linux-kernel - Re: [PATCH] gpio: pcf857x: restore the initial line state of all pcf lines

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Date:	Thu, 18 Dec 2014 11:48:17 +0530
From:	Kishon Vijay Abraham I <kishon@...com>
To:	Nishanth Menon <nm@...com>, <linus.walleij@...aro.org>,
	<gnurou@...il.com>, <linux-gpio@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <nsekhar@...com>
CC:	<linux-omap@...r.kernel.org>, <linux-mmc@...r.kernel.org>,
	Benoit Parrot <bparrot@...com>
Subject: Re: [PATCH] gpio: pcf857x: restore the initial line state of all
 pcf lines



On Tuesday 16 December 2014 02:20 AM, Nishanth Menon wrote:
> On 12/12/2014 02:06 AM, Kishon Vijay Abraham I wrote:
>> The reset values for all the PCF lines are high and hence on shutdown
>> we should drive all the lines high in order to bring it to the reset state.
>>
>> This is actually required since pcf doesn't have a reset line and even after
>> warm reset (by invoking "reboot" in prompt) the pcf lines maintains it's
>> previous programmed state. This becomes a problem if the boards are designed
>> to work with the default initial state.
>>
>> DRA7XX_evm uses PCF8575 and one of the PCF output lines feeds to MMC/SD and
>> this line should be driven high in order for the MMC/SD to be detected.
>> This line is modelled as regulator and the hsmmc driver takes care of enabling
>> and disabling it. In the case of 'reboot', during shutdown path as part of it's
>> cleanup process the hsmmc driver disables this regulator. This makes MMC boot
>> not functional.
>>
>> Fixed it by driving high all the pcf lines.
>>
>> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
>> ---
>>  drivers/gpio/gpio-pcf857x.c |    9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/drivers/gpio/gpio-pcf857x.c b/drivers/gpio/gpio-pcf857x.c
>> index 236708a..00b15b2 100644
>> --- a/drivers/gpio/gpio-pcf857x.c
>> +++ b/drivers/gpio/gpio-pcf857x.c
>> @@ -448,6 +448,14 @@ static int pcf857x_remove(struct i2c_client *client)
>>  	return status;
>>  }
>>  
>> +static void pcf857x_shutdown(struct i2c_client *client)
>> +{
>> +	struct pcf857x *gpio = i2c_get_clientdata(client);
>> +
>> +	/* Drive all the I/O lines high */
>> +	gpio->write(gpio->client, BIT(gpio->chip.ngpio) - 1);
> 
> you might force a contention here - depending on System configuration.
> example:
> +-------+
> |       |
> |  U1   |         +------+      +-----------+
> |       +--------->      |      |           |
> +-------+         |      |      |           |
>                   | Switch<-----+    SoC    |
> +-------+         |      |      |           |
> |       |         |      |      |           |
> | U2    <---------+--^---+      +-----------+
> |       |            |
> |       |            |
> +-------+            |
>                   +--+--+
>                   |     |
>                   | PCF |
>                   |     |
>                   +-----+
> 
> At low, SoC pin is connected to U2 as drive. when reset to high, you
> now have U1 driving to the same pin that SoC has, potentially
> resulting in contention.
> 
> 
> Unfortunately, at this level, you do not know what the state of the
> system is, blindly forcing a pin level will potentially cause
> contention risk depending on pin configuration.

Assume we are doing a reset when the system is powered on, irrespective of the
state of the system, we'll be forcing the pin level to the default state.

The same thing happens here so I don't think this would be a problem.

Thanks
Kishon
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