[U-Boot] Early I2C bus availability

[Joakim Tjernlund]

>
> Hi,
>
> We're contemplating a new embedded design based on the MPC8641. This design is
> going to have the processor potentially running at two different clock
> frequencies and therefore (or so we're told by Freescale) the memory will need
> to run at different clock rates (same physical memory, but configured to run
> at different clock speed by uboot). All of this needs to be determined by
> jumpers (GPIO?) and then uboot needs to bring-up the memory using this
> different clock rate. All of this needs to happen before the memory is 'up',
> so early in the uboot init process.
>
> Presuming all of that is possible, the problem becomes how to read the jumpers
> (GPIO) to get the configuration information to tell uboot what speed this
> memory should be run at. We are not going to have access to any GPIO directly
> located on the MPC8641. We are considering whether we can put a GPIO device on
> the I2C bus and have it read by the processor early in the uboot init process
> to get the information we need. My question is whether uboot will have enough
> resources early in the init process to do this?

Sure, just think about how DRAM is setup in many cases. First SPD data
is read from an I2C EEPROM, then the DRAM controller is setup.

I read GPIO devices on the I2C bus very early too, it is no problem.

 Jocke