linux-kernel - Re: [PATCH] i2c: pca954x: Add property to skip disabling PCA954x MUX device

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Date:   Sun, 29 Sep 2019 21:21:40 +0000
From:   Peter Rosin <peda@...ntia.se>
To:     Biwen Li <biwen.li@....com>,
        "leoyang.li@....com" <leoyang.li@....com>
CC:     "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i2c: pca954x: Add property to skip disabling PCA954x MUX
 device

On 2019-09-29 12:36, Biwen Li wrote:
> On some Layerscape boards like LS2085ARDB and LS2088ARDB,
> input pull-up resistors on PCA954x MUX device are missing on board,
> So, if MUX are disabled after powered-on, input lines will float
> leading to incorrect functionality.

Hi!

Are you saying that the parent bus of the mux is relying on some
pull-ups inside the mux?

> Hence, PCA954x MUX device should never be turned-off after
> power-on.
> 
> Add property to skip disabling PCA954x MUX device
> if device tree contains "i2c-mux-never-disable"
> for PCA954x device node.
> 
> Errata ID: E-00013 on board LS2085ARDB and LS2088ARDB
> (Board revision found on Rev.B, Rev.C and Rev.D)

I think you should follow the example of the i2c-mux-gpio driver
and implement the idle-state property instead.

That is a lot more consistent, assuming it solves the problem
at hand?

> 
> Signed-off-by: Biwen Li <biwen.li@....com>
> ---
>  drivers/i2c/muxes/i2c-mux-pca954x.c | 33 +++++++++++++++++++++++++----
>  1 file changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c b/drivers/i2c/muxes/i2c-mux-pca954x.c
> index 923aa3a5a3dc..ea8aca54d572 100644
> --- a/drivers/i2c/muxes/i2c-mux-pca954x.c
> +++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
> @@ -93,6 +93,7 @@ struct pca954x {
>  	struct irq_domain *irq;
>  	unsigned int irq_mask;
>  	raw_spinlock_t lock;
> +	u8 disable_mux;		/* do not disable mux if val not 0 */

Awful number of negations there. The name is also backwards given that
a non-zero value means that the mux should *not* be disabled. I would
have reused the name from the binding.

	bool never_disable;

A bit less confusing...

>  };
>  
>  /* Provide specs for the PCA954x types we know about */
> @@ -258,6 +259,11 @@ static int pca954x_deselect_mux(struct i2c_mux_core *muxc, u32 chan)
>  	struct i2c_client *client = data->client;
>  	s8 idle_state;
>  
> +	if (data->disable_mux != 0) {

Please drop " != 0" and use the variable as a truth value. More
instances below...

> +		data->last_chan = data->chip->nchans;
> +		return pca954x_reg_write(muxc->parent, client, data->disable_mux);
> +	}
> +
>  	idle_state = READ_ONCE(data->idle_state);
>  	if (idle_state >= 0)
>  		/* Set the mux back to a predetermined channel */
> @@ -462,16 +468,32 @@ static int pca954x_probe(struct i2c_client *client,
>  		}
>  	}
>  
> +	/* Errata ID E-00013 on board LS2088ARDB and LS2088ARDB:
> +	 * The point here is that you must not disable a mux if there
> +	 * are no pullups on the input or you mess up the I2C. This
> +	 * needs to be put into the DTS really as the kernel cannot
> +	 * know this otherwise.
> +	 */
> +
> +	data->disable_mux = np &&
> +		of_property_read_bool(np, "i2c-mux-never-disable") &&

i2c-mux-never-disable needs to be documented. However see above for my
point that you should do it like the i2c-mux-gpio driver. Any usage
of idle-state still needs to be documented for pca954x binding.

> +		data->chip->muxtype == pca954x_ismux ?
> +		data->chip->enable : 0;

Why do you not allow never-disable for switches?

Cheers,
Peter

> +
>  	/* Write the mux register at addr to verify
>  	 * that the mux is in fact present. This also
>  	 * initializes the mux to disconnected state.
>  	 */
> -	if (i2c_smbus_write_byte(client, 0) < 0) {
> +	if (i2c_smbus_write_byte(client, data->disable_mux) < 0) {
>  		dev_warn(dev, "probe failed\n");
>  		return -ENODEV;
>  	}
>  
> -	data->last_chan = 0;		   /* force the first selection */
> +	if (data->disable_mux != 0)
> +		data->last_chan = data->chip->nchans;
> +	else
> +		data->last_chan = 0;               /* force the first selection */
> +
>  	data->idle_state = MUX_IDLE_AS_IS;
>  
>  	idle_disconnect_dt = np &&
> @@ -531,8 +553,11 @@ static int pca954x_resume(struct device *dev)
>  	struct i2c_mux_core *muxc = i2c_get_clientdata(client);
>  	struct pca954x *data = i2c_mux_priv(muxc);
>  
> -	data->last_chan = 0;
> -	return i2c_smbus_write_byte(client, 0);
> +	if (data->disable_mux != 0)
> +		data->last_chan = data->chip->nchans;
> +	else
> +		data->last_chan = 0;
> +	return i2c_smbus_write_byte(client, data->disable_mux);
>  }
>  #endif
>  
>