[PATCH] ARM: pxa: mfp: Force gpio direction for MFP_LPM_KEEP_OUTPUT

[Haojian Zhuang]

>> > But since DRIVE_HIGH or DRIVE_LOW currently don't
>> prevent that,
>>
>> Don't prevent what?
>
> Don't prevent a GPIO input being changed to an output when entering
> suspend().
>
PGSRx register allow for selecting the output state (the driven value)
of each GPIO pin when the processor enters and while it is in sleep
mode.

Each bit of PGSRx register is named as SS (Sleep State of GPIO<n>).

SS '0' -- If GPIO is programmed as an output, the pin is driven and held
low during the transition to and while in sleep mode.

SS '1' -- If GPIO is programmed as an output, the pin is driven and held
high during the transition to and while in sleep mode.

So pin must be configured to GPIO output first before entering sleep mode.

Now the issue comes. If you use the pin as input, why do you need to change
it into output mode in run time?

> By the way, there is still the issue of DRIVE_HIGH and DRIVE_LOW
> forcing the GPIO direction to output in pxa2xx_mfp_suspend()
> *before* the PGSR values take effect. Surely a bug?
>
It's evil to change pin from input mode to output mode if it's in active mode.

Actually we only need to configure control pins to peripherals on pxa27x. It's
a little different from pxa3xx. For pxa3xx, we can even configure input pin
in low power mode. PXA3xx has standby mode that internal device keeps
its state. PXA27x doesn't have this feature. If we want to save power while
PXA27x is suspend, the peripheral should be shutdown too.

Since we need to shutdown peripheral, control pin should always connect to
 reset or chip select signal of peripheral, we can disable the peripheral. So
we needn't to configure low power mode of those input pins.

Thanks
Haojian