Power Domain Subsystem Details

Power Domain Subsystem Details

genpd stay_on and sync_state Interaction

Domains that are powered on at pm_genpd_init() time stay powered indefinitely until sync_state fires, unless GENPD_FLAG_NO_STAY_ON is set. If consumers never probe, sync_state never fires (in the default strict mode), and the domain stays on forever -- wasting power and potentially blocking regulator cleanup.

When CONFIG_PM_GENERIC_DOMAINS_OF is enabled and a domain is initialized as on (is_off=false), pm_genpd_init() in drivers/pmdomain/core.c calls genpd_set_stay_on(), which sets genpd->stay_on = true unless GENPD_FLAG_NO_STAY_ON is set. While stay_on is true, genpd_power_off() refuses to power off the domain (checked at the top of that function).

The stay_on flag is cleared when the provider's sync_state callback runs. For OF-based providers, this is handled by genpd_provider_sync_state() or of_genpd_sync_state(), both in drivers/pmdomain/core.c. These functions set genpd->stay_on = false and then attempt genpd_power_off().

The sync_state callback is only invoked after all consumer device links reach DL_STATE_ACTIVE (i.e., all consumers have probed). If a consumer never probes (no driver, deferred indefinitely, etc.), sync_state never fires in strict mode. The fw_devlink.sync_state=timeout option (or CONFIG_FW_DEVLINK_SYNC_STATE_TIMEOUT) changes this behavior to give up waiting after deferred_probe_timeout expires or at late_initcall() if !CONFIG_MODULES.

GENPD_FLAG_NO_STAY_ON prevents stay_on from being set at all, allowing the domain to be powered off as soon as it has no active consumers -- without waiting for sync_state. Platforms using this flag include:

• Renesas R-Car: drivers/pmdomain/renesas/rcar-sysc.c
• Renesas R-Mobile: drivers/pmdomain/renesas/rmobile-sysc.c
• Rockchip: drivers/pmdomain/rockchip/pm-domains.c
• Tegra BPMP: drivers/pmdomain/tegra/powergate-bpmp.c

Without CONFIG_PM_GENERIC_DOMAINS_OF, genpd_set_stay_on() unconditionally sets stay_on = false, so the stay-on mechanism is inactive.

genpd_power_off_unused and Regulator Cleanup Ordering

If genpd_power_off_unused() is delayed (e.g., by the stay_on mechanism), domains remain powered past the point where regulators expect them to be off. On platforms where a regulator supplies a power domain, this can cause the regulator to be disabled while its domain is still active, leading to hardware malfunction.

genpd_power_off_unused() runs at late_initcall_sync (drivers/pmdomain/core.c). It iterates all registered genpds and queues genpd_power_off_work for each, which asynchronously attempts to power off domains with no active consumers. Domains with stay_on == true are skipped.

regulator_init_complete() also runs at late_initcall_sync (drivers/regulator/core.c), but it does not disable regulators synchronously. It schedules regulator_init_complete_work as a delayed work with a 30-second timeout, which eventually calls regulator_late_cleanup() to disable unused regulators.

Even with the 30-second delay, if domains are held on by stay_on awaiting a sync_state callback that never arrives, the domain may still be powered when the regulator is finally disabled. The Rockchip power domain driver sets GENPD_FLAG_NO_STAY_ON specifically to avoid this scenario.

Platform-Specific Workaround Conditionals

Applying workarounds unconditionally causes unnecessary code execution on unaffected platforms and may introduce regressions on hardware that does not need the workaround.

Workarounds for bootloader state handover (e.g., splash-screen handover) must be gated on the affected platform:

• Architecture checks (e.g., IS_ENABLED(CONFIG_ARM) for ARM32-only issues)
• Device compatibility checks (of_device_is_compatible())
• Platform-specific device tree properties

Timing also matters: power domain reset operations for bootloader handover should occur early in probe, before pm_genpd_init() initializes the domain. After pm_genpd_init(), the domain is registered in the global gpd_list and managed by the genpd framework, so manual reset may conflict with framework state.

For targeted resets, explicit per-driver power-off functions (e.g., exynos_pd_power_off() in drivers/pmdomain/samsung/exynos-pm-domains.c) are preferred over of_genpd_sync_state(), which iterates all domains belonging to a provider and attempts to power off each one.

Platform Default Domain States

Forcing domains on at boot when they should default to off wastes power and may violate hardware constraints.

Each vendor has its own mechanism for expressing default-off state:

• MediaTek uses MTK_SCPD_KEEP_DEFAULT_OFF (defined in drivers/pmdomain/mediatek/mtk-pm-domains.h) in per-domain capability flags. When set, the domain is initialized as off via pm_genpd_init() with is_off=true.
• Renesas and Rockchip use GENPD_FLAG_NO_STAY_ON to allow domains to power off without waiting for sync_state.

Core genpd changes that alter default on/off behavior must be verified against platform drivers that rely on these mechanisms.

Quick Checks

• of_genpd_sync_state() as sync_state callback: This function iterates all domains belonging to a provider and powers off each one. If a platform driver uses it as a sync_state callback, verify this is appropriate for all domains the provider manages, or whether only specific domains should be powered off.
• GENPD_FLAG_NO_STAY_ON on new drivers: Drivers for platforms with regulator-supplied power domains or unreliable sync_state invocation should set this flag.
• Core genpd timing changes: New default behavior that delays or prevents domain power-off must provide a GENPD_FLAG_* opt-out for platforms that cannot tolerate the change.