Message-ID: <Z4d8nrJy-h9EwzsJ@pluto>
Date: Wed, 15 Jan 2025 09:15:10 +0000
From: Cristian Marussi <cristian.marussi@....com>
To: Ranjani Vaidyanathan <ranjani.vaidyanathan@....com>
Cc: Sudeep Holla <sudeep.holla@....com>, Peng Fan <peng.fan@....com>,
	"Peng Fan (OSS)" <peng.fan@....nxp.com>,
	"cristian.marussi@....com" <cristian.marussi@....com>,
	"ulf.hansson@...aro.org" <ulf.hansson@...aro.org>,
	"arm-scmi@...r.kernel.org" <arm-scmi@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [EXT] Re: [PATCH] pmdomain: arm: scmi_pm_domain: Initialize
 state as off

On Tue, Jan 14, 2025 at 04:09:13PM +0000, Ranjani Vaidyanathan wrote:
> Hello Sudeep,
> 
> Comments below.
> 
> Regards,
> Ranjani Vaidyanathan
> 
> -----Original Message-----
> From: Sudeep Holla [mailto:sudeep.holla@....com] 
> Sent: Tuesday, January 14, 2025 9:24 AM
> To: Ranjani Vaidyanathan <ranjani.vaidyanathan@....com>
> Cc: Peng Fan <peng.fan@....com>; Peng Fan (OSS) <peng.fan@....nxp.com>; cristian.marussi@....com; Sudeep Holla <sudeep.holla@....com>; ulf.hansson@...aro.org; arm-scmi@...r.kernel.org; linux-arm-kernel@...ts.infradead.org; linux-pm@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [EXT] Re: [PATCH] pmdomain: arm: scmi_pm_domain: Initialize state as off
> 
> Caution: This is an external email. Please take care when clicking links or opening attachments. When in doubt, report the message using the 'Report this email' button
> 
> 
> Hi Ranjani,
> 
> On Mon, Jan 13, 2025 at 07:54:06PM +0000, Ranjani Vaidyanathan wrote:
> > Hello Sudeep,
> >
> > Will try to explain the situation we are facing.
> > 1. We have multiple agents running, Agent-A is booted up first before 
> > Linux is booted and powers up a shared power domain PD-X.
> > 2. Linux boots and gets the power state of PD-X. And its already ON. 
> > And then PD -X is initialized with a default ON state.
> > 3. When the driver that needs PD-X  is probed, Linux sees that the 
> > power domain status is ON and never makes an SCMI call to power up the 
> > PD-X for Linux Agent.
> > 4. Agent-A now is shutdown/suspends. Linux will crash because the 
> > platform disables PD-X because it has no other requests for PD-X.
> >
> 
> Thanks for the detailed explanation. I understand the issue now.
> 
> I would like to discuss if the below alternative approach works for you.
> We can debate the pros and cons. I see with the approach in this patch proposed by Peng we would avoid querying and setting genpd all together during the genpd initialisation which is good. But if there are any genpd left on by the platform or bootloader(same agent), it will not get turned off when Linux tries to turn off the unused genpds(IIRC this could be the reason for the current state of code). While your platform may find sending those commands unnecessary, there was some usecase where SCMI platform kept all resources ON by default for faster boot and expects OSPM to turn off unused resources. So we need to support both the cases. I hope my below patch should suffice.
> 
> [RV] Linux can still make the call to disable unused power domains, even if it never explicitly made a request to power it on. The platform will aggregate the request from all agents and will power off the resource if no other agent has enabled it. From Linux point of view it has disabled all unused power domains. 
> Your patch below may also work, but feels like a workaround to artificially (for lack of a better word) enable a resource. And also makes unnecessary SCMI calls (expensive) for every resource immaterial of it power state (maybe can be improved by a conditional check). 
> 

...sincerely, both of these solutions seem to me hacks/workarounds to
counteract the fundamental issue that derives from having allowed (IMPDEF)
to implement the get operations to return the real physical state of a
resource instead of its virtual per-agent state as maintained by the platform,
while, at the same time, having allowed to implement the set-operations to
operate in a 'virtual-fashion'...

...so, when Peng's patch forcibly set the state to OFF on genpd init, you
are indeed artificially forcing the kernel internal state to align with
what would have been the virtual-per-agent state of the resource in your
specific particular configuration....

...on the other side Sudeep's proposed patch tries really to play the same
trick, just on the other way around, by instead forcibly/artificially aligning
the state on the platform side by issuing a redundant ON request to bump the
refcount and take hold of that resource from the Kernel agent point of view...

... but Peng's proposed patch will broke immediately the moment you have
instead a system with an SCMI-capable bootloader that instead left the
resource ON for the Kernel to inherit, since the kernel will now forcibly
see this anyway as OFF, and so you wont be ever be able to switch that resource
REALLY OFF in the future, if ever needed, because the bootloader/Kernel agent
will never see it as ON in genPD, since, at least in the genPD case, AFAICS
correct me if wrong, there is no callback to peek at the real state later on:
so, after the initialization value has been chosen at genpd_init time, genPd
subsystem maintains the PD state on its own based on the issued ON/OFF genPD
requests, so your forced-initial-OFF-state will be, in this specific alternative
scenario, wrong and forever.

...I think at least Sudeep's solution could survive this scenario
because it is more general and can cope with both scenarios

I know this phys-vs-virtual state is a lost battle at this point, but the
IMPDEF possible scenarios that derive from this choice now have to be supported
kernel-side as best as we can both ways...with the additional headache that,
from the Kernel agent perspective, we cannot infer what kind of IMPDEF get_state
is implemented by the platform we are talking to AND various different
subsystem in the Linux kernel handle this "initial-state-matter" in
different ways (e.g. see  clocks subsystem)

Thanks
Cristian

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