Re: [PATCH] i2c-mux-pca954x: Force reset on probe if available

[Mike Looijmans]

ïOn 08-05-18 08:36, Peter Rosin wrote:
> On 2018-05-01 13:42, Mike Looijmans wrote:
> > Instead of just hogging the reset GPIO into deactivated state, activate and
> > then de-activate the reset. This allows for better recovery if the CPU was
> > reset halfway through an I2C transaction for example.
>
> I can't see any problems with this, and a reset at load time can certainly
> be a benefit. Some questions below though...
>
> Cheers,
> Peter
>
> > Signed-off-by: Mike Looijmans <mike.looijmans@xxxxxxxx>
> > ---
> >   drivers/i2c/muxes/i2c-mux-pca954x.c | 12 ++++++++++--
> >   1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c b/drivers/i2c/muxes/i2c-mux-pca954x.c
> > index 09bafd3..13e10d0 100644
> > --- a/drivers/i2c/muxes/i2c-mux-pca954x.c
> > +++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
> > @@ -36,6 +36,7 @@
> >    */
> >   
> >   #include <linux/device.h>
> > +#include <linux/delay.h>
> >   #include <linux/gpio/consumer.h>
> >   #include <linux/i2c.h>
> >   #include <linux/i2c-mux.h>
> > @@ -389,10 +390,17 @@ static int pca954x_probe(struct i2c_client *client,
> >   	i2c_set_clientdata(client, muxc);
> >   	data->client = client;
> >   
> > -	/* Get the mux out of reset if a reset GPIO is specified. */
> > -	gpio = devm_gpiod_get_optional(&client->dev, "reset", GPIOD_OUT_LOW);
> > +	/* Reset the mux if a reset GPIO is specified. */
> > +	gpio = devm_gpiod_get_optional(&client->dev, "reset", GPIOD_OUT_HIGH);
> >   	if (IS_ERR(gpio))
> >   		return PTR_ERR(gpio);
> > +	if (gpio) {
> > +		/* Datasheet specifies a 4 ns reset-low time */
> > +		udelay(1);
>
> ndelay(4) ?

I have several reasons for making it much much longer:
- A true 4ns pulse might not make it through the GPIO controller (on some 
chips, the GPIO controller runs at 1/10th of the CPU speed or even less)
- Board designers tend to treat RESET signals as "DC", so a 250MHz pulse might 
not make it through. There may be level converters and open-drain drives in 
the signal path as well.
- I've only looked it op on a few chips, others may have different values

I think best is to just remove the comment line, it doesn't really help.

> > +		gpiod_set_value_cansleep(gpio, 0);
> > +		/* Datasheet specifies a 500 ns reset recovery time */
> > +		udelay(1);
>
> ndelay(500) ?

Similar reasons as above.

> > +	}
> >   
> >   	match = of_match_device(of_match_ptr(pca954x_of_match), &client->dev);
> >   	if (match)



Kind regards,

Mike Looijmans
System Expert

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