 means no console redirection. In that case using 0 is
better.