arm64: dts: qcom: msm8916: Pull-up the UART RX pin

[samcday]

With UART disconnected, no one is driving the UART RX line. It will be floating, which can end up producing garbage on stdin.

Reliable UART is particularly important in early boot stages, otherwise an autoboot sequence might be interrupted, so bootph-all is also added to the RX pin.

[samcday]

Hmm, maybe this is more appropriate to send upstream to u-boot as a tweak rather than to the kernel? Problem is I don't think this fix could be achieved there without adding a million *-u-boot.dtsi files for every individual msm8916 device :\

[stephan-gh]

We should definitely add the bias-pull-up upstream in Linux, since you could potentially hit the garbage input there as well. I'm not sure yet if the bootph-all is acceptable upstream. It is documented in dtschema as a somewhat generic property (i.e. not entirely specific to U-Boot). But whether this is actually accepted needs to be tried.

[stephan-gh]

(I'm planning to send the patch upstream to Linux, just didn't get to it yet)

[samcday]

But whether this is actually accepted needs to be tried.

I wondered this too, but I noticed there's a few upstream DTs with bootph-all now (and even some mentions of it in commit messages). So ... 🤞

[samcday]

By the way, if you accept this PR here in the meantime, it'll mean that gen-uboot-img, which pulls DTs from this repo (in both CI and the written instructions), will subsequently pick up the fixed DTs.