linux-kernel - Re: [PATCH 3/3] i2c: muxes: i2c-mux-gpmux: Use of_get_i2c_adapter_by

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Message-ID: <4b641f48-7673-9d55-0963-35b52189b6ad@axentia.se>
Date:   Thu, 19 Oct 2023 11:26:41 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Herve Codina <herve.codina@...tlin.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        Allan Nielsen <allan.nielsen@...rochip.com>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Steen Hegelund <steen.hegelund@...rochip.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH 3/3] i2c: muxes: i2c-mux-gpmux: Use
 of_get_i2c_adapter_by_node()

Hi!

2023-10-19 at 09:35, Herve Codina wrote:
> i2c-mux-gpmux uses the pair of_find_i2c_adapter_by_node() /
> i2c_put_adapter(). These pair alone is not correct to properly lock the
> I2C parent adapter.
> 
> Indeed, i2c_put_adapter() decrements the module refcount while
> of_find_i2c_adapter_by_node() does not increment it. This leads to an
> overflow over zero of the parent adapter user counter.

Perhaps "... underflow of the parent module refcount."?

> 
> Use the dedicated function, of_get_i2c_adapter_by_node(), to handle
> correctly the module refcount.
> 
> Signed-off-by: Herve Codina <herve.codina@...tlin.com>

Good find, and sorry about that!

Fixes: ac8498f0ce53 ("i2c: i2c-mux-gpmux: new driver")
Acked-by: Peter Rosin <peda@...ntia.se>

Cheers,
Peter

> ---
>  drivers/i2c/muxes/i2c-mux-gpmux.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/muxes/i2c-mux-gpmux.c b/drivers/i2c/muxes/i2c-mux-gpmux.c
> index baccf4bfaf02..8305661e1253 100644
> --- a/drivers/i2c/muxes/i2c-mux-gpmux.c
> +++ b/drivers/i2c/muxes/i2c-mux-gpmux.c
> @@ -52,7 +52,7 @@ static struct i2c_adapter *mux_parent_adapter(struct device *dev)
>  		dev_err(dev, "Cannot parse i2c-parent\n");
>  		return ERR_PTR(-ENODEV);
>  	}
> -	parent = of_find_i2c_adapter_by_node(parent_np);
> +	parent = of_get_i2c_adapter_by_node(parent_np);
>  	of_node_put(parent_np);
>  	if (!parent)
>  		return ERR_PTR(-EPROBE_DEFER);