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Some boards like the AM335x EVM-SK and AM437x GP EVM provide software control via a GPIO pin to toggle the DDR VTT regulator to reduce power consumption in low power states. The VTT regulator should be disabled after enabling self-refresh on suspend, and should be enabled before disabling self-refresh on resume. This is to allow proper self-refresh entry/exit commands to be transmitted to the memory. The "ti,vtt-gpio-pin" device tree property in the wkup_m3_ipc node specifies which GPIO pin to use. This property is communicated to the Wakeup Cortex M3 co-processor where the actual toggling of the GPIO pin happens in CM3 firmware [1]. Please note that the GPIO pin must be on the GPIO0 module as that module is in the wakeup power domain. [1] https://git.ti.com/cgit/processor-firmware/ti-amx3-cm3-pm-firmware/tree/src/pm_services/ddr.c?h=08.02.00.006#n190 Signed-off-by: Dave Gerlach <d-gerlach@ti.com> Signed-off-by: Keerthy <j-keerthy@ti.com> [dfustini: remove the unnecessary "ti,needs-vtt-toggle" property] Signed-off-by: Drew Fustini <dfustini@baylibre.com> Signed-off-by: Nishanth Menon <nm@ti.com> Link: https://lore.kernel.org/r/20220409211215.2529387-3-dfustini@baylibre.com