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

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Date:	Thu, 18 Dec 2014 08:11:43 -0600
From:	Nishanth Menon <nm@...com>
To:	Kishon Vijay Abraham I <kishon@...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 12/18/2014 12:18 AM, Kishon Vijay Abraham I wrote:
> 
> 
> 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.

Yes, I dont deny that system will be fine *after* reset sequence is
started or completed. However there is a duration between the pcf
shutdown handler is called and the final reset handler is invoked -
that is the duration when  the contention might cause device behavior.
Essentially ignoring the state various drivers have asked PCF to setup
the pins and doing a hands down configuration may have side effects we
cant properly expect.

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


-- 
Regards,
Nishanth Menon
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