+static int wcd934x_slim_status(struct slim_device *sdev,
enum slim_device_status status)+{
- switch (status) {
- case SLIM_DEVICE_STATUS_UP:
return wcd934x_slim_status_up(sdev);- case SLIM_DEVICE_STATUS_DOWN:
mfd_remove_devices(&sdev->dev);break;- default:
return -EINVAL;- }
- return 0;
+}
this is interesting/surprising - I just noticed what looks like a significant change in how probe/initialization are handled.
It was my understanding that in SLIMbus the Linux devices are created at probe time, and when the device reports present this 'device_status' callback is used to notify the codec driver of a change. The rationale for this was that the codec driver may control power switches/gpios that are necessary for the device to appear on the bus.
This argument was used to require an change in the SoundWire implementation, so we followed this model of creating devices at probe time based on information reported by ACPI/DT, and used the 'update_status' callback when the device is present on the bus (which may happen after a delay or controlled by an external power switch). This approach can lead to 'ghost devices' described in firmware but not populated in hardware, and power management opens on how long a bus needs to remain active if no devices report present.
What I understand from the code above is that the devices are actually created when the SLIMbus device reports PRESENT, which seems a 180 degree change in directions?
I actually prefer it this way, and all current discussions in MIPI circles point to the fact that when the bus starts all devices on that bus should already be powered and have the ability to report present immediately (if the bus starts in a 'safe' mode and then later programs different PHY parameters, a device can no longer join the bus)
I would however not remove the devices when the status is down but only on an explicit .remove.
Feedback welcome. -Pierre