linux-kernel - Re: [PATCH 7/7] media: usb: uvc: no need to check return value of debugfs

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Date:   Thu, 17 Sep 2020 14:46:00 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     linux-media@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] media: usb: uvc: no need to check return value of
 debugfs_create functions

On Thu, Sep 17, 2020 at 03:37:57PM +0300, Laurent Pinchart wrote:
> Hi Greg,
> 
> On Thu, Sep 17, 2020 at 02:34:26PM +0200, Greg Kroah-Hartman wrote:
> > On Thu, Sep 17, 2020 at 03:25:50PM +0300, Laurent Pinchart wrote:
> > > On Wed, Aug 19, 2020 at 02:47:19AM +0300, Laurent Pinchart wrote:
> > > > On Tue, Aug 18, 2020 at 03:36:08PM +0200, Greg Kroah-Hartman wrote:
> > > > > When calling debugfs functions, there is no need to ever check the
> > > > > return value.  The function can work or not, but the code logic should
> > > > > never do something different based on this.
> > > > 
> > > > Is there no value in warning the user that something went wrong ? Silent
> > > > failures are harder to debug.
> > > 
> > > Could yous share your opinion about this ?
> > 
> > For debugfs, this isn't an issue, what can a user do with something like
> > "debugfs isn't working?  What does that mean???"
> > 
> > And if we _really_ want warnings like this, it should go into the
> > debugfs core, not require this to be done for every debugfs user, right?
> > 
> > debugfs is just there for kernel developers to help debug things, it's
> > not a dependancy on any userspace functionality, so if it works or not
> > should not be an issue for any user.
> > 
> > Unless that user is a kernel developer of course :)
> 
> Exactly my point :-)
> 
> I'm fine moving the error message to the debugfs core itself instead of
> duplicating it in drivers. Maybe it's already there though, I haven't
> checked. Not printing any message isn't a great idea in my opinion, it
> makes debugging more difficult. I can't count the number of times where
> I've had to add printk's and recompile the kernel to debug issues that
> really should have generated at least a dev_dbg().

There are a lot of error messages that debugfs will print out if it can
not create a file:

inode.c 329 pr_err("Unable to pin filesystem for file '%s'\n", name);
inode.c 348 pr_err("Directory '%s' with parent '%s' already present!\n",
inode.c 351 pr_err("File '%s' in directory '%s' already present!\n",
inode.c 402 pr_err("out of free dentries, can not create file '%s'\n",
inode.c 563 pr_err("out of free dentries, can not create directory '%s'\n",
inode.c 610 pr_err("out of free dentries, can not create automount '%s'\n",
inode.c 668 pr_err("out of free dentries, can not create symlink '%s'\n",

So I think you are safe here.  If we are missing any, I'll gladly add
them.

Also given that you've never noticed this being a real error, means it's
probably not an issue :)

thanks,

greg k-h