linux-kernel - Re: [RFC PATCH v2 01/12] Add sys_hotplug.h for system device hotplug framework

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Message-ID: <3007489.fG0fDZGHrB@vostro.rjw.lan>
Date:	Mon, 04 Feb 2013 20:45:11 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Toshi Kani <toshi.kani@...com>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"isimatu.yasuaki@...fujitsu.com" <isimatu.yasuaki@...fujitsu.com>,
	"jiang.liu@...wei.com" <jiang.liu@...wei.com>,
	"wency@...fujitsu.com" <wency@...fujitsu.com>,
	"guohanjun@...wei.com" <guohanjun@...wei.com>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	"srivatsa.bhat@...ux.vnet.ibm.com" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v2 01/12] Add sys_hotplug.h for system device hotplug framework

On Monday, February 04, 2013 09:46:18 AM Toshi Kani wrote:
> On Mon, 2013-02-04 at 04:46 -0800, Greg KH wrote:
> > On Sun, Feb 03, 2013 at 05:28:09PM -0700, Toshi Kani wrote:
> > > On Sat, 2013-02-02 at 16:01 +0100, Greg KH wrote:
> > > > On Fri, Feb 01, 2013 at 01:40:10PM -0700, Toshi Kani wrote:
> > > > > On Fri, 2013-02-01 at 07:30 +0000, Greg KH wrote:
> > > > > > On Thu, Jan 31, 2013 at 06:32:18PM -0700, Toshi Kani wrote:
> > > > > >  > This is already done for PCI host bridges and platform devices and I don't
> > > > > > > > see why we can't do that for the other types of devices too.
> > > > > > > > 
> > > > > > > > The only missing piece I see is a way to handle the "eject" problem, i.e.
> > > > > > > > when we try do eject a device at the top of a subtree and need to tear down
> > > > > > > > the entire subtree below it, but if that's going to lead to a system crash,
> > > > > > > > for example, we want to cancel the eject.  It seems to me that we'll need some
> > > > > > > > help from the driver core here.
> > > > > > > 
> > > > > > > There are three different approaches suggested for system device
> > > > > > > hot-plug:
> > > > > > >  A. Proceed within system device bus scan.
> > > > > > >  B. Proceed within ACPI bus scan.
> > > > > > >  C. Proceed with a sequence (as a mini-boot).
> > > > > > > 
> > > > > > > Option A uses system devices as tokens, option B uses acpi devices as
> > > > > > > tokens, and option C uses resource tables as tokens, for their handlers.
> > > > > > > 
> > > > > > > Here is summary of key questions & answers so far.  I hope this
> > > > > > > clarifies why I am suggesting option 3.
> > > > > > > 
> > > > > > > 1. What are the system devices?
> > > > > > > System devices provide system-wide core computing resources, which are
> > > > > > > essential to compose a computer system.  System devices are not
> > > > > > > connected to any particular standard buses.
> > > > > > 
> > > > > > Not a problem, lots of devices are not connected to any "particular
> > > > > > standard busses".  All this means is that system devices are connected
> > > > > > to the "system" bus, nothing more.
> > > > > 
> > > > > Can you give me a few examples of other devices that support hotplug and
> > > > > are not connected to any particular buses?  I will investigate them to
> > > > > see how they are managed to support hotplug.
> > > > 
> > > > Any device that is attached to any bus in the driver model can be
> > > > hotunplugged from userspace by telling it to be "unbound" from the
> > > > driver controlling it.  Try it for any platform device in your system to
> > > > see how it happens.
> > > 
> > > The unbind operation, as I understand from you, is to detach a driver
> > > from a device.  Yes, unbinding can be done for any devices.  It is
> > > however different from hot-plug operation, which unplugs a device.
> > 
> > Physically, yes, but to the driver involved, and the driver core, there
> > is no difference.  That was one of the primary goals of the driver core
> > creation so many years ago.
> > 
> > > Today, the unbind operation to an ACPI cpu/memory devices causes
> > > hot-unplug (offline) operation to them, which is one of the major issues
> > > for us since unbind cannot fail.  This patchset addresses this issue by
> > > making the unbind operation of ACPI cpu/memory devices to do the
> > > unbinding only.  ACPI drivers no longer control cpu and memory as they
> > > are supposed to be controlled by their drivers, cpu and memory modules.
> > 
> > I think that's the problem right there, solve that, please.
> 
> We cannot eliminate the ACPI drivers since we have to scan ACPI.  But we
> can limit the ACPI drivers to do the scanning stuff only.   This is
> precisely the intend of this patchset.  The real stuff, removing actual
> devices, is done by the system device drivers/modules.

In case you haven't realized that yet, the $subject patchset has no future.

Let's just talk about how we can get what we need in more general terms.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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