linux-kernel - Re: [PATCH v2 2/2] i2c: mux: pca954x: Support multiple devices on a single reset line

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Date:   Mon, 2 Aug 2021 14:46:02 -0600
From:   Rob Herring <robh@...nel.org>
To:     Eddie James <eajames@...ux.ibm.com>
Cc:     linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, peda@...ntia.se
Subject: Re: [PATCH v2 2/2] i2c: mux: pca954x: Support multiple devices on a
 single reset line

On Tue, Jul 27, 2021 at 11:03:15AM -0500, Eddie James wrote:
> Some systems connect several PCA954x devices to a single reset GPIO. For
> these devices to get out of reset and probe successfully, each device must
> defer the probe until the GPIO has been hogged. Accomplish this by
> attempting to grab a new "reset-shared-hogged" devicetree property, but
> expect it to fail with EPROBE_DEFER or EBUSY.
> 
> Signed-off-by: Eddie James <eajames@...ux.ibm.com>
> ---
>  drivers/i2c/muxes/i2c-mux-pca954x.c | 46 +++++++++++++++++++++++------
>  1 file changed, 37 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c b/drivers/i2c/muxes/i2c-mux-pca954x.c
> index 4ad665757dd8..376b54ffb590 100644
> --- a/drivers/i2c/muxes/i2c-mux-pca954x.c
> +++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
> @@ -434,15 +434,43 @@ static int pca954x_probe(struct i2c_client *client,
>  	i2c_set_clientdata(client, muxc);
>  	data->client = client;
>  
> -	/* Reset the mux if a reset GPIO is specified. */
> -	gpio = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_HIGH);
> -	if (IS_ERR(gpio))
> -		return PTR_ERR(gpio);
> -	if (gpio) {
> -		udelay(1);
> -		gpiod_set_value_cansleep(gpio, 0);
> -		/* Give the chip some time to recover. */
> -		udelay(1);
> +	/*
> +	 * Grab the shared, hogged gpio that controls the mux reset. We expect
> +	 * this to fail with either EPROBE_DEFER or EBUSY. The only purpose of
> +	 * trying to get it is to make sure the gpio controller has probed up
> +	 * and hogged the line to take the mux out of reset, meaning that the
> +	 * mux is ready to be probed up. Don't try and set the line any way; in
> +	 * the event we actually successfully get the line (if it wasn't
> +	 * hogged) then we immediately release it, since there is no way to
> +	 * sync up the line between muxes.
> +	 */
> +	gpio = gpiod_get_optional(dev, "reset-shared-hogged", 0);
> +	if (IS_ERR(gpio)) {
> +		ret = PTR_ERR(gpio);
> +		if (ret != -EBUSY)
> +			return ret;

Why can't you just do this with the existing 'reset-gpios' property? 
What's the usecase where you'd want to fail probe because EBUSY other 
than an error in your DT.

> +	} else {
> +		if (gpio) {
> +			/* This is really a problem since now we don't know the
> +			 * state of the gpio. Log a warning and keep trying to
> +			 * probe the mux just in case it works.
> +			 */
> +			dev_warn(dev, "got hogged reset line, expect error\n");
> +			gpiod_put(gpio);
> +		} else {
> +			/* Reset the mux if a reset GPIO is specified. */
> +			gpio = devm_gpiod_get_optional(dev, "reset",
> +						       GPIOD_OUT_HIGH);
> +			if (IS_ERR(gpio))
> +				return PTR_ERR(gpio);
> +
> +			if (gpio) {
> +				udelay(1);
> +				gpiod_set_value_cansleep(gpio, 0);
> +				/* Give the chip some time to recover. */
> +				udelay(1);
> +			}
> +		}
>  	}
>  
>  	data->chip = device_get_match_data(dev);
> -- 
> 2.27.0
> 
>