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

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Date:   Wed, 16 Feb 2022 12:42:50 +0100
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Doug Anderson <dianders@...omium.org>
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 16.02.2022 10:35, Javier Martinez Canillas wrote:
> 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.

Nice, didn't know debugfs started using this pattern. I hope the pattern 
will be used broader, as it allows to save lot of redundant checks.

Regards
Andrzej


>
> Best regards,