Re: [PATCH v3 04/15] clk: qcom: gcc-sm6375: Add runtime PM

[Johan Hovold]

On Wed, Dec 20, 2023 at 01:26:55PM +0100, Konrad Dybcio wrote:
> On 20.12.2023 10:26, Johan Hovold wrote:
> > On Wed, Dec 20, 2023 at 01:30:45AM +0100, Konrad Dybcio wrote:
> >> The GCC block on SM6375 is powered by the VDD_CX rail. We need to ensure
> >> that CX is enabled to prevent unwanted power collapse 
> > 
> > As I pointed out earlier, this bit of the commit message is incorrect
> > and misleading as the power domain will never be disabled until you
> > enable runtime PM as part of this very patch:
> > 
> > 	https://lore.kernel.org/all/ZLaSpFFBzP_Yz5yY@xxxxxxxxxxxxxxxxxxxx/
> > 
> > Specifically, genpd will not power off CX (at runtime) while the driver
> > is bound when runtime PM is left disabled.

> OK I only now see what you really meant.
> 
> What this bit says is true, but it may be confusing within the context
> of this patch.

I'd say it's misleading since it suggests that something can currently
cause an "unwanted power collapse" which is not the case.

> The CX domain must be turned on [for the SoC to function], however this
> patch does not solve the issue of it being powered down [like you've said
> just binding the PD will keep it always-active for RPM-disabled devices].
> It complements this process, by allowing it to shut down when unnecessary.

Right, so just skip the misleading bits about "unwanted power collapse".

> >> and that the
> >> reference is dropped when unused so that the system can enter a
> >> firmware-managed lower power state.
> >>
> >> Enable runtime PM to keep the power flowing only when necessary.
> > 
> > The rest is correct.

> Let me try to reword this and see if you like it:
> 
> 
> The GCC block on SM6375 is powered by the VDD_CX rail. The Device Tree
> description of this dependency lets Linux keep the rail online to prevent
> power outages. It is however undesirable to keep it enabled at all times,
> as that consumes additional power.

I'd skip or rephrase the second sentence myself.
 
> Moreover, failing to drop the "enabled" vote prevents firmware-managed,
> SoC-wide power collapse in suspend, which leads to even more wasted power.

However if this is what you meant by "firmware-managed lower power
state" then this is not correct either. genpd will still power off the
power domain during system suspend, regardless of whether a driver
implements runtime PM.

> Enable runtime PM to keep the power flowing only when necessary.

So I'm starting to question whether we need this at all. AFAIK CX is
never going to be disabled at runtime and this patch is not needed to
disable CX during system suspend.

Johan