Date:	Wed, 21 Jan 2015 15:56:41 +0000
From:	Graeme Gregory <gg@...mlogic.co.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Jon Masters <jcm@...hat.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"hanjun.guo@...aro.org" <hanjun.guo@...aro.org>,
	Mark Rutland <Mark.Rutland@....com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	Will Deacon <Will.Deacon@....com>,
	"wangyijing@...wei.com" <wangyijing@...wei.com>,
	Rob Herring <robh@...nel.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Al Stone <al.stone@...aro.org>,
	Timur Tabi <timur@...eaurora.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@....com>,
	"phoenix.liyi@...wei.com" <phoenix.liyi@...wei.com>,
	Robert Richter <rric@...nel.org>,
	Jason Cooper <jason@...edaemon.net>,
	Arnd Bergmann <arnd@...db.de>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Mark Brown <broonie@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"graeme.gregory@...aro.org" <graeme.gregory@...aro.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Olof Johansson <olof@...om.net>,
	"christoffer.dall@...aro.org" <christoffer.dall@...aro.org>,
	"parth.dixit@...aro.org" <parth.dixit@...aro.org>,
	Leif Lindholm <leif.lindholm@...aro.org>
Subject: Re: [PATCH v7 04/17] ARM64 / ACPI: Introduce early_param for "acpi"
 and pass acpi=force to enable ACPI

On Wed, Jan 21, 2015 at 03:42:43PM +0000, Catalin Marinas wrote:
> On Wed, Jan 21, 2015 at 03:29:52PM +0000, Jon Masters wrote:
> > On 01/21/2015 10:23 AM, Catalin Marinas wrote:
> > > I have some questions for the ACPI and EFI folk:
> > > 
> > > 1. When booting with ACPI, are the EFI run-time services required for
> > >    anything? If yes, Xen may have a bigger problem
> > 
> > Yes. At least for some things. For example, installing an Operating
> > System would require that you make runtime services calls to set the
> > BootOrder/BootNext variables, and so on. Further, we use the GetTime
> > service and EFI based reboot to avoid having special drivers. I had
> > those added to SBBR as requirements for that reason.
> 
> So what would a kexec'ed kernel do here? Or we usually expect it to be
> short lived and doesn't need reboot, nor GetTime.
> 
> Xen is slightly more problematic but I wonder whether it could run a
> (paravirtualised) UEFI.
> 
> > > 2. Could a boot loader (either kernel doing kexec or Xen) emulate the
> > >    EFI system/config tables and still make them useful to the kernel but
> > >    without EFI_BOOT or EFI_RUNTIME_SERVICES?
> > 
> > Yes. But again, without the other required pieces (including the
> > services function pointers in the systab which are required) you'd crash
> > soon after boot trying to make those calls.
> 
> My point was whether you can still pass information like RSDP address
> via EFI tables but explicitly disable runtime services so that the
> kernel won't try to make such calls (and crash).
> 
There is no specific dependency from ACPI->EFI its just the only current
method defind to get the RSDP pointer. It would work just as well
getting the pointer from /chosen/ if we just pick a node and document it
for Xen/kexec/other usage.

We were running ACPI on machine from u-boot doing exactly this for
a long time.

Graeme

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