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

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Date:   Sat, 19 May 2018 23:58:14 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Mike Looijmans <mike.looijmans@...ic.nl>, linux-i2c@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i2c-mux-pca954x: Force reset on probe if available

On 2018-05-08 10:06, Mike Looijmans wrote:
> On 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@...ic.nl>
>>> ---
>>>   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.

Agreed, sounds sane.

Applied to i2c-mux/for-next with minor tweaks to the subject and comments.

Cheers,
Peter

>>
>>> +	}
>>>   
>>>   	match = of_match_device(of_match_ptr(pca954x_of_match), &client->dev);
>>>   	if (match)
>>>
>>
> 
> 
> 
> Kind regards,
> 
> Mike Looijmans
> System Expert
> 
> TOPIC Products
> Materiaalweg 4, NL-5681 RJ Best
> Postbus 440, NL-5680 AK Best
> Telefoon: +31 (0) 499 33 69 79
> E-mail: mike.looijmans@...icproducts.com
> Website: www.topicproducts.com
> 
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