Re: [PATCH 0/3] Allow specifying an S2RAM sleep on pre-SYSTEM_SUSPEND PSCI impls

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Tue, Nov 12, 2024 at 07:32:36PM +0100, Konrad Dybcio wrote:
> 
> 
> On 11/12/24 19:01, Manivannan Sadhasivam wrote:
> > On Mon, Oct 28, 2024 at 03:22:56PM +0100, Konrad Dybcio wrote:
> > > Certain firmwares expose exactly what PSCI_SYSTEM_SUSPEND does through
> > > CPU_SUSPEND instead. Inform Linux about that.
> > > Please see the commit messages for a more detailed explanation.
> > > 
> > 
> > It is still not PSCI_SYSTEM_SUSPEND though...
> 
> It *literally* does the same thing on devices where it's exposed.
> 

But still...

> > 
> > > This is effectively a more educated follow-up to [1].
> > > 
> > > The ultimate goal is to stop making Linux think that certain states
> > > only concern cores/clusters, and consequently setting
> > > pm_set_suspend/resume_via_firmware(), so that client drivers (such as
> > > NVMe, see related discussion over at [2]) can make informed decisions
> > > about assuming the power state of the device they govern.
> > > 
> > > If this series gets green light, I'll push a follow-up one that wires
> > > up said sleep state on Qualcomm SoCs across the board.
> > > 
> > 
> > Sorry. I don't think PSCI is the right place for this. Qcom SoCs have a common
> > firmware across all segments (mostly),
> 
> This ^
> 
> > so there is no S2R involved and only S2Idle.
> 
> is not at all related to this ^, the "so" makes no sense.
> 
> (also you're wrong, this *is* S2RAM)
> 

What? Qcom SoCs supporting S2R? I'm unheard of.

> > If you use PSCI to implement suspend_via_firmware(), then all the SoCs
> > making use of the PSCI implementation will have the same behavior. I don't think
> > we would want that.
> 
> This is an issue with the NVMe framework that is totally unrelated to this
> change, see below. Also, the code only sets that on targets where such state
> exists and is described.
> 

Well, you are doing it just because you want the NVMe device to learn about the
platform requirement.

> > For instance, if a Qcom SoC is used in an android tablet with the same firmware,
> > then this would allow the NVMe device to be turned off during system suspend all
> > the time when user presses the lock button. And this will cause NVMe device to
> > wear out faster. The said approach will work fine for non-android usecases
> > though.
> 
> The NVMe framework doesn't make a distinction between "phone screen off" and
> "laptop lid closed & thrown in a bag" on *any* platform. The usecase you're
> describing is not supported as of today since nobody *actually* has NVMe on a
> phone that also happens to run upstream Linux.
> I'm not going to solve imaginary problems.
> 

Not just phone, NVMe device could be running on an android tablet. I'm not
talking about an imaginary problem, but a real problem that is in a forseeable
future (that is also the reason why NVMe developers doesn't want to put the
device into power down mode always during system suspend).

And with this change, you are just going to make the NVMe lifetime miserable on
those platforms.

- Mani

> Besides, userspace already has sysfs to tune device power state knobs. Which
> Android uses very extensively on market devices.
> 
> Konrad

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