Messages in this thread | | | Date | Thu, 3 Feb 2022 09:36:28 -0800 | Subject | Re: [PATCH 0/4] Broadcom STB PM PSCI extensions | From | Florian Fainelli <> |
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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
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