When A7 is suspended on sabre IMX7D platform using "echo mem > /sys/power/state" , M4 is not working. It simply halts without executing any code. After A7 is resumed, it starts working back again from the point where it stopped. Is it possible to keep M4 active while A7 is suspended on IMX7D ? We initially thought Megamix/FastMix PGC is powered down, but looking at power management code (arch/arm/mach-imx/pm-imx7.c) and dumping GPC related registers, we observed that it is not the case.
On M4, we are running hello world demo app provided in FreeRTOS_BSP_1.0.1_iMX7D and for linux, we are using 4.14.98 yocto release. We are loading our code from uboot and we are using imx7d-sdb-m4.dtb. We also tried standby mode using "echo standby > /sys/power/state" as well and observed the same issue. Could you please confirm that whether M4 works when A7 is placed in standby/mem mode ?
Thanks for your prompt response and valuable information. Is it possible to control the m4 clock gating in the low power mode via software(registers) ? We are aiming for a solution where M4 needs to run when A7 is suspended and it's a big blocker for us. Can you strongly confirm that M4 will not run(clock gated) when A7 is suspended ? Also does this mean that if A7 is in WAIT/STOP lpm mode, M4 is not usable ? Thanks for your great help.
> Is it possible to control the m4 clock gating in the low power mode via software(registers) ?
seems not as per Table 5-19. CCGR Mapping Table : "Not all CCGRs are mapped."
Table 5-19 gives peripherals which has programmable CCGR Interface described in
sect.22.214.171.124 CCGR Interface and Table 5-12. System Clocks and Gating.
Looks like its not the case and M4 will work when A7 is suspended. I have found low_power_imx7d demo app in FreeRTOS_BSP_1.0.1_imx7D/examples/imx7d_sdb_m4/demo_apps folder and it seems to be the app demonstrating the feature.
>Could you please confirm that whether M4 works when A7 is placed in standby/mem mode ?
seems not as in low power mode M4 Core "Clock Gated" as described in
Table 5-5 "System IDLE" i.MX 7Dual Applications Processor Reference Manual