Fixed Bluetooth breaking on Raspbian Stretch and Buster

[Kostadin]

Adding dtoverlay=pi3-miniuart-bt to /boot/config.txt without anything else breaks all Bluetooth functionality on Raspberry Pi models with a Bluetooth chip. Setting core_freq=250 and core_freq_min=250 fixes the problem.

p3-miniuart-bt has been renamed to miniuart-bt but the previous name was kept as an alias for compatibility reasons.

More info on miniuart-bt:
https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README#L1571

[Gadgetoid]

I can see what you're trying to achieve. But this, as far as I understand this may have other unintended side-effects to performance which may be an extremely bold choice for an easy-installer particularly for the Pi 4.

I'm going to wager that more users would expect their Pi to run full-speed than expect functional bluetooth in this case, but I could be wrong. I think this fix needs to be communicated, and maybe even offered as an option for install, but not without clearly documenting the tradeoff (which, I must admit, I'm not well-versed on since the documentation with respect to Pi 4 is limited.)

From what I can tell (source: https://www.raspberrypi.org/documentation/configuration/config-txt/overclocking.md):

1. core_freq might default to 400 on the Pi 4, but I'm struggling to find confirmation of this?
2. Changing core_freq on the Pi 4 does not affect the SDRAM frequency
3. Does changing core_freq affect the L2 cache performance on Pi 4?

[Gadgetoid]

I'm leaving this unmerged and marked as notice so that others running into this issue can decide if they want to use this fix.