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

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Date:   Mon, 02 Aug 2021 16:51:11 -0500
From:   Eddie James <eajames@...ux.ibm.com>
To:     Rob Herring <robh@...nel.org>
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 Mon, 2021-08-02 at 14:46 -0600, Rob Herring wrote:
> 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.

Hi, thanks for the reply.

Are you suggesting I use "reset-gpios" and change the driver to ignore
EBUSY? I don't know any other usecase, I just didn't think it would be
acceptable to ignore EBUSY on that, but perhaps it is a better
solution.

Thanks,
Eddie

> 
> > +	} 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
> > 
> >