linux-kernel - Re: [PATCH RFD] alternative kobject release wait mechanism

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Date:	Thu, 19 Apr 2007 09:13:43 -0400
From:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
To:	"Cornelia Huck" <cornelia.huck@...ibm.com>
Cc:	"Tejun Heo" <htejun@...il.com>,
	"Alan Stern" <stern@...land.harvard.edu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Greg K-H" <greg@...ah.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>
Subject: Re: [PATCH RFD] alternative kobject release wait mechanism

On 4/19/07, Cornelia Huck <cornelia.huck@...ibm.com> wrote:
> On Wed, 18 Apr 2007 12:41:36 -0400,
> "Dmitry Torokhov" <dmitry.torokhov@...il.com> wrote:
>
> > I am still do not understand why this is needed. Would it not be
> > simplier just to use a reference to struct device instead of embedding
> > it in a larger structure if their lifetimes are different and one does
> > not have a subsystem that takes care of releasing logic.
>
> Why are their lifetimes different? Usually, if I hold on to the device,
> I also want to be able to use the structure that embeds the device.
>

Because they are managed by 2 different entities. the struct device
objects are managed by device core and driver-specific objects are
managed by their respective driver.

> > Pretty much drivers have 2 options:
> >
> > struct my_device {
> >         void *private_data;
> >         struct device dev;
> > };
> >
> > In this case ->release must live in a subsystem code; individual
> > drivers kfree(my_dev->private) and do any additional cleanup after
> > calling device_unregister(&my_dev->dev);
>
> They must do this in the ->remove callback.

Why? If the driver truly stops hardware then any driver-specific data
is not needed. With sysfs severing access to removed attributes there
is no need to gave "global release", cleanup can be done in stages.

>
> >
> > Second option:
> >
> > struct my_device {
> >         type member1;
> >         type member2;
> >
> >        struct device *dev;
> > };
> >
> > dev is coming from _device_create(). Driver core takes care of
> > releasing dev structure; driver does cleanup of my_device.
>
> device_create() would need to not expect a class then, or it's not
> universally usable. Also, the driver would need a method to get back
> from the device to my_device. We're practically back at the first
> option again, only that now the ->release function is sitting in the
> driver core instead of the subsystem.
>

To a degree.

-- 
Dmitry
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