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

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Message-ID: <rveb47frebdzhpcxdt4jnyasiztrhfnee3pzgl5ndafh2jbrsf@za4o23cw3lch>
Date:   Mon, 28 Aug 2023 09:34:40 +0200
From:   Jacopo Mondi <jacopo.mondi@...asonboard.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        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

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.

>
> 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
>