On Fri, Nov 16, 2012 at 11:46:40PM -0700, Bjorn Helgaas wrote:

On Fri, Nov 16, 2012 at 10:28 AM, Mika Westerberg
[off-list ref] wrote:

quoted

...
From: Mika Westerberg <mika.westerberg@linux.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
      }
    }

Yes.

_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."

Yes.

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.

Yes, if we only matched the PNP IDs we would get bunch of PNP devices which
certainly doesn't help us to reuse the existing I2C drivers. So instead of
creating a new glue driver for ACPI or PNP device we added this enumeration
method that then creates the I2C devices, just like DT does.

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.

If you remember the concrete example, I gave you few mails back, it shows
how we are planning to the matching in an existing (or new driver). So we
match with _HID and _CID but that matching is done in the core of each bus
in question (platform, I2C and SPI).

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?

The matching is done using those _HID and _CIDs in the ACPI namespace, we
just now allow any driver to match any device using and not limit it to
ACPI device drivers (this is done with the help of drv->acpi_match_table).

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.

We had similar discussion internally about not using that
drivers/acpi/glue.c but we decided to use it for now, even if it really
backwards and makes things hard (like you observed as well). A much better
way would be just to assign the handle once we make the device but it
seemed not to be that simple after all.