Miyoo Flip BaseOS v0.7.0

Pre-release for BaseOS for Miyoo Flip. Read the installation instructions in INSTALL.md.

This is the biggest update to BaseOS for Miyoo Flip yet, it allows to reduce boot time by a whopping 1.8 s. To do so, I gave up the vendor U-boot and compiled a mainline U-boot with several customizations and optimizations:

• CPU boosted at 1800 MHz during decompression and handed off to kernel at 1104 MHz (0.34 s)
• zstd compressed kernel, decoder built without -mstrict-align or MINIFY and with -O2 optimization (0.22 s)
• SD clock fix (card really at 50 MHz, 12 → 23.8 MB/s, 0.58 s)
• data cache on before relocation (0.56 s)
• blkcache holding the GPT (0.15 s)
• Flip-specific control tree (0.03 s)

In addition the following optimizations, independent from U-boot, were also made in the branch:

• WiFi probe race: usbcore.authorized_default=0, with rcS authorizing devices, removes a ~0.3 s stall that hit 11 of 20 boots (about 0.16 s on average, and less variance).
• SDR104 tuning in 10° steps: the 113 ms timeout now hits 6 of 22 boots instead of 13 of 16 (about 65 ms on average)

Overall, the boot time improvements are as follows:

• Hand-off: 3.73 → 1.94 s.
• Frontend starts: 4.2 → 2.4 s.

The branch also brings fixes and new features, some made possible with the U-boot change:

• HDMI hot-plugging is now possible on the device
• The boot card does not drop out anymore out of suspend
• adb reconnects after a suspend
• A crash leaves a record in the logs
• Faster and cooler charge while off because of lower CPU frequency and voltage

However, switching to mainline U-boot also has some downsides:

• U-boot has no VOP2 driver and therefore cannot draw a bootlogo. It is therefore drawn by rcS, about one second later. With the reductions in boot time, this feels acceptable.
• Similarly the charge screen while off is gone. Instead, the charge LED blinks three times to indicate the battery is too low to boot.

BSP U-boot was implementing a lot of the batter and state of charge logic, which had to be reimplemented as patches over mainline. I have tested those to work properly, but issues could appear, Be vigilant and report issues if they appear.