linux-kernel - Re: [PATCH V2] Change ACPI IPMI support to "default y"

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Date:	Fri, 21 Feb 2014 10:53:59 -0600
From:	Russ Anderson <rja@....com>
To:	Matthew Garrett <matthew.garrett@...ula.com>
Cc:	"lenb@...nel.org" <lenb@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"minyard@....org" <minyard@....org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	rja@....com
Subject: Re: [PATCH V2] Change ACPI IPMI support to "default y"

On Fri, Feb 21, 2014 at 12:13:13AM +0000, Matthew Garrett wrote:
> On Thu, 2014-02-20 at 17:59 -0600, Russ Anderson wrote:
> > On Thu, Feb 20, 2014 at 11:09:42PM +0000, Matthew Garrett wrote:
> > > On Thu, 2014-02-20 at 16:45 -0600, Russ Anderson wrote:
> > > > 
> > > > The ACPI spec requires IPMI functionality before a module loads at
> > > > boot time?  And the kernel is *broken* if it does not support ACIP IPMI
> > > > functionality before module load time?  Really?
> > > 
> > > There's no mechanism to ensure that IPMI support will be loaded before
> > > ACPI calls attempt to access IPMI operation regions. Really.
> > 
> > And no mechanism can be added to ensure that ACPI call are
> > not attempted before IPMI is initialized?  A flag or lock
> > or exported symbol indicating IPMI support is ready.
> 
> ACPI functions are a black box to drivers. You make an ACPI call, the
> AML code does something. We could block there, but what's the driver
> supposed to do at that point? The core could call out to a module
> loader, but if the driver is built in and IPMI isn't then you'll end up
> with a 60 second pause in boot and a driver that doesn't work. 

When you say "if the driver is built in" which driver are you
talking about?  I thought the issue was making sure ipmi_si 
driver was loaded before power meter driver.



> > > > > ACPI 4.0 includes support for IPMI operation regions. Modular IPMI means
> > > > > that the kernel will spend a significant amount of time (potentially
> > > > > until a user manually loads a driver) failing to implement part of the
> > > > > IPMI specification. That's a problem, and the correct fix is to ensure
> > > > > that the kernel always implements IPMI support.
> > > > 
> > > > The ACPI spec says ipmi_si cannot be a driver?  Really?
> > > > What is the real problem you are trying to solve?
> > > 
> > > The most straightforward case is that of an ACPI power meter.
> > 
> > So it is just a matter of making sure ipmi_si modules loads before
> > the ACPI power meter module loads, right?  module dependency issue.
> 
> No, because the power meter driver has no way of knowing that a vendor
> has implemented this interface via IPMI. *Any* ACPI entry point could
> theoretically reference IPMI code, even the _INI method that's called
> during ACPI core init. If it does, and if you don't have built-in ACPI
> support, you'd fail ACPI initialisation and things would go downhill
> from there.

Again, it sounds like as long as ipmi_si driver is loaded before
power meter driver, power meter driver is fine.



-- 
Russ Anderson,  Kernel and Performance Software Team Manager
SGI - Silicon Graphics Inc          rja@....com
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