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SubjectRe: [alsa-devel] [PATCH v6 02/11] mfd: wcd934x: add support to wcd9340/wcd9341 codec
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> +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

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