In my experience JTAG/SWD is pretty useless when used with operating systems or firmware that uses virtual memory and multiple cores/processes/threads? There are more advanced debug systems which are often embedded in those SoC to do OS level hardware debugging, but JTAG might still be useful if you want to work on bootloader stuff or bare metal applications...
PS: JTAG via sdcard-pins seems to be pretty common because of tablets and phones, Allwinner SoC have that as well and often you can use an adapter like this:
https://www.cubietruck.com/products/usd-breakout-board
PS: JTAG via sdcard-pins seems to be pretty common because of tablets and phones, Allwinner SoC have that as well and often you can use an adapter like this:
https://www.cubietruck.com/products/usd-breakout-board
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