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

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Message-ID: <CAPcyv4h-zw5iGQHErbU9py1LLdPhXbxE3H5i7-KxwEqMNTXGUg@mail.gmail.com>
Date:   Mon, 13 Feb 2017 12:47:30 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Logan Gunthorpe <logang@...tatee.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

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 *);