Re: [PATCH RFC] pmdomain: ti-sci: Set PD on/off state according to the HW state

[Tomi Valkeinen]

Hi,

On 30/10/2024 22:04, Kevin Hilman wrote:
> Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx> 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.

In current upstream, if the bootloader has enabled the display, we most 
likely won't hit the DSS hang issue as the PD will stay on until the DSS 
driver has had a chance to probe, and the driver takes actions to avoid 
the hang issue.

With this patch applied, the PD may be turned off before the DSS driver 
has had a chance to probe, causing the board to hang when the DSS driver 
probes the first time.

That's why I'm a bit hesitant to apply this. It could mean that for some 
people their board stops booting.

I'm not even sure what would be the perfect fix for this hang problem...

We could have some built-in early boot code which checks if the DSS is 
enabled, and disables it, so that the hang issue won't happen. But 
that's not good if we try to keep the boot splash on the screen until 
the userspace takes over.

Alternatively we could, somehow, mark the DSS powerdomain to be handled 
in a special way: if the PD is enabled at boot time, it will be kept 
enabled until the DSS driver (somehow) changes the PD back to normal 
operation (and if DSS driver is never loaded, PD will stay on).

 Tomi

> > 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@xxxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx>
> Tested-by: Kevin Hilman <khilman@xxxxxxxxxxxx>
>
> Kevin