Tomi Valkeinen [off-list ref] writes:

At the moment the driver sets the power state of all the PDs it creates
to off, regardless of the actual HW state. This has two drawbacks:

1) The kernel cannot disable unused PDs automatically for power saving,
   as it thinks they are off already

2) A more specific case (but perhaps applicable to other scenarios
   also): bootloader enabled splash-screen cannot be kept on the screen.

The issue in 2) is that the driver framework automatically enables the
device's PD before calling probe() and disables it after the probe().
This means that when the display subsystem (DSS) driver probes, but e.g.
fails due to deferred probing, the DSS PD gets turned off and the driver
cannot do anything to affect that.

Solving the 2) requires more changes to actually keep the PD on during
the boot, but a prerequisite for it is to have the correct power state
for the PD.

The downside with this patch is that it takes time to call the 'is_on'
op, and we need to call it for each PD. In my tests with AM62 SK, using
defconfig, I see an increase from ~3.5ms to ~7ms. However, the added
feature is valuable, so in my opinion it's worth it.

The performance could probably be improved with a new firmware API which
returns the power states of all the PDs.

Agreed.  I think we have to pay this performance price for correctness,
and we can optimizie it later with improvements to the SCI firmware and
a new API.

There's also a related HW issue at play here: if the DSS IP is enabled
and active, and its PD is turned off without first disabling the DSS
display outputs, the DSS IP will hang and causes the kernel to halt if
and when the DSS driver accesses the DSS registers the next time.

Ouch.

With the current upstream kernel, with this patch applied, this means
that if the bootloader enables the display, and the DSS driver is
compiled as a module, the kernel will at some point disable unused PDs,
including the DSS PD. When the DSS module is later loaded, it will hang
the kernel.

The same issue is already there, even without this patch, as the DSS
driver may hit deferred probing, which causes the PD to be turned off,
and leading to kernel halt when the DSS driver is probed again. This
issue has been made quite rare with some arrangements in the DSS
driver's probe, but it's still there.

So, because of the DSS hang issues, I think this patch is still an RFC.

Like you said, I think that DSS hang is an issue independently of this
patch, so it shouldn't hold this up IMO.

Hopefully we can sort out all the issues, but this patch (or similar)
will be part of the solution so I'd like to get some acks/nacks/comments
for this. Also, this change might have side effects to other devices
too, if the drivers expect the PD to be on, so testing is needed.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

We already discussed this a bit off-list, but for the record, I agree
with the approach.

I also tested it on k3-am62a7-sk where I've been doing the other TI SCI
pmdomain work and everything still working fine.

Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>

Kevin