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

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Date:	Mon, 5 Jan 2015 11:56:57 +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

Hi,

On Thursday 18 December 2014 07:41 PM, Nishanth Menon wrote:
> 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 solution might be to invoke the shutdown handler of the various drivers
using the PCF before the shutdown handler of the PCF driver itself gets
invoked? But I'm not sure how can that be achieved in linux kernel :-(

Thanks
Kishon
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