Wake-up from suspend by UARTs

[enginlee]

Hi,

I found a strange issue on my i.MX6ULL board:

There is a MCU connect to CPU with UART3, and UART2 is unused.

Application access ttymxc2 without any problem, and also set "enabled" to "/sys/class/tty/ttymxc2/power/wakeup".

But after suspend to RAM (echo mem > /sys/power/state), ttymxc2 becomes abnormal, I cannot read/write anymore.

And I can see the power consumption during suspend mode is higher than it should be, about 10% increase.

I tried lots of settings (BSP/USB/SAI/GPIOs), and finally I found a solution: set "enabled" to "/sys/class/tty/ttymxc1/power/wakeup".

Even through there is no signal connect to UART2, but it just working: ttymxc2 works fine after waking up, and power consumption is normal.

I am not sure the reason why it happened, but I can reproduce this issue on both SDKs (4.14.78_ga_1.0.0 and 4.14.98_ga_2.0.0)

Thanks,

-Engin

[igorpadykov]

Hi Engin

>But after suspend to RAM (echo mem > /sys/power/state), ttymxc2

>becomes abnormal, I cannot read/write anymore.

in suspend state uart is not intended to operate and

if it receives data it will cause processor to exit from low power mode.

Also if uart clocks are gated in low power mode (using CCM_CCGRn)

it is not possible to read uart registers.

Best regards
igor
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[enginlee]

Very USEFUL answer!!! HAHAHA