linux-kernel - Re: [RFC PATCH] pmdomain: arm: scmi_pm_domain: Do lazy init as part of probe

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Message-ID: <20250530-pelican-of-rampant-greatness-d80aec@sudeepholla>
Date: Fri, 30 May 2025 17:37:16 +0100
From: Sudeep Holla <sudeep.holla@....com>
To: Dhruva Gole <d-gole@...com>
Cc: Cristian Marussi <cristian.marussi@....com>,
	Sudeep Holla <sudeep.holla@....com>,
	Ulf Hansson <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: [RFC PATCH] pmdomain: arm: scmi_pm_domain: Do lazy init as part
 of probe

On Fri, May 30, 2025 at 05:31:08PM +0100, Sudeep Holla wrote:
> On Fri, May 30, 2025 at 04:05:27PM +0530, Dhruva Gole wrote:
> > Optimize the SCMI power domain driver to only initialize domains that are
> > actually referenced in the device tree. Previously, the driver would
> > initialize all possible domains up to the maximum ID, which could lead to
> > unnecessary firmware calls and longer probe times.
> > 
> > Key changes:
> > - Scan device tree to identify which power domains are actually referenced
> 
> How do this work with runtime DT overlays ?
> 
> > - Use bitmap to track needed domains instead of initializing all
> > - Only perform state queries and initialization for referenced domains
> > - Maintain proper array sizing for power domain framework compatibility
> > - Keep full provider structure to support late binding
> > 
> > This optimization reduces probe time and unnecessary firmware interactions
> > by only initializing power domains that are actually used in the system.
> 
> Why is this very specific to power domains only ? This must apply for other
> domains like perf or clock or reset ?
> 
> > For example, in a system with 100 possible domains but only 3 referenced
> > in the device tree, we now only initialize those 3 domains instead of
> > all 100.
> > 
> 
> Well, how much of these PD will get used in the final products ? I can
> understand the need to use just 3 in devel platforms. Just trying to see
> how realistic is the scenario ? Is there any other optimisation possible
> from the firmware ? Does getting the state of a PD takes so much time
> on the platform in question ?
> 

And I missed another point. Someone will soon complain Linux no longer
turns off unused power domains. So I am inclined against this optimisation.
We need to consider all the above point before .

-- 
Regards,
Sudeep