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[0/3] arm64: tegra: Add earlycon support on Tegra194

Message ID 20210326174641.1612738-1-thierry.reding@gmail.com
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Series arm64: tegra: Add earlycon support on Tegra194 | expand

Message

Thierry Reding March 26, 2021, 5:46 p.m. UTC 
From: Thierry Reding <treding@nvidia.com>

Hi Rob,

I've been looking at adding support for earlycon on Tegra194 which uses
the "combined UART" as console. This isn't a real device but rather is
composed of two mailboxes that are used to communicate with a processor
that multiplexes multiple data streams before they are sent over the HW
UART.

It's possible to make this work by passing earlycon with extra options
that select the TCU and pass a TX mailbox address, but I'd prefer if we
could also make this work without any options and instead infer the TX
mailbox address to use from /chosen/stdout-path. I can make that work
with the regular OF_EARLYCON_DECLARE and just a bit of additional code
to write data to the TX mailbox if I change the device tree node to take
a "reg" property. This set of patches implements those changes (there's
a separate patch to the tegra-tcu driver that adds OF_EARLYCON_DECLARE
and a bit of code, but it's not directly relevant to this discussion).

Given that this isn't a real device and the address in the "reg"
property is also a subset of the mailbox device that will take over for
the real console later on, I'm wondering if this is acceptable, even if
it is bending the rules a tiny bit.

The only other alternative I could think of would be to obtain the node
offset to the TCU node (either by passing it to the earlycon ->setup()
callback, or looking it up manually) and then parsing the mboxes
property and find the TX mailbox provider and then compute the TX
mailbox address. But that all seems very involved for something that we
can make work out of the box by simply adding this "reg" property, even
if it may not be strictly correct.

What do you think?

Thierry

Thierry Reding (3):
  dt-bindings: serial: tegra-tcu: Convert to json-schema
  dt-bindings: serial: tegra-tcu: Document "reg" property
  arm64: tegra: Add "reg" property for TCU on Tegra194

 .../bindings/serial/nvidia,tegra194-tcu.txt   | 35 -----------
 .../bindings/serial/nvidia,tegra194-tcu.yaml  | 60 +++++++++++++++++++
 arch/arm64/boot/dts/nvidia/tegra194.dtsi      |  4 +-
 3 files changed, 63 insertions(+), 36 deletions(-)
 delete mode 100644 Documentation/devicetree/bindings/serial/nvidia,tegra194-tcu.txt
 create mode 100644 Documentation/devicetree/bindings/serial/nvidia,tegra194-tcu.yaml