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At the moment, msm8916/39.dtsi have two inconsistent UART pinctrl templates
that are used by all the boards:
- &blsp_uart1_default configures all 4 pins (TX, RX, CTS, RTS), some
boards then limit this to just RX and TX
- &blsp_uart2_default only configures 2 pins (TX, RX), even though UART2
also supports CTS/RTS
It's difficult to define a generic pinctrl template for all UART use cases,
since they are quite different in practice. The main use case for most of
the 40+ MSM8916/39-based boards upstream is the UART debug console. The
current generic template is lacking some properties to work properly:
- bias-pull-up for RX: Generally, UART is push-pull and does not need pull
up/down. Both sides drive TX, so RX should never be floating. This is
why the current pinctrl in msm8916/39.dtsi uses bias-disable. However,
this assumes that UART is always connected. For the debug console this
will be rarely the case on mobile devices, only during debugging
sessions. The rest of the time, the RX pin is floating.
This has never caused massive problems, but it's obvious now that this
needs fixing:
(1) In U-Boot, we have been fighting with problems with autoboot for
years. Most of the time, there is a single \0 byte ("break event")
read during boot, which interrupts the autoboot process. I tried to
work around that by inserting some random delay [1], but it turned
out this is also not working reliably on all boards.
What happens is: Since RX is floating, it switches randomly between
high or low. A long low state is interpreted as "break event" (\0).
(2) In postmarketOS, we used to have the "magic SysRq key" enabled by
default for the serial console. We had to disable this at some
point, because there was a small number of users who were reporting
sysrq spam in the kernel log, possibly even crashes/panics
triggered by sysrq.
What likely happened is: SysRq is triggered by sending a "break
event", like in (1). With enough luck, you could even trigger any
of the SysRq actions if the RX pin switches between high and low
(e.g. because of noise introduced by the LTE radio close by).
We can fix this using bias-pull-up, but this may be unneeded,
unexpected, or even unwanted for other UART use cases.
- bootph-all: U-Boot needs to know which pinctrl to apply during early
boot stages, so we should specify "bootph-all" for the console UART
pinctrl. Without bootph-all, the bias-pull-up won't be applied early
enough in U-Boot to avoid the problem with autoboot in point (1) above.
It doesn't make sense to specify this for the other UART instances.
bootph-all is a generic property documented in dt-schema bootph.yaml.
Define these two additional properties only for the debug UART console, by
defining a new pinctrl template specifically for that. In the following
commits, boards will be converted to use these where appropriate.
[1]: ad7e967738
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Link: https://lore.kernel.org/r/20250422-msm8916-console-pinctrl-v2-2-f345b7a53c91@linaro.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Linux kernel
============
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