linux-kernel - Re: [PATCH 2/2] ACPI / platform: Initialize ACPI handles of platform devices in advance

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Message-ID: <4256205.7vQh2av97P@vostro.rjw.lan>
Date:	Mon, 19 Nov 2012 19:25:08 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jean Delvare <khali@...ux-fr.org>, ben-linux@...ff.org,
	w.sang@...gutronix.de, linux-kernel@...r.kernel.org,
	lenb@...nel.org, rafael.j.wysocki@...el.com,
	broonie@...nsource.wolfsonmicro.com, grant.likely@...retlab.ca,
	linus.walleij@...aro.org, mathias.nyman@...ux.intel.com,
	linux-acpi@...r.kernel.org
Subject: Re: [PATCH 2/2] ACPI / platform: Initialize ACPI handles of platform devices in advance

On Monday, November 19, 2012 06:32:06 PM Rafael J. Wysocki wrote:
> On Monday, November 19, 2012 08:23:34 AM Greg Kroah-Hartman wrote:
> > On Sun, Nov 18, 2012 at 10:13:59PM +0100, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > 
> > > The current platform device creation and registration code in
> > > acpi_create_platform_device() is quite convoluted.  This function
> > > takes an ACPI device node as an argument and eventually calls
> > > platform_device_register_resndata() to create and register a
> > > platform device object on the basis of the information contained
> > > in that code.  However, it doesn't associate the new platform
> > > device with the ACPI node directly, but instead it relies on
> > > acpi_platform_notify(), called from within device_add(), to find
> > > that ACPI node again with the help of acpi_platform_find_device()
> > > and acpi_platform_match() and then attach the new platform device
> > > to it.  This causes an additional ACPI namespace walk to happen and
> > > is clearly suboptimal.
> > > 
> > > Use the observation that it is now possible to initialize the ACPI
> > > handle of a device before calling device_add() for it to make this
> > > code more straightforward.  Namely, add a new field to struct
> > > platform_device_info allowing us to pass the ACPI handle of interest
> > > to platform_device_register_full(), which will then use it to
> > > initialize the new device's ACPI handle before registering it.
> > > This will cause acpi_platform_notify() to use the ACPI handle from
> > > the device structure directly instead of using the .find_device()
> > > routine provided by the device's bus type.  In consequence,
> > > acpi_platform_bus, acpi_platform_find_device(), and
> > > acpi_platform_match() are not necessary any more, so remove them.
> > 
> > Why can't you use the platform_data * that is already in struct device
> > for this, instead of adding an acpi-specific field to the
> > platform_device structure?
> 
> Hmm, I kind of don't understand the question. :-)
> 
> Yes, we have acpi_handle in struct device (it actually is being added by a
> patch you've acked) and we use it.  The whole point here is to streamline
> of the initalization of that field.
> 
> > If not that, surely there is another field in struct device that you
> > could use that is free for this type of device?
> 
> Yes, there is one and as I said above. :-)
> 
> I'd be happy to use the struct device's field directly, but
> platform_device_register_full() allocates memory for the struct device in
> question, so that field actually doesn't exist yet when it is called.

So what happens is we want to use platform_device_register_full(), because
it initializes a struct platform_device for use and registers it for us, so we
don't need to worry about all that.  However, platform_device_register_full()
also creates the struct platform_device it registers and we need to let it
know what value to put into the acpi_handle field of that struct platform_device
object's struct device component.  That's what this all is about. :-)

Thanks,
Rafael


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