linux-kernel - Re: [PATCH 4/6] dma: pxa_dma: no need to check return value of debugfs

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Date:   Sun, 11 Aug 2019 09:03:50 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:     dan.j.williams@...el.com, vkoul@...nel.org,
        Daniel Mack <daniel@...que.org>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        linux-arm-kernel@...ts.infradead.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] dma: pxa_dma: no need to check return value of
 debugfs_create functions

On Sat, Aug 10, 2019 at 09:27:26PM +0200, Robert Jarzmik wrote:
> Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
> 
> Hi Greg,
> 
> > 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.
> >
> > Also, because there is no need to save the file dentry, remove the
> > variable that was saving it as it was never even being used once set.
> >
> > Cc: Daniel Mack <daniel@...que.org>
> > Cc: Haojian Zhuang <haojian.zhuang@...il.com>
> > Cc: Robert Jarzmik <robert.jarzmik@...e.fr>
> > Cc: Vinod Koul <vkoul@...nel.org>
> > Cc: Dan Williams <dan.j.williams@...el.com>
> > Cc: linux-arm-kernel@...ts.infradead.org
> > Cc: dmaengine@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > ---
> >  drivers/dma/pxa_dma.c | 56 +++++++++----------------------------------
> >  1 file changed, 11 insertions(+), 45 deletions(-)
> >
> > diff --git a/drivers/dma/pxa_dma.c b/drivers/dma/pxa_dma.c
> > index b429642f3e7a..0f698f49ee26 100644
> > --- a/drivers/dma/pxa_dma.c
> > +++ b/drivers/dma/pxa_dma.c
> > @@ -132,7 +132,6 @@ struct pxad_device {
> >  	spinlock_t			phy_lock;	/* Phy association */
> >  #ifdef CONFIG_DEBUG_FS
> >  	struct dentry			*dbgfs_root;
> > -	struct dentry			*dbgfs_state;
> >  	struct dentry			**dbgfs_chan;
> >  #endif
> >  };
> > @@ -326,31 +325,18 @@ static struct dentry *pxad_dbg_alloc_chan(struct pxad_device *pdev,
> >  					     int ch, struct dentry *chandir)
> >  {
> >  	char chan_name[11];
> > -	struct dentry *chan, *chan_state = NULL, *chan_descr = NULL;
> > -	struct dentry *chan_reqs = NULL;
> > +	struct dentry *chan;
> >  	void *dt;
> >  
> >  	scnprintf(chan_name, sizeof(chan_name), "%d", ch);
> >  	chan = debugfs_create_dir(chan_name, chandir);
> >  	dt = (void *)&pdev->phys[ch];
> >  
> > -	if (chan)
> > -		chan_state = debugfs_create_file("state", 0400, chan, dt,
> > -						 &chan_state_fops);
> > -	if (chan_state)
> > -		chan_descr = debugfs_create_file("descriptors", 0400, chan, dt,
> > -						 &descriptors_fops);
> > -	if (chan_descr)
> > -		chan_reqs = debugfs_create_file("requesters", 0400, chan, dt,
> > -						&requester_chan_fops);
> > -	if (!chan_reqs)
> > -		goto err_state;
> > +	debugfs_create_file("state", 0400, chan, dt, &chan_state_fops);
> > +	debugfs_create_file("descriptors", 0400, chan, dt, &descriptors_fops);
> > +	debugfs_create_file("requesters", 0400, chan, dt, &requester_chan_fops);
> 
> This is not strictly equivalent.
> Imagine that the debugfs_create_dir() fails and returns NULL :

How can that happen?

>  - in the former case, neither "state", "descriptors" nor "requesters" would be
>    created
>  - in the new code, "state", "descriptors" nor "requesters" will be created in
>    the debugfs root directory

I agree, but debugfs_create_dir() does not return a NULL on an error
since many kernel releases.  Neither can debugfs_create_file() so really
this test is not working at all as-is :)

thanks,

greg k-h