[U-Boot] [PATCH 1/3] i2c: add i2c_core and prepare for new multibus support

[Heiko Schocher]

Hello Timur,

Timur Tabi wrote:
> Wolfgang Denk wrote:
>> As mentioned before, this is what we currently have as "device model"
>> in U-Boot - not only I2C: we have the same "current device" concept
>> with IDE, USB, ...
> 
> The difference is that I2C operations are typically done internally by
> other code, whereas IDE, USB, etc are done by the user on the command
> line.  It's not unusual for boot code to access multiple I2C devices on
> different buses, so we're switching I2C buses a lot.  People generally
> don't try to access two networks or two USB devices back-to-back, but
> that's exactly what we do with I2C.
>
> The other problem is that I2C operations are necessary prior to
> relocation, but IDE, USB, etc generally are not.  That's why we have this:
> 
> static unsigned int i2c_bus_num __attribute__ ((section (".data"))) =
> CONFIG_SYS_SPD_BUS_NUM;

Yep, correct. For that case, I need the "trickery" with the "cur_i2c_adap"
pointer in gd_t in the "new" framework, as gd_t is writeable before
relocation ...

> We need to initialize i2c_bus_num to the I2C bus that SPD is on, because
> i2c_bus_num is not writable until after relocation, and DDR initialization
> requires I2C.
> 
> A board that has SPD on two different I2C buses could not be supported by
> U-Boot today.

... and so with the "new" framework, it should be possible to switch to
other i2c busses (incl. i2c muxes on that), before relocation.

bye,
Heiko
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