linux-kernel - Re: [PATCH 3/8] platform: x86: dell-laptop: no need to check return value of debugfs

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Date:   Wed, 12 Jun 2019 14:36:04 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Pali Rohár <pali.rohar@...il.com>
Cc:     dvhart@...radead.org, andy@...radead.org,
        Matthew Garrett <mjg59@...f.ucam.org>,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] platform: x86: dell-laptop: no need to check return
 value of debugfs_create functions

On Wed, Jun 12, 2019 at 02:21:05PM +0200, Pali Rohár wrote:
> On Wednesday 12 June 2019 14:12:53 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.
> > 
> > Cc: Matthew Garrett <mjg59@...f.ucam.org>
> > Cc: "Pali Rohár" <pali.rohar@...il.com>
> > Cc: Darren Hart <dvhart@...radead.org>
> > Cc: Andy Shevchenko <andy@...radead.org>
> > Cc: platform-driver-x86@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > ---
> >  drivers/platform/x86/dell-laptop.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/platform/x86/dell-laptop.c b/drivers/platform/x86/dell-laptop.c
> > index a561f653cf13..94a2f259031c 100644
> > --- a/drivers/platform/x86/dell-laptop.c
> > +++ b/drivers/platform/x86/dell-laptop.c
> > @@ -2176,9 +2176,8 @@ static int __init dell_init(void)
> >  	kbd_led_init(&platform_device->dev);
> >  
> >  	dell_laptop_dir = debugfs_create_dir("dell_laptop", NULL);
> > -	if (dell_laptop_dir != NULL)
> > -		debugfs_create_file("rfkill", 0444, dell_laptop_dir, NULL,
> > -				    &dell_debugfs_fops);
> > +	debugfs_create_file("rfkill", 0444, dell_laptop_dir, NULL,
> > +			    &dell_debugfs_fops);
> 
> Hi!
> 
> So... debugfs_create_dir() can return NULL, right?

Nope.

> And it is then OK to call
> debugfs_create_file("rfkill", 0444, dell_laptop_dir, ...) with
> dell_laptop_dir = NULL?

Yes.

> Where would be that "rfkill" file created?

The root of debugfs.

But, if debugfs_create_dir() return an error, and you pass that value
into debugfs_create_file() it will happily just return an error back
again, and move on.

So it is always safe to pass the return value of one debugfs call into
another, no need to check anything.  If the system is so messed up that
debugfs_create_dir() fails (i.e. you are out of memory), failing to
create a debugfs file is the least of your worries :)

And even then, no need to change your code logic, the functionality of
your code should never depend on if debugfs is working properly at the
moment or not.

thanks,

greg k-h