linux-kernel - Re: [PATCH] drivers: mpt3sas: mpt3sas_debugfs: return value check of `mpt3sas_debugfs

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Message-ID: <3c4b372f-db4b-43b4-b5ab-7f4860cf6f20@kili.mountain>
Date:   Tue, 23 May 2023 17:57:03 +0300
From:   Dan Carpenter <dan.carpenter@...aro.org>
To:     Tomas Henzl <thenzl@...hat.com>
Cc:     Dongliang Mu <dzm91@...t.edu.cn>, Jing Xu <U202112064@...t.edu.cn>,
        Sathya Prakash <sathya.prakash@...adcom.com>,
        Sreekanth Reddy <sreekanth.reddy@...adcom.com>,
        Suganath Prabu Subramani 
        <suganath-prabu.subramani@...adcom.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        hust-os-kernel-patches@...glegroups.com,
        MPT-FusionLinux.pdl@...adcom.com, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers: mpt3sas: mpt3sas_debugfs: return value check of
 `mpt3sas_debugfs_root`

On Tue, May 23, 2023 at 04:48:12PM +0200, Tomas Henzl wrote:
> On 5/8/23 16:38, Dan Carpenter wrote:
> > On Mon, May 08, 2023 at 09:40:41PM +0800, Dongliang Mu wrote:
> >>>>> diff --git a/drivers/scsi/mpt3sas/mpt3sas_debugfs.c b/drivers/scsi/mpt3sas/mpt3sas_debugfs.c
> >>>>> index a6ab1db81167..c92e08c130b9 100644
> >>>>> --- a/drivers/scsi/mpt3sas/mpt3sas_debugfs.c
> >>>>> +++ b/drivers/scsi/mpt3sas/mpt3sas_debugfs.c
> >>>>> @@ -99,8 +99,6 @@ static const struct file_operations mpt3sas_debugfs_iocdump_fops = {
> >>>>>   void mpt3sas_init_debugfs(void)
> >>>>>   {
> >>>>>   	mpt3sas_debugfs_root = debugfs_create_dir("mpt3sas", NULL);
> >>>>> -	if (!mpt3sas_debugfs_root)
> >>>>> -		pr_info("mpt3sas: Cannot create debugfs root\n");
> >>>> Hi Jing,
> >>>> most drivers just ignore the return value but here the author wanted to
> >>>> have the information logged.
> >>>> Can you instead of removing the message modify the 'if' condition so it
> >>>> suits the author's intention?
> >>>
> >>> This code was always just wrong.
> >>>
> >>> The history of this is slightly complicated and boring.  These days it's
> >>> harmless dead code so I guess it's less bad than before.
> >>
> >> Hi Dan and Tomas,
> >>
> >> Any conclusion about this patch? The student Jing Xu is not sure about how
> >> to revise this patch.
> > 
> > The correct fix is to delete the code.
> > 
> > Debugfs code has error checking built in and was never supposed to be
> > checked for errors in normal driver code.
> > 
> > Originally, debugfs returned a mix of error pointers and NULL.  In the
> > kernel, when you have a mix of error pointers and NULL, then the NULL
> > means that the feature has been disabled deliberately.  It's not an
> > error, we should not print a message.
> > 
> > So a different, correct-ish way to write write debugfs error handling
> > was to say:
> > 
> > 	mpt3sas_debugfs_root = debugfs_create_dir("mpt3sas", NULL);
> > 	if (IS_ERR(mpt3sas_debugfs_root))
> > 		return PTR_ERR(mpt3sas_debugfs_root);
> I'm fine with this as well, I could wish we get a fix for the exact same
> case of debugfs_create_dir in mpt3sas_setup_debugfs and ideally all the
> debugfs_create* in  mpt3sas_debugfs.c in a single patch. But this patch
> is ok even if that wasn't possible.
> tomash

No, you didn't read until the end.  That will break the driver badly.

This *used* to be a correct-ish way that *used* to work but it was never
the what Greg wanted.  So to discourage people from doing it, Greg made
it *impossible* to check for if debugfs has failed.  Literally, the only
correct thing to do now is to delete the debugfs checking.

regards,
dan carpenter