Re: [PATCH v3 1/4] ACPI: Support system notify handler via .sys_notify

Rafael J. Wysocki
2012-11-26T23:36:10Z

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.

> 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.

> 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.

> > > Besides, acpi_os_execute_deferred() is always run on CPU0, because of some
> > > SMI-related peculiarity, which is not very efficient as far as the events
> > > handling is concerned, but we can improve the situation a bit by queing the
> > > execution of the registered handlers in a different workqueue. Maybe it's
> > > worth considering if we're going to change this code anyway?
>
> In my experience, serializing hot-plug operations within an OS instance
> is not typically an issue, and makes it much easier for testing and
> diagnosing an issue.
>
>
> > Well, perhaps we really don't need to change it after all? Maybe we can just
> > switch everyone to using acpi_install_notify_handler() and then we can just
> > drop that code entirely?
>
> I am concerned with the approach of each driver calling
> acpi_install_notify_handler() directly, as described above.

Depending on how it is implemented, it shouldn't be much more computationally
expensive than using .sys_notify() as proposed and the benefit would be
everyone using the same well tested interface that's being used already by
almost everyone anyway.

To make things clear, I'm actually going to drop that whole useless system
notification code from bus.c shortly. We can add something in its place later,
but this one is not worth fixing in my opinion. Let alone extending it.

And as far as acpi_drivers are concerned, please consider them as an obsolete
thing and try to avoid using them and extending their interfaces. If you have
problems with that, please let me know.

Thanks,
Rafael

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