linux-kernel - Re: [PATCH 0/4] Broadcom STB PM PSCI extensions

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Date:   Thu, 3 Feb 2022 09:36:28 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Sudeep Holla <sudeep.holla@....com>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        "maintainer:BROADCOM BCM7XXX ARM ARCHITECTURE" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] Broadcom STB PM PSCI extensions



On 2/3/2022 3:14 AM, Sudeep Holla wrote:
> On Fri, Jan 21, 2022 at 07:54:17PM -0800, Florian Fainelli wrote:
>> Hi all,
>>
>> This patch series contains the Broadcom STB PSCI extensions which adds
>> some additional functions on top of the existing standard PSCI interface
>> which is the reason for having the driver implement a custom
>> suspend_ops.
>>
>> These platforms have traditionally supported a mode that is akin to
>> ACPI's S2 with the CPU in WFI and all of the chip being clock gated
>> which is entered with "echo standby > /sys/power/state". Additional a
>> true suspend to DRAM as defined in ACPI by S3 is implemented with "echo
>> mem > /sys/power/state".
> 
> How different is the above "standby" state compare to the standard "idle"
> (a.k.a suspend-to-idle which is different from system-to-ram/S3) ?

There are a few differences:

- s2idle does not power gate the secondary CPUs

- s2idle requires the use of in-band interrupts for wake-up

The reasons for implementing "standby" are largely two fold:

- we need to achieve decent power savings (typically below 0.5W for the 
whole system while allowing Wake-on-WLAN, GPIO, RTC, infrared, etc.)

- we have a security subsystem that requires the CPUs to be either power 
gated or idle in order the hardware state machine that lets the system 
enter such a state and allows the out of band interrupts from being 
wake-up sources


> Suspend to idle takes all the CPUs to lowest possible power state instead
> of cpu-hotplug in S2R. Also I assume some userspace has to identify when
> to enter "standby" vs "mem" right ? I am trying to see how addition of
> "idle" changes that(if it does). Sorry for too many questions.
> 

Right that user-space in our case is either custom (like RDK, or 
completely custom), or is Android. For Android it looks like we are 
carrying a patch that makes "mem" de-generate into "standby" but this is 
largely because we had historically problems with "mem" that are being 
addressed (completely orthogonal).

I did not consider it as a viable option at the time, but if we were to 
implement "standby" in drivers/firmware/psci/psci.c would that be 
somewhat acceptable?
-- 
Florian