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

[Thierry Reding <thierry.reding@avionic-design.de>]

* 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.

Thierry
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