linux-kernel - Re: [PATCH v2 2/3] drm: Plumb debugfs

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Date:   Wed, 16 Feb 2022 10:35:53 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Doug Anderson <dianders@...omium.org>,
        Andrzej Hajda <andrzej.hajda@...el.com>
Cc:     Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        David Airlie <airlied@...ux.ie>,
        Robert Foss <robert.foss@...aro.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Thomas Zimmermann <tzimmermann@...e.de>, lschyi@...omium.org,
        Sam Ravnborg <sam@...nborg.org>, jjsu@...omium.org
Subject: Re: [PATCH v2 2/3] drm: Plumb debugfs_init through to panels

On 2/16/22 10:25, Jani Nikula wrote:

[snip]

>>
>> I actually wrote said follow-up patches and they were ready to go, but
>> when I was trying to come up with the right "Fixes" tag I found commit
>> b792e64021ec ("drm: no need to check return value of debugfs_create
>> functions"). So what's being requested is nearly the opposite of what
>> Greg did there.
>>
>> I thought about perhaps only checking for directories but even that
>> type of check was removed by Greg's patch. Further checking shows that
>> start_creating() actually has:
>>
>> if (IS_ERR(parent))
>>   return parent;
>>
>> ...so I guess that explains why it's fine to skip the check even for parents?
>>
>> Sure enough I confirmed that if I pass `ERR_PTR(-EINVAL)` as the root
>> for `panel->funcs->debugfs_init()` that nothing bad seems to happen...
> 
> Yeah, the idea is that you don't need to check for debugfs function
> return values and you can safely pass error pointers to debugfs
> functions. The worst that can happen is you don't get the debugfs, but
> hey, it's debugfs so you shouldn't fail anything else because of that
> anyway.
> 

Thanks a lot Doug and Jani for the explanations. That makes sense and it
explains why most code I looked was not checking for the return value.

I guess we should write something about this in the debugfs functions
kernel doc so it's mentioned explicitly and people don't have to guess. 

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat