linux-kernel - Re: [PATCH] media: i2c: max9286: Fix some redundant of_node

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Message-ID: <169321991310.137962.4278774029844229066@ping.linuxembedded.co.uk>
Date:   Mon, 28 Aug 2023 11:51:53 +0100
From:   Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Jacopo Mondi <jacopo.mondi@...asonboard.com>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
        Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH] media: i2c: max9286: Fix some redundant of_node_put() calls

Quoting Jacopo Mondi (2023-08-28 08:34:40)
> Hi Christophe
> 
> On Sat, Aug 26, 2023 at 12:13:40AM +0200, Christophe JAILLET wrote:
> > This is odd to have a of_node_put() just after a for_each_child_of_node()
> > or a for_each_endpoint_of_node() loop. It should already be called
> > during the last iteration.
> 
> Let's unwrap the calls:
> 
> #define for_each_child_of_node(parent, child) \
>      for (child = of_get_next_child(parent, NULL); child != NULL; \
>           child = of_get_next_child(parent, child))
> 
> static struct device_node *__of_get_next_child(const struct device_node *node,
>                                                 struct device_node *prev)
> {
>         struct device_node *next;
> 
>         if (!node)
>                 return NULL;
> 
>         next = prev ? prev->sibling : node->child;
>         of_node_get(next);
>         of_node_put(prev);
>         return next;
> }
> 
> Let's express the C for loop semantic as a while to help following the
> code:
> 
>         child = of_get_next_child(parent, NULL);
>         while (child != NULL)
>                 child = of_get_next_child(parent, child);
> 
> I concur that the last loop iteration the call to
> __of_get_next_child() will expand to
> 
>         next = NULL;
>         of_node_get(NULL);
>         of_node_put(prev)
> 
> So it seems to me it is not necessary to put the node after
> for_each_child_of_node() ?
> 
> In facts none of the other usages of for_each_child_of_node() in the
> kernel (the ones i checked at least) have a put() after the loop.

I agree. As long as the loops don't use any break statement - there
shouldn't be any _put() after the completion of the loop.

That would make a good cocci script - make sure these iterators do not
use 'break' internally - as that would then conflict!


Reviewed-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>

> 
> >
> > Remove these calls.
> >
> > Fixes: 66d8c9d2422d ("media: i2c: Add MAX9286 driver")
> > Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> 
> Reviewed-by: Jacopo Mondi <jacopo.mondi@...asonboard.com>
> 
> Thanks
>   j
> 
> > ---
> > /!\  This patch is speculative, review with case  /!\
> > ---
> >  drivers/media/i2c/max9286.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/drivers/media/i2c/max9286.c b/drivers/media/i2c/max9286.c
> > index 20e7c7cf5eeb..f27a69b1b727 100644
> > --- a/drivers/media/i2c/max9286.c
> > +++ b/drivers/media/i2c/max9286.c
> > @@ -1450,7 +1450,6 @@ static int max9286_parse_dt(struct max9286_priv *priv)
> >
> >               i2c_mux_mask |= BIT(id);
> >       }
> > -     of_node_put(node);
> >       of_node_put(i2c_mux);
> >
> >       /* Parse the endpoints */
> > @@ -1514,7 +1513,6 @@ static int max9286_parse_dt(struct max9286_priv *priv)
> >               priv->source_mask |= BIT(ep.port);
> >               priv->nsources++;
> >       }
> > -     of_node_put(node);
> >
> >       of_property_read_u32(dev->of_node, "maxim,bus-width", &priv->bus_width);
> >       switch (priv->bus_width) {
> > --
> > 2.34.1
> >