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I've checked the documentation for this chip.
gpio20, gpio21 = blsp0_uart0
gpio28, gpio29 = blsp0_uart0
These pins are muxed to UART0, I agree, the u-boot dts also indicates this also.
If we open the documentation further we see
0x78AF000 = BLSP1_BLSP_UART0
0x79b0000 = BLSP1_BLSP_UART1
So for starters the dtsi has the _wrong_ label.
Here/anseo
grep uart0: arch/arm64/boot/dts/qcom/*
arch/arm64/boot/dts/qcom/ipq5332.dtsi: blsp1_uart0: serial@78af000 {
arch/arm64/boot/dts/qcom/ipq9574.dtsi: blsp1_uart0: serial@78af000 {
That's how that label ought to be the main hint something is askance is assigning a pin named "blsp0_uart0" to a dts entry named "blsp1_uart1".
Please update the label in your next revision.
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bod