linux-kernel - Re: [PATCH v2 1/4] mfd: add viperboard driver

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Date:	Thu, 18 Oct 2012 16:13:18 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Lars Poeschel <poeschel@...onage.de>
CC:	Lars Poeschel <larsi@....tu-dresden.de>, sameo@...ux.intel.com,
	linux-kernel@...r.kernel.org, jic23@....ac.uk, khali@...ux-fr.org,
	ben-linux@...ff.org, w.sang@...gutronix.de,
	grant.likely@...retlab.ca, linus.walleij@...aro.org
Subject: Re: [PATCH v2 1/4] mfd: add viperboard driver

On 10/18/2012 09:29 AM, Lars Poeschel wrote:
> On Tuesday 16 October 2012 at 12:58:48, Lars-Peter Clausen wrote:
>> On 10/16/2012 11:43 AM, Lars Poeschel wrote:
>>> On Tuesday 16 October 2012 at 10:40:26, Lars-Peter Clausen wrote:
>>>> On 10/12/2012 04:34 PM, Lars Poeschel wrote:
>>>> Btw. I'm wondering why is the extra platform device required? Can't you
>>>> not just use the usb device as the parent device for the mfd cells?
>>>
>>> This is what I first did, but this does not work. You can read about my
>>> first thoughts why this is not working here: (To sum it up: The device
>>> is housed in an usb_device, not a platform_device and This usb_device
>>> has no mfd_cell member.)
>>>
>>> https://lkml.org/lkml/2012/9/28/327
>>>
>>> As I got a bit more deeper I also noticed, that mfd_add_devices
>>> (obviously) adds the devices "as childs" to the parent device.
>>> mfd_remove_devices then removes ALL "child" devices from the parent, not
>>> only those added by mfd_add_devices before. This does not work in the
>>> case of the usb parent device, because it has other childs that the usb
>>> layer added before (some endpoints and stuff). So I had to construct an
>>> "empty" (in sense of childs) mock platform_device between the usb and
>>> mfd.
>>
>> Ah, ok that makes sense. I was a bit confused, because there are other mfd
>> drivers with for example i2c or spi devices as parents and these work fine,
>> but I guess this is because non of them registers any additional child
>> devices. I guess it makes sense to create a mfd cell device type and assign
>> this type to newly created mfd cells and only unregister a device in
>> mfd_remove_devices if it has the correct device type.
>>
>> E.g. something along the lines of:
>>
>>
>> --- a/drivers/mfd/mfd-core.c
>> +++ b/drivers/mfd/mfd-core.c
>> @@ -21,6 +21,10 @@
>>  #include <linux/irqdomain.h>
>>  #include <linux/of.h>
>>
>> +static struct device_type mfd_device_type = {
>> +	.name = "mfd-cell",
>> +};
>> +
>>  int mfd_cell_enable(struct platform_device *pdev)
>>  {
>>  	const struct mfd_cell *cell = mfd_get_cell(pdev);
>> @@ -91,6 +95,7 @@ static int mfd_add_device(struct device *parent, int id,
>>  		goto fail_device;
>>
>>  	pdev->dev.parent = parent;
>> +	pdev->dev.type = &mfd_device_type;
>>
>>  	if (parent->of_node && cell->of_compatible) {
>>  		for_each_child_of_node(parent->of_node, np) {
>> @@ -204,10 +209,16 @@ EXPORT_SYMBOL(mfd_add_devices);
>>
>>  static int mfd_remove_devices_fn(struct device *dev, void *c)
>>  {
>> -	struct platform_device *pdev = to_platform_device(dev);
>> -	const struct mfd_cell *cell = mfd_get_cell(pdev);
>> +	struct platform_device *pdev;
>> +	const struct mfd_cell *cell;
>>  	atomic_t **usage_count = c;
>>
>> +	if (dev->type != &mfd_device_type)
>> +		return 0;
>> +
>> +	pdev = to_platform_device(dev);
>> +	cell = mfd_get_cell(pdev);
>> +
>>  	/* find the base address of usage_count pointers (for freeing) */
>>  	if (!*usage_count || (cell->usage_count < *usage_count))
>>  		*usage_count = cell->usage_count;
> 
> I thought about this and I am not fully happy with it:
> If we add the mfd devices to the usb_interface parent they are at the same 
> level in the device tree as the usb endpoints and stuff. I would consider this 
> logically wrong.
> Is this something we should take care of ?

I wouldn't worry to much about it. If you use the the container platform
device the container platform device would be at the same level as the usb
endpoints. I did a quick search and it seams that other subsystems also
register the child devices directly on the usb interface device. E.g. the
media subsystem uses this a lot.

- Lars
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