AM6548: WKUP_UART0 use from A53

Hi Tony,

I talked with Bin, let me jump in here.

Tony Tang said:

WKUP_UART0 used by MCU during boot, after boot up A53 Linux, the MCU firmware still in running, does it still control WKUP_UART0?

Just to clarify, WKUP_UART0 is used as a debug UART by the "System Firmware" (SYSFW) running on the DMSC sub-system, to output log traces, if enabled. Since SYSFW is a fundamental piece to the operation of AM654x-type devices, yes this firmware will always be running. You can control whether it prints traces to the WKUP_UART0 through the board configuration data that gets built into the sysfw.itb blob by the System Firmware Image Generator (SIG) project, see http://software-dl.ti.com/tisci/esd/latest/3_boardcfg/BOARDCFG.html.

Note that by default we have the SYSFW traces disabled, however there will still be 4-5 lines of initial print as U-Boot is bringing up SYSFW, but with traces disabled nothing will follow after that. If you need to you could suppress those initial prints by simply not performing the WKUP_UART0 pinmux initialization in U-Boot in the device-specific init file in U-Boot.

Tony Tang said:

if Linux use WKUP_UART0, will the MCU release control of WKUP_UART0? or How?

You should be able to use/request WKUP_UART0 from Linux and use it without issues. I can't double-check it right now but I remember before I was using it on a J721E device which is architecturally very similar without issues. And as explained as SYSFW debug traces are disabled by default there won't be any interference from the SYSFW accessing that UART as well (if you were to turn on traces you would see the WKUP_UART0 output getting mixed yup).

Tony Tang said:

Customer worries when use WKUP_UART0 in A53 Linux, if MCU firmware is still running, will it output something occasionally to WKUP_UART0?

With SYSFW debug traces disabled that should not be an issue.

Regards, Andreas