So headphones don't work, but bluetooth works and it plays it on the BT headset AND speaker. Lovely.

yum

So it seems as if the leaked factory firmware for begonia allows preloader to expose its serial connection in case something goes wrong.
(Again, could be wrong on specifics but this is what I've read from leaks and heard from people)
Why is this a good thing? Preloader runs after BROM is done doing its security config. This includes SLA and/or DAA. This allows us to use SP Flash Tools to write partitions! Normally, on the stock MIUI Preloader and LK, preloader wouldn't enable serial at all. The only way to restore the device was by holding volume up and entering BROM download.
(Huge thanks to Nikolay for finding this out, and Sagar & Harshit for trying to brick the device with this firmware!)

Some interesting kernel behaviour with the engineering LK and Preloader:

Connect the cable and that's the serial output of bootloader & kernel via type C

As I doubted

Bootloader sets debug props that fuck with kernel drivers to make it behave that way to cmdline

So, MTK fastboot and BROM/Preloader can check preloader's partition type. When you give it preloader.bin, it'll read existing preloader, and seek to after the header to then start overwriting from there. Pretty neat.

Yep, as expected, there is no funky footer/sig inside the preloader partition. It's literally just the preloader bin appended to the header. hexdiff of generated_preloader_ufs.img and the partition dd'd out: (where generated is using cat header_ufs preloader.bin > generated_preloader_ufs.img)

im actually epic at commit messages

So, Xiaomi's r16p0 Mali driver had changes weirdly knit in from r18 and r20. Thankfully, it wasn't too hard to port and get r20 working, thus :D

new driver new blobs yay

That's one angry driver

Charging -1% hahayes

Well, Realme just made my life a lot easier. They released Realme 6 sources, which contains the 11.17 mali bifrost driver! Xiaomi still hasn't opensourced the kernel they use on their A10 builds :/

So at least gralloc doesn't scream anymore