ACPI/FFH gaps around exposing suspend-to-RAM capability.

[andreiw]

          We discussed this topic in BRS meeing today: https://docs.google.com/presentation/d/1bWS_YUZAZwwDDgHOWf2S1vyoP_Eh7y6090-NjoJQTSo/edit#slide=id.g21d5651495d_0_17

Suspend shouldn't be mandatory, but we don't have the ACPI framework in place to allow arbitrary sbi calls?

Originally posted by @adurbin-rivos in https://github.com/riscv-non-isa/riscv-brs/issues/52#issuecomment-1668182024

[andreiw]

Suspend-to-RAM is not a BRS-I requirement today. It may become a requirement in a platform-specific section (e.g. client, BRS-I-C?), but for that perhaps it would be preferable to expose this in some ACPI-specific manner instead of just having the OS use SBI SUSP?

Need to study the ACPI spec and understand the necessary plumbing (that needs to be created) before making a call.

[adurbin-rivos]

@atishp04 This topic was supposed to be discussed in the PRS meeting today. Atish, could you share what transpired then?

[atishp04]

We did not have a PRS meeting last week due to holidays in India. It is on next week's meeting agenda. I will update the summary after the meeting.

[adurbin-rivos]

Sorry. I was thinking there was one today, but there wasn't.

[jones-drew]

This was discussed in the PRS call on August 29, 2023. There may be a recording of that session and below is a summary of the slides

For HW-reduced systems, the ACPI-specific manner in which an OSPM initiates suspend-to-ram is with the FADT.SLEEP_CONTROL_REG, which is an MMIO address (it's not a generic address descriptor, so it can't be an FFH defined SBI call). While any OSPM that implements suspend-to-ram using the FADT register is able to do so in an ACPI-specific way, the trade-off is that the platform must provide the MMIO addresses. Regardless of the mechanism used for suspend-to-ram, it probably shouldn't be BRS-I mandatory as the ACPI spec also provides ways to describe when a platform is low power s0 idle capable and states that that means "...that the system will achieve no power benefit by making a sleep transition to S3". IOW, some platforms may want to only provide LPI levels and not system suspend (suspend-to-ram).