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

From: Lars-Peter Clausen
Date: Tue Oct 16 2012 - 06:57:28 EST

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:
>>> [...]
>>> +static void vprbrd_dev_release(struct device *dev)
>>> +{
>>> + return;
>> A empty release callback is usually a good indicator that something is
>> wrong. The release callback will be called once the last reference to the
>> device has been called, so the memory associated with the device should not
>> be freed before the release callback has been called, otherwise the memory
>> might be accessed after it has been freed...
>>> +}
>>> +
>>> +static void vprbrd_free(struct vprbrd *dev)
>>> +{
>>> + usb_put_dev(dev->usb_dev);
>>> + kfree(dev);
>> ..., so this kfree should be moved from here to the release callback.
> Thank you for catching that one!
>> 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.)
> 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;
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