i2c child devices: binding to uio device driver

[Thomas De Schampheleire]

Hi,

On Mon, May 23, 2011 at 10:13 AM, Thomas De Schampheleire
<patrickdepinguin+devicetree at gmail.com> wrote:
> Hi Grant,
>
> On Thu, May 19, 2011 at 5:58 PM, Grant Likely <grant.likely at secretlab.ca> wrote:
>> On Thu, May 19, 2011 at 05:32:32PM +0200, Thomas De Schampheleire wrote:
>>> Hi,
>>>
>>> I am using the uio framework (userspace i/o) to handle devices in
>>> userspace. This includes memory access and interrupt handling.
>>> For i2c devices, I'm using the i2c-dev interface, to talk to these
>>> devices from userspace. This already works fine.
>>>
>>> One of my i2c devices also has an interrupt line connected to the
>>> processor. To be able to handle this interrupt with uio, I need to
>>> have a kernel driver matched against the i2c device. However, the i2c
>>> device described in the device tree does not bind with the driver. Is
>>> this behavior supported, and if so, what am I doing wrong? Do I have
>>> to create a dummy bus (non-i2c) and add a shadow node for the
>>> interrupt?
>>
>> You're wandering into new territory here.  It is definitely the right
>> thing to do to describe your i2c device in the device tree with the
>> interrupt fully specified.
>>
>> What you probably need to do is create a uio helper driver for your
>> device that takes care of finding the device node, mapping the
>> interrupt, and proving the interrupt handling UIO hook to userspace.
>> The easiest way to work this out would be to make your uio helper
>> actually bind to the i2c device created by the device node (this is
>> probably the best way too), but you don't have to.  As long as you
>> arrange for you uio helper to get called at some point, you can get it
>> to find the appropriate node in the DT, map the interrupt, and export
>> it to userspace.
>
> Binding the helper driver with the i2c device in the device tree is
> indeed what I am trying to achieve. Unfortunately, the binding does
> not happen, the probe function of my driver never gets called.
>
> For the other UIO devices in this system (non-I2C), the mechanism is as follows:
> 1. the devices are described in the device tree, as children under
> their respective bus node (e.g. localbus)
> 2. the platform code (arch/powerpc/platforms/85xx/corenet_ds.c)
> iterates over all buses it understands, among which 'simple-bus'.
> 3. for each child device on the bus, a bind operation with a driver is
> attempted, based on the device 'compatible' string
>
> This works perfectly.
> However, the i2c bus does not have any of the matched bus types, like
> simple-bus, and as such it is handled differently. The i2c bus driver
> (drivers/i2c/busses/i2c-mpc.c) calls of_register_i2c_devices (in
> drivers/of/of_i2c.c), which handles the i2c-specific part of the
> device, but does not (as far as I understand and observe) treat the
> device as a regular device with a match table.
>
> Maybe I misunderstand something. Is this where I need to play with
> modaliases? How does this work in the context of device trees?
>
>>
>> g.
>>
>>>
>>> My device tree nodes look like this:
>>>
>>>         soc at fe000000 {
>>>                 i2c at 118100 {
>>>                         #address-cells = <1>;
>>>                         #size-cells = <0>;
>>>                         cell-index = <1>;
>>>                         compatible = "fsl-i2c";
>>>                         reg = <0x118100 0x100>;
>>>                         interrupts = <38 2 0 0>;
>>>                         dfsrr;
>>>                         mydevice at 68 {
>>>                                 compatible = "mymanufacturer,mydevice";
>>>                                 reg = <0x68>;
>>>                                 interrupts = <7 1 0 0>; /* External
>>> IRQ7, active-low level */
>>>                         };
>>>                 };
>>
>> This looks to be correct.
>>
>>>
>>> The device driver then has:
>>>
>>> static const struct of_device_id mydevice_ids[] = {
>>>         {
>>>                 .compatible = "mymanufacturer,mydevice"
>>>         },
>>>         {},
>>> };
>>> static struct of_platform_driver mydevice_driver = {
>>
>> You don't want to do this.  This is an i2c device, not a platform
>> device.  If anything, this should be an i2c_driver and you should use
>> the normal creation of i2c devices from the DT.
>
> Ok, so this may be the reason why my driver does not get registered
> against the device...


I have it working now. The bottom line was indeed, as you hinted, to
transform the platform driver into an i2c driver, and make sure that
it gets bound to the device. Here indeed, I need to specify the right
modalias in the compatible string.

For the record, here is what needed to be done:
* In the device tree, add a child node to the i2c controller node. The
model part of the first compatible string, should correspond to the
'type' string you will specify in the driver. In this node, you can
also specify any interrupt assignments.

* The driver should be a regular i2c driver, but the 'type' field in
struct i2c_device_id should match what you specified in the device
tree. For example, if the device tree has 'mymanufacturer,mydevice' as
compatible string, then:

static struct i2c_device_id mydevice_ids[] = {
        { "mydevice", 0 },
        { }
};

static struct i2c_driver mydevice_driver = {
        .driver = {
                .name = "mydevice",
        },

        .id_table       = mydevice_ids,
        .probe          = mydevice_probe,
        .remove         = __devexit_p(mydevice_remove),
};

* In the probe function, to get the interrupt information out of the
device tree, you first have to find the correct node, for example
based on compatible string:

        np = of_find_compatible_node(NULL, NULL, "mymanufacturer,mydevice");
        if (NULL == np) {
                dev_err(&client->dev, "couldn't find corresponding
node in device tree\n");
                goto err_find_node;
        }

        {
                int irq = of_irq_to_resource(np, 0, NULL);
                if (unlikely(irq == NO_IRQ)) {
                        dev_warn(&client->dev, "couldn't obtain
interrupt information from device tree\n");
                } else {
                        ... (handle the UIO part)
                }
        }
        of_node_put(np);


Best regards,
Thomas