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Need to update UART documentation for Pi5 #3239

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@metachip

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@metachip

The existing documentation makes no mention of Pi5 or how it differs in terms of UART usage and we've reported at least one new bug (affecting older models but introduced by code changes to support Pi5). While we get the big picture apropos the new connector (all good!) it appears the logical meaning or intended usage of the alias names (serial0 and serial1) and the confounding bluetooth complexities of the past may be changed in Pi5, or there are unresolved bugs, or both?

For example, on Pi4 with bullseye, specifying console=serial0,115200 in cmdline.txt when uart_enable=1 and/or dtoverlay=disable-bt is asserted in config.txt results in serial0 being linked as primary (i.e. ttyAMA0) and the console being enabled on pins 14/15 on J8 enabling firmware boot and kernel messages on boot and the system console via the J8 pins. In terms of aliases we have /dev/serial0 -> ttyAMA0 and /dev/serial1 -> ttyS0. All good.

Not so on Pi5.

Instead on Pi5 with bookworm this configuration seems to enable all this on new UART connector. A different setup is needed to replicate backward compatible behaviour which setup seems to be: specify console=ttyAMA0,115200 in cmdline.txt when dtparam=uart0=on is asserted in config.txt. All good, but confusing as now /dev/serial0 -> ttyAMA10 where (I'm guessing ttyAMA10 is the new port) and /dev/serial1 is not assigned at all? If we also assert dtoverlay=disable-bt in config.txt we get aliases /dev/serial0 -> ttyAMA10 and /dev/serial1 -> ttyS0.

So there are changes and bugs here and we're having difficulty disambiguating between them.

Bottom line: updated documentation so we know what the intended behaviour is, even if bugs may get in the way :)

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