[U-Boot] [PATCH v2 5/7] tegra: Implement gpio_early_init() on Tamonten

[Stephen Warren <swarren@wwwdotorg.org>]

On 05/29/2012 08:57 AM, Thierry Reding wrote:
> * Kai Poggensee wrote:
>> 
>> Hi Thierry,
>> 
>> On 25.05.2012 19:40, Thierry Reding wrote:
>>> * Stephen Warren wrote:
>>>> On 05/25/2012 07:46 AM, Thierry Reding wrote:
>>>>> The PI4 GPIO is used on Tamonten to reset carrier board
>>>>> peripherals. Power sequencing hardware on the carrier pulls
>>>>> the reset low before powering up the Tegra, and the CPU is
>>>>> supposed to signal readiness, and therefore bring
>>>>> peripherals out of reset by pulling PI4 high.
>>>> 
>>>>> +void gpio_early_init(void) +{ +	gpio_request(GPIO_PI4,
>>>>> NULL); +	gpio_direction_output(GPIO_PI4, 1); +
>>>>> gpio_free(GPIO_PI4); +}
>>>> 
>>>> Do you really mean to free the GPIO here?
>>>> 
>>>> While gpio_free() does not do this at present, it seems
>>>> perfectly reasonable for someone to modify gpio_free() so
>>>> that instead of leaving the HW in some random state when
>>>> free, it actively reprograms the pin to be an input instead,
>>>> since that's the most conflict-free setting when the pin is
>>>> actively unused.
>>> 
>>> I believe that even a pulse would be enough for the undelying
>>> mechanisms to take effect, but I'll have to check with our
>>> hardware engineers to be sure.
>> 
>> As an aside: A pulse wont be enough in this specific case as a
>> pull-down on the carrier would lead to the nRST going low thus
>> resetting the baseboard peripherals.
>> 
>> Plus there is a status LED on the Tamonten Evaluation carrier 
>> that would then indicate the red "system not up".
> 
> So this means we'll have to hand this over to the kernel somehow.
> Stephen, I'm not aware of any mechanism to pass static GPIO
> configuration via the FDT. Are there any plans to add something
> like it? The problem I see is that the pinmux binding might
> reconfigure the corresponding pin and therefore reset the
> peripherals.

The pinctrl and GPIO drivers will only reconfigure groups/pins they're
asked to, so I don't believe there will be any problem here, assuming
the DT contains tables that are correct.