linux-kernel - Re: [PATCH] device-dax: don't set kobj parent during cdev init

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Message-ID: <e041cf1a-d40b-7138-29fd-442996530504@deltatee.com>
Date:   Mon, 13 Feb 2017 15:38:07 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Sajjan Vikas C <vikas.cha.sajjan@....com>,
        Arnd Bergmann <arnd@...db.de>, Jan Kara <jack@...e.cz>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] device-dax: don't set kobj parent during cdev init

Hey,

I like the interface. It's just Greg that needs to comment on whether
using the kobj.parent for this purpose is actually sane. That was his
argument from the beginning.

Logan

On 13/02/17 01:47 PM, Dan Williams wrote:
> On Sat, Feb 11, 2017 at 9:42 PM, Logan Gunthorpe <logang@...tatee.com> wrote:
>> On 11/02/17 11:58 AM, Dan Williams wrote:
>>> Also when using an embedded cdev how would you recommend avoiding this problem?
>>
>> I don't know. Hopefully, Greg has a good idea. But it sounds like a
>> general problem that a lot of cdev's actually suffer from without
>> realizing. Perhaps we need a more general solution. Some way for the
>> cdev to reference its containing structure in a way that it's designed
>> for such that anyone writing a driver will do the right thing without
>> needing to dive into the kobjects.
>>
> 
> How about something like the below? I.e. hide the details with a new
> helper api so that all driver writers need to worry about is the
> parent device and cdev_del(). This is similar to the device_add_disk()
> and del_gendisk() pairing that we have for block-device drivers.
> 
> diff --git a/fs/char_dev.c b/fs/char_dev.c
> index 3bc97002c86f..4c5d51cdd353 100644
> --- a/fs/char_dev.c
> +++ b/fs/char_dev.c
> @@ -471,6 +471,12 @@ int cdev_add(struct cdev *p, dev_t dev, unsigned count)
>         return 0;
>  }
> 
> +int device_add_cdev(struct device *dev, struct cdev *p)
> +{
> +       p->kobj.parent = &dev->kobj;
> +       return cdev_add(p, dev->devt, 1);
> +}
> +
>  static void cdev_unmap(dev_t dev, unsigned count)
>  {
>         kobj_unmap(cdev_map, dev, count);
> diff --git a/include/linux/cdev.h b/include/linux/cdev.h
> index f8763615a5f2..043168df1d8f 100644
> --- a/include/linux/cdev.h
> +++ b/include/linux/cdev.h
> @@ -25,6 +25,7 @@ struct cdev *cdev_alloc(void);
>  void cdev_put(struct cdev *p);
> 
>  int cdev_add(struct cdev *, dev_t, unsigned);
> +int device_add_cdev(struct device *dev, struct cdev *);
> 
>  void cdev_del(struct cdev *);
>