Hi Pete,

I've submitted this patch because the mini UART serial console produces garbage since UEFI v1.25 (due to the wrong baud rate).
It has been tested on the firmware shipped with UEFI v1.24 and it works fine. I don't see any reason to test it on older versions than this.

The core clock is not fixed. I've made PcdSerialClockRate patchable here:

> 
> @@ -465,6 +464,9 @@ [PcdsFixedAtBuild.common]
> gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVendor|L"EDK2"
> gEfiMdeModulePkgTokenSpaceGuid.PcdSetNxForStack|TRUE
> 
> +[PcdsPatchableInModule]
> + gEfiMdeModulePkgTokenSpaceGuid.PcdSerialClockRate|500000000
> +
> 

500 MHz is the default clock rate (assuming it hasn't been changed in config.txt).
The PCD gets patched in both the PEI and DXE phases to the value read from mailbox.