Hi Mark,
>(...) Usage, e.g.:
>
>static struct i2c_adapter *xyz_real_adap;
>static struct i2c_adapter *xyz_virt_adap;
>
>static int __init xyz_mux_init(void)
>{
>
> int err;
>
> /* register myself as a driver/client */
> if ((err = i2c_add_driver(&xyz_mux_driver))
> goto ERROR_0;
Except that you cannot register the client at this point. If you do,
it'll get kicked as soon as you "lock" the adapter.
> /* find the real bus */
> if (!((xyz_real_adap = i2c_get_adapter(<some id>))) {
> err = -EINVAL;
> goto ERROR_1;
> }
>
> /* lock all other clients out of it */
> if ((err = i2c_lock_adapter(&xyz_real_adap))
> goto ERROR_2;
And register the client manually at this point. The mux client is the
only client attached to the real bus.
> /* build the virtual adapter */
> /* ... */
> if ((err = i2c_add_adapter(&xyz_virt_adap))
> goto ERROR_3;
>
> return 0;
>1) The above carries a built-in assumption: that the mux itself, if it
>is controlled by I2C/SMBus, must be a client of the immediate parent
>segment. Does anyone know of a mux that doesn't work that way? E.g.
>a 2x1 mux with a fourth bus connection for mux control only?
The S4882 itself has an interesting architecture. The Philips PCA9556
itself is not an I2C mux chip. It is an 8-bit I/O. On the S4882 each pin
is used as an output, and the pins control two PCA9516 chips, which are
the real (5-way) I2C muxes. The PCA9516 chips don't show on the I2C bus.
This shows that the multiplexing is highly board-dependant. It also shows
that we could see PCA9556 chips which are not means for bus
multiplexing, or PCA9516 chips which are controled by other means (such
as LPC GPIO chips).
However, I don't see how the model you propose wouldn't cover such
cases. What prevents the virtual busses from doing e.g. ISA I/O to
control the multiplexer(s)?
>2) Doesn't address the sysfs hierarchy. It would be nice to have a
>sysfs structure that is isomorphic to the i2c bus structure... I think
>USB is like that so I will look into it.
I wonder if it is worth the effort. There aren't that many multiplexed
SMBus systems out there, and we still have to see multilevel muxes. The
problem I see is that a sysfs structure change would most likely require
changes to user-space tools such as i2cdetect. While not impossible to
do, I think we have to think about these changes before taking the
actual move.
Thanks,
--
Jean Delvare
