linux-kernel - Re: [PATCH 6/9] debugfs: Implement debugfs_create

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Date:   Fri, 26 Mar 2021 12:30:24 +0100
From:   Greg KH <greg@...ah.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, mgorman@...e.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, bristot@...hat.com,
        joshdon@...gle.com, valentin.schneider@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/9] debugfs: Implement debugfs_create_str()

On Fri, Mar 26, 2021 at 12:18:47PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 26, 2021 at 12:05:07PM +0100, Greg KH wrote:
> > On Fri, Mar 26, 2021 at 11:33:58AM +0100, Peter Zijlstra wrote:
> > > 
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > 
> > No changelog text?  :(
> 
> Yeah, didn't really know what to say that the Subject didn't already do.
> 
> > > +/**
> > > + * debugfs_create_str - create a debugfs file that is used to read and write a string value
> > > + * @name: a pointer to a string containing the name of the file to create.
> > > + * @mode: the permission that the file should have
> > > + * @parent: a pointer to the parent dentry for this file.  This should be a
> > > + *          directory dentry if set.  If this parameter is %NULL, then the
> > > + *          file will be created in the root of the debugfs filesystem.
> > > + * @value: a pointer to the variable that the file should read to and write
> > > + *         from.
> > > + *
> > > + * This function creates a file in debugfs with the given name that
> > > + * contains the value of the variable @value.  If the @mode variable is so
> > > + * set, it can be read from, and written to.
> > > + *
> > > + * This function will return a pointer to a dentry if it succeeds.  This
> > > + * pointer must be passed to the debugfs_remove() function when the file is
> > > + * to be removed (no automatic cleanup happens if your module is unloaded,
> > > + * you are responsible here.)  If an error occurs, ERR_PTR(-ERROR) will be
> > > + * returned.
> > > + *
> > > + * NOTE: when writing is enabled it will replace the string, string lifetime is
> > > + * assumed to be RCU managed.
> > > + *
> > > + * If debugfs is not enabled in the kernel, the value ERR_PTR(-ENODEV) will
> > > + * be returned.
> > > + */
> > > +struct dentry *debugfs_create_str(const char *name, umode_t mode,
> > > +				   struct dentry *parent, char **value)
> > 
> > Please have this return void, no need for me to have to clean up
> > afterward later on :)
> 
> That would make it inconsistent with debugfs_create_{bool,blob}() from
> which I copiously 'borrowed'.

As mentioned on IRC, I am trying to get rid of the return value, those
are the only 2 holdouts given their current use in some of the cruftier
areas of the kernel at the moment :(

> Also, the return dentry might be useful with debugfs_create_symlink(),
> but you're right in that most other create_file wrappers return void.

Great, change that and limit the size of the string that can be written
and it looks good to me, thanks for adding this.

greg k-h