From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: linux-acpi@vger.kernel.org
Cc: Toshi Kani <toshi.kani@hp.com>, lenb@kernel.org,
        linux-kernel@vger.kernel.org, bhelgaas@google.com,
        isimatu.yasuaki@jp.fujitsu.com, liuj97@gmail.com
Subject: Re: [PATCH v3 1/4] ACPI: Support system notify handler via .sys_notify
Date: Wed, 28 Nov 2012 00:57:49 +0100
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On Monday, November 26, 2012 02:09:54 PM Toshi Kani wrote:
> On Mon, 2012-11-26 at 21:44 +0100, Rafael J. Wysocki wrote:
> > On Monday, November 26, 2012 12:06:39 PM Toshi Kani wrote:
> > > On Sat, 2012-11-24 at 23:37 +0100, Rafael J. Wysocki wrote:
> > > > On Saturday, November 24, 2012 11:01:56 PM Rafael J. Wysocki wrote:
> > > > > On Thursday, November 08, 2012 01:23:44 PM Toshi Kani wrote:
> > > > > > Added a new .sys_notify interface, which allows ACPI drivers to
> > > > > > register their system-level (ex. hotplug) notify handlers through
> > > > > > their acpi_driver table.  This removes redundant ACPI namespace
> > > > > > walks from ACPI drivers for faster booting.
> > > > > > 
> > > > > > The global notify handler acpi_bus_notify() is called for all
> > > > > > system-level ACPI notifications, which then calls an appropriate
> > > > > > driver's handler if any.  ACPI drivers no longer need to register
> > > > > > or unregister driver's handler to each ACPI device object.  It also
> > > > > > supports dynamic ACPI namespace with LoadTable & Unload opcode
> > > > > > without any modification in ACPI drivers.
> > > > > > 
> > > > > > Added a common system notify handler acpi_bus_sys_notify(), which
> > > > > > allows ACPI drivers to set it to .sys_notify when this function is
> > > > > > fully implemented.
> > > > > 
> > > > > I don't really understand this.
> > > > > 
> > > > > > It removes functional conflict between driver's
> > > > > > notify handler and the global notify handler acpi_bus_notify().
> > > > > > 
> > > > > > Note that the changes maintain backward compatibility for ACPI
> > > > > > drivers.  Any drivers registered their hotplug handler through the
> > > > > > existing interfaces, such as acpi_install_notify_handler() and
> > > > > > register_acpi_bus_notifier(), will continue to work as before.
> > > > > 
> > > > > I really wouldn't like to add new callbacks to struct acpi_device_ops, because
> > > > > I'd like that whole thing to go away entirely eventually, along with struct
> > > > > acpi_driver.
> > > > > 
> > > > > Moreover, in this particular case, it really is not useful to have to define
> > > > > a struct acpi_driver so that one can register for receiving system
> > > > > notifications from ACPI.  It would be really nice if non-ACPI drivers, such
> > > > > as PCI or platform, could do that too.
> > > > 
> > > > Which they do by using acpi_install_notify_handler() directly.
> > > 
> > > By using acpi_install_notify_handler(), each driver needs to walk
> > > through the entire ACPI namespace to find its associated ACPI devices
> > > and call it to register one by one.  I think this is more work for
> > > non-ACPI drivers than defining acpi_driver.
> > 
> > I'm not really sure what you mean.  The drivers in question already know
> > what the relevant ACPI device nodes are (because they need them anyway
> > for other purposes), so they don't need to look for them specifically and
> > acpi_install_notify_handler() doesn't do any namespace walking.  So what
> > you said above simply doesn't make sense from this viewpoint.
> 
> Yes, if drivers already know the relevant ACPI devices, then walking the
> ACPI namespace is not necessary.  I was referring the case like
> processor_driver.c, acpi_memhotplug.c, and container.c in my statement. 

OK, I need to have a deeper look at those things and I'm not sure when that
happens (everybody is sending me patches like mad right now).

> > > Furthermore, this approach
> > > will impose some major issues below.  (NOTE: Hot-plug implementation
> > > varies in platforms/virtual machines.  These are examples from our IA64
> > > platforms supported by other OS, but I hope Linux would support similar
> > > capability in future.) 
> > >
> > > a) Node Hot-plug
> > > When a new node is added, the FW may extend the ACPI namespace by
> > > loading SSDT for the new node.  Therefore, if we rely on drivers to call
> > > acpi_install_notify_handler(), we need to make the drivers to walk the
> > > namespace again to call it for new devices.  Similarly, the drivers need
> > > to call acpi_remove_notify_handler() when a node is removed. 
> > 
> > I'm not sure how adding .sys_notify() is going to address this issue.
> > In order to use .sys_notify() your driver has to bind to a struct
> > acpi_device in the first place, so you need to know that object to use it
> > anyway.  This isn't any different from having a struct device whose
> > ACPI_HANDLE() has been populated by the core and using
> > acpi_install_notify_handler() directly on that.
> 
> No, .sys_notify() does not take acpi_device as an argument.

acpi_bus_notify() finds a struct acpi_device for the given handle, however,
and calls .sys_notify() for the driver attached to it.  So a driver has to
attach to a struct device to use it (except in the broken
acpi_lookup_drv_notify() case, but that's really broken, so let's just not
consider it even).

> So, the driver does not have to bind to an acpi_device previously.  The only
> requirement is that the driver needs to call acpi_bus_register_driver()
> previously.
> 
> 
> > > b) Memory hot-plug
> > > The FW may slice up the memory range with multiple memory device objects
> > > so that logical hot-add/removal can be performed in finer granularity
> > > for better resource balancing.  For example, a system with 4TB memory
> > > sliced up with 1GB memory device objects will have (4 * 1024) memory
> > > device objects in ACPI namespace.  If each driver walks ACPI namespace,
> > > it can lead noticeable delay in such environment.  The number of objects
> > > can easily go up when supporting more finer granularity or more amount
> > > of memory.
> > 
> > Again, I don't see why drivers would have to walk the namespace.
> > 
> > It would be great if you could give a specific example of that.
> 
> Again, processor_driver.c, acpi_memhotplug.c, and container.c are
> examples of such case.

OK, I'll have a look.

Thanks,
Rafael


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