linux-kernel - Re: [PATCH 1/2] mux: add mux_chip

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Message-ID: <94069d56-0981-2d69-65c2-901a05758806@axentia.se>
Date: Tue, 3 Sep 2024 15:22:52 +0200
From: Peter Rosin <peda@...ntia.se>
To: Thomas Richard <thomas.richard@...tlin.com>
Cc: linux-kernel@...r.kernel.org, gregory.clement@...tlin.com,
 theo.lebrun@...tlin.com, thomas.petazzoni@...tlin.com, u-kumar1@...com,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 1/2] mux: add mux_chip_resume() function

Hi!

Sorry for being unresponsive. And for first writing this in the older v4
thread instead of here.

2024-06-13 at 15:07, Thomas Richard wrote:
> The mux_chip_resume() function restores a mux_chip using the cached state
> of each mux.
> 
> Signed-off-by: Thomas Richard <thomas.richard@...tlin.com>
> ---
>  drivers/mux/core.c         | 29 +++++++++++++++++++++++++++++
>  include/linux/mux/driver.h |  1 +
>  2 files changed, 30 insertions(+)
> 
> diff --git a/drivers/mux/core.c b/drivers/mux/core.c
> index 78c0022697ec..0858cacae845 100644
> --- a/drivers/mux/core.c
> +++ b/drivers/mux/core.c
> @@ -215,6 +215,35 @@ void mux_chip_free(struct mux_chip *mux_chip)
>  }
>  EXPORT_SYMBOL_GPL(mux_chip_free);
>  
> +/**
> + * mux_chip_resume() - restores the mux-chip state
> + * @mux_chip: The mux-chip to resume.
> + *
> + * Restores the mux-chip state.
> + *
> + * Return: Zero on success or a negative errno on error.
> + */
> +int mux_chip_resume(struct mux_chip *mux_chip)
> +{
> +	int ret, i;
> +
> +	for (i = 0; i < mux_chip->controllers; ++i) {
> +		struct mux_control *mux = &mux_chip->mux[i];
> +
> +		if (mux->cached_state == MUX_CACHE_UNKNOWN)
> +			continue;
> +
> +		ret = mux_control_set(mux, mux->cached_state);

mux_control_set() is an internal helper. It is called from
__mux_control_select() and mux_control_deselect() (and on init...)

In all those cases, there is no race to reach the mux_control_set()
function, by means of the mux->lock semaphore (or the mux not being
"published" yet).

I fail to see how resume is safe when mux->lock is ignored?

Cheers,
Peter

> +		if (ret < 0) {
> +			dev_err(&mux_chip->dev, "unable to restore state\n");
> +			return ret;
> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(mux_chip_resume);
> +
>  static void devm_mux_chip_release(struct device *dev, void *res)
>  {
>  	struct mux_chip *mux_chip = *(struct mux_chip **)res;
> diff --git a/include/linux/mux/driver.h b/include/linux/mux/driver.h
> index 18824064f8c0..2a7e5ec5d540 100644
> --- a/include/linux/mux/driver.h
> +++ b/include/linux/mux/driver.h
> @@ -88,6 +88,7 @@ struct mux_chip *mux_chip_alloc(struct device *dev,
>  int mux_chip_register(struct mux_chip *mux_chip);
>  void mux_chip_unregister(struct mux_chip *mux_chip);
>  void mux_chip_free(struct mux_chip *mux_chip);
> +int mux_chip_resume(struct mux_chip *mux_chip);
>  
>  struct mux_chip *devm_mux_chip_alloc(struct device *dev,
>  				     unsigned int controllers,
>