Re: RFC: Platform data for onboard USB assets

Andy Green
2011-03-11T16:52:34Z

On 03/11/2011 04:45 PM, Somebody in the thread at some point said:

Hi -

> Or to put it another way... With external, hot-plugged USB devices,
> there is no need to know "how it is wired". The fact that it is on a
> USB bus is the only information necessary. Why does anyone need to
> know more than this for on-board USB devices?

For example, the USB device is a chip with option pins. On the board it
is placed on, some of the option pins are tied in a particular way that
impacts its actual function, but can't be seen from the chip itself.
The driver covers all the options, but it needs to be told which mode
the chip was wired up for.

Another example, it's a USB chip with GPIO pins, analogous to a I2C GPIO
extender. Some of the GPIO are wired to LEDs also on the board, which
you want to expose as generic GPIO. The board definition file is in a
position to do all that because it knows what the board is and what it
is wired up to.

That the USB chips in these examples are 'discoverable' has nothing to
do with anything. In fact the board definition file has knowledge about
the "functional implemntation" of the instances of those chips -- just
exactly those instances soldered to the board. If you plugged another
of these chips, the board definition file has nothing to say about it
because they are not "on the board" and in-scope for it.

-Andy