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

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Message-ID: <CAD=FV=Ut3N9syXbN7i939mNsx3h7-u9cU9j6=XFkz9vrh0Vseg@mail.gmail.com>
Date:   Tue, 15 Feb 2022 15:11:41 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Andrzej Hajda <andrzej.hajda@...el.com>
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jonas Karlman <jonas@...boo.se>,
        David Airlie <airlied@...ux.ie>,
        Neil Armstrong <narmstrong@...libre.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Robert Foss <robert.foss@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>, jjsu@...omium.org,
        lschyi@...omium.org, Sam Ravnborg <sam@...nborg.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v2 2/3] drm: Plumb debugfs_init through to panels

Hi,

On Tue, Feb 15, 2022 at 2:20 PM Andrzej Hajda <andrzej.hajda@...el.com> wrote:
>
> On 15.02.2022 23:09, Javier Martinez Canillas wrote:
> > Hello Doug,
> >
> > On 2/5/22 01:13, Douglas Anderson wrote:
> >
> > [snip]
> >
> >> +static void panel_bridge_debugfs_init(struct drm_bridge *bridge,
> >> +                                  struct dentry *root)
> >> +{
> >> +    struct panel_bridge *panel_bridge = drm_bridge_to_panel_bridge(bridge);
> >> +    struct drm_panel *panel = panel_bridge->panel;
> >> +
> >> +    root = debugfs_create_dir("panel", root);
> > This could return a ERR_PTR(-errno) if the function doesn't succeed.
> >
> > I noticed that most kernel code doesn't check the return value though...
> >
> >> +    if (panel->funcs->debugfs_init)
> > Probably if (!(IS_ERR(root) && panel->funcs->debugfs_init) ?
> >
> >> +            panel->funcs->debugfs_init(panel, root);
> >> +}
> > [snip]
> >
> >> @@ -436,6 +436,9 @@ void drm_debugfs_connector_add(struct drm_connector *connector)
> >>      /* vrr range */
> >>      debugfs_create_file("vrr_range", S_IRUGO, root, connector,
> >>                          &vrr_range_fops);
> > Same here, wonder if the return value should be checked.

My plan (confirmed with Javier over IRC) is to land my patches and we
can address as needed with follow-up patches.

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...

> I've seen sometimes that file/dir was already created with the same
> name, reporting error in such case will be helpful.

It sure looks like start_creating() already handles that type of
reporting... Sure enough, I tried to create the "force" file twice,
adding no error checking myself, and I see:

debugfs: File 'force' in directory 'eDP-1' already present!
debugfs: File 'force' in directory 'DP-1' already present!

So tl;dr is that I'm going to land the patches and now am _not_
planning on doing followup patches. However, if I'm confused about any
of the above then please let me know and I'll dig more / can send
follow-up patches.

-Doug