linux-kernel - Re: [PATCH v2 3/3 UPDATED] i2c / ACPI: add ACPI enumeration support

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Date:	Fri, 16 Nov 2012 23:46:40 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:	Jean Delvare <khali@...ux-fr.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, 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 v2 3/3 UPDATED] i2c / ACPI: add ACPI enumeration support

On Fri, Nov 16, 2012 at 10:28 AM, Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
> ...
> From: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Date: Mon, 10 Sep 2012 12:12:32 +0300
> Subject: [PATCH] i2c / ACPI: add ACPI enumeration support
>
> ACPI 5 introduced I2cSerialBus resource that makes it possible to enumerate
> and configure the I2C slave devices behind the I2C controller. This patch
> adds helper functions to support I2C slave enumeration.
>
> An ACPI enabled I2C controller driver only needs to call acpi_i2c_register_devices()
> in order to get its slave devices enumerated, created and bound to the
> corresponding ACPI handle.

I must admit I don't understand the strategy here.  Likely it's only
because I haven't been paying enough attention, but I'll ask anyway in
case anybody else is similarly confused.

The callchain when we enumerate these slave devices looks like this:

    acpi_i2c_register_devices(struct i2c_adapter *)
      acpi_walk_namespace(adapter->dev.acpi_handle, acpi_i2c_add_device)
        acpi_i2c_add_device
          acpi_bus_get_device
          acpi_bus_get_status
          acpi_dev_get_resources(..., acpi_i2c_add_resource, ...)
          <find IRQ, addr>
          acpi_dev_free_resources
          i2c_new_device
            client = kzalloc
            client->dev = ...
            device_register(&client->dev)

Is the ACPI namespace in question something like the following?

    Device {                    # i2C master, i.e., the i2c_adapter
      _HID PNPmmmm
      Device {                  # I2C slave 1, i.e.,  a client
        _HID PNPsss1
        _CRS
          SerialBus/I2C addr addr1, mode mode1
          IRQ irq1
      }
      Device {                  # I2C slave 2
        _HID PNPsss2
        _CRS
          SerialBus/I2C addr addr2, mode mode2
          IRQ irq2
      }
    }

_CRS is a device configuration method, so I would expect that it
exists within the scope of a Device() object.  The way I'm used to
this working is for a driver to specify "I know about PNPsss1
devices."

But it looks like acpi_i2c_register() walks the namespace below an i2c
master device, registering a new i2c device (a slave) for every ACPI
device node with a _CRS method that contains a SERIAL_BUS/TYPE_I2C
descriptor.  It seems like you're basically claiming those devices
nodes based on the contents of their _CRS, not based on their PNP IDs,
which seems strange to me.

We have to be able to hook device enumeration into udev so we can
autoload drivers.  It's obvious how to do that with _HID and _CID --
we just emit a uevent saying "we found a new device with PNP IDs
x,y,z".  I don't see how to do anything similar based on the _CRS
contents.  Again, probably I'm completely missing the point here, and
I'm sorry to be dense.

I guess this isn't really "enumeration" -- the ACPI core has
previously walked this namespace and built acpi_devices for the
Device() nodes, and I think it emits uevents at that time.  So this is
more of a "claim" than an "enumerate."  But the Device() node for the
I2C slave still exists, and it has _HID/_CID, doesn't it?  Do we use
that _HID anywhere?

In any event, after acpi_i2c_register(), I think we have a set of
i2c_client devices (with the above namespace, I assume we'd have two
of them).  I guess acpi_i2c_find_device() is useful now -- it looks
like it takes a "struct device *" (e.g., &client->dev from a struct
i2c_client), and and gives you back the acpi_handle corresponding to
it?

Here's the callchain of that path:

    acpi_i2c_find_device(struct device *)       # acpi_i2c_bus.find_device
      i2c_verify_client(dev)
      acpi_walk_namespace
        acpi_i2c_find_child
          acpi_bus_get_device
          acpi_bus_get_status
          acpi_dev_get_resources(..., acpi_i2c_find_child_address, ...)
            acpi_i2c_find_child_address
            found if (SERIAL_BUS && SERIAL_TYPE_I2C && slave_address == xx)
          acpi_dev_free_resource_list
      *handle = handle

That seems like an awful lot of work to do just to map a struct device
* back to the acpi_handle.  But I don't have any suggestion; just that
observation.

Bjorn
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