Hi all,
For Tegra we have an i2c device for display port, namely the display
port auxiliary channel (or dpaux) as specified by the display port
standard. If an design using Tegra does not utilise the display port
interface, then the pads assigned to the dpaux can be re-assigned to
another generic i2c controller (i2c6 for Tegra124/210). In other words,
the pads can be re-used for a generic i2c interface.
The registers that control whether the pads are mapped to the dpaux or
i2c6 are located in the dpaux register space. Therefore, I am looking at
adding pin controller support for dpaux so that i2c6 can request these
pads if it is enabled and I was hoping to add a pinmux node the to dpaux
device in device-tree to do this. For example, something like ...
dpaux@0,545c0000 {
...
/* pinctrl node */
pinmux {
...
};
};
Although the above works, when doing this I noticed that when the device
booted, I would seeing the following error messages on boot ...
i2c i2c-5: of_i2c: modalias failure on ...
These error messages being caused by the new pinmux node because it is
not recognised as an i2c device. To avoid this error messages we have
come up with a couple solutions but wanted to get some feedback on the
best approach.
1. Add a i2c-bus sub-node to the dpaux binding (suggested by Stephen
Warren), so we would have something like the below. Then i2c devices
for dpaux would be place in the i2c-bus sub-node.
dpaux@0,545c0000 {
...
/* pinctrl node */
pinmux {
...
};
/* place-holder for i2c devices */
i2c-bus {
...
};
};
To make the above work ideally we would like to make the 'i2c-bus'
node a generic solution for all i2c devices, so the i2c core would
check for the presence of this node and if it is found then would
default to this node for looking for i2c-devices.
2. When registering i2c devices via device-tree, the function
of_i2c_register_devices() checks to see if OF_POPULATED flag is set
for a given node. If it is set, then the node is skipped. I believe
this was added for device-tree overlays (commit 4f001fd30145 i2c:
Mark instantiated device nodes with OF_POPULATE). Another option is
for the dpaux driver to mark the pinmux node as populated before
registering the i2c adapter and this will prevent the i2c core from
trying to parse the pinmux node. I am not sure if this would be
frowned upon in anyway or if we can guarantee that no future changes
to DT overlays would change this in a way where it would not work.
Let me know your thoughts.
Cheers
Jon
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