platform device as parent of a miscdevice

platform device as parent of a miscdevice

[Martin Kaiser]

Hi all,

I have a driver that is bound to a platform device. It creates a
miscdevice to interact with user space. And it has a "global" structure
that stores info which are used throughout the driver. 

This works ok as far as I can see. The only issue is how to make the
"global" struct available to the miscdevice's file operations. Of
course, I can make the struct a global variable.

Recently, I realized that I could make the platform device the parent of
the miscdevice. I came up with something like

static int example_open(struct inode *inode, struct file *file)
{
    struct miscdevice *m = file->private_data;
    struct example_global_info *gbl =  dev_get_drvdata(m->parent);
    ...
}

static struct file_operations example_fops =
{
    .owner   = THIS_MODULE,
    .open    = example_open,
    ...
};

static struct miscdevice example_miscdev = {
    ...
    .fops  = &example_fops,
};

static int example_probe(struct platform_device *pdev)
{
    struct example_global_info *gbl = devm_kzalloc(&pdev->dev, ....);

    platform_set_drvdata(pdev, gbl);

    example_miscdev.parent = &pdev->dev;
    ret = misc_register(&example_miscdev);
    ...
}


Yet again, this seems to work ok. I was surprised to see that there's
very few (if any) mainline drivers that do something similar. Does this
approach make sense? Does it have any implications that I should be
aware of (e.g.  does the link between pdev and miscdevice cause problems
for suspend/resume)?

Thanks,
   Martin

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Re: platform device as parent of a miscdevice

[Greg KH]

On Tue, Mar 30, 2021 at 06:04:09PM +0200, Martin Kaiser wrote:
> Yet again, this seems to work ok. I was surprised to see that there's
> very few (if any) mainline drivers that do something similar. Does this
> approach make sense? Does it have any implications that I should be
> aware of (e.g.  does the link between pdev and miscdevice cause problems
> for suspend/resume)?

Not at all, this is what you should be done, and is what many misc
devices do, that's why the parent pointer is there :)

What in-kernel misc drivers do not do that that you feel should?

thanks,

greg k-h

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Re: platform device as parent of a miscdevice

[Martin Kaiser]

Hi Greg and all,

Thus wrote Greg KH (greg@kroah.com):

> Not at all, this is what you should be done, and is what many misc
> devices do, that's why the parent pointer is there :)

thanks for the confirmation.

> What in-kernel misc drivers do not do that that you feel should?

My misunderstanding was that quite a few drivers set the parent pointer
but hardly anyone uses the parent pointer in the file_operations.

It seems that most drivers have their miscdevice as part of their global
structure and use container_of in the file_operations, e.g. ipmb_read in
drivers/char/ipmi/ipmb_dev_int.c.

Best regards,
Martin

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Re: platform device as parent of a miscdevice

[Greg KH]

On Tue, Mar 30, 2021 at 09:27:17PM +0200, Martin Kaiser wrote:
> > What in-kernel misc drivers do not do that that you feel should?
> 
> My misunderstanding was that quite a few drivers set the parent pointer
> but hardly anyone uses the parent pointer in the file_operations.

What parent pointer in struct file_operations?  I don't see one...

> It seems that most drivers have their miscdevice as part of their global
> structure and use container_of in the file_operations, e.g. ipmb_read in
> drivers/char/ipmi/ipmb_dev_int.c.

That's one way to use it, there are I think at least 3 ways to use it,
maybe more...

Don't confuse the struct device parent pointer with the private pointer
to get back to your local device-specific data.  They are different
things.

thanks,

greg k-h

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