[PATCH 11/11] doc: bcm283x: document use of mini-uart on Raspberry Pi Zero W / CM3

[Ahmad Fatoum]

As described in the UART configuration[1] article in the Raspberry Pi
Foundation documentation, Raspberry Pi 3 & 4 as well as Zero W use the
mini-uart as primary (easily user-accessible) UART. At least on the
Raspberry Zero W and CM3, we need to pass uart_2ndstage=1, so the
BootROM leaves the 8250 IP in a suitable state for use by barebox.
Document this.

[1]: https://www.raspberrypi.org/documentation/configuration/uart.md

Cc: Roland Hieber <rhi at pengutronix.de>
Cc: Rouven Czerwinski <rcz at pengutronix.de>
Cc: Robert Carnecky <robert at neopsis.com>
Cc: Andrew John <andrew at mobius-embedded-systems.co.uk>
Signed-off-by: Ahmad Fatoum <ahmad at a3f.at>
---
 Documentation/boards/bcm2835.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/boards/bcm2835.rst b/Documentation/boards/bcm2835.rst
index c896871e0d82..dbdfc2633173 100644
--- a/Documentation/boards/bcm2835.rst
+++ b/Documentation/boards/bcm2835.rst
@@ -23,6 +23,13 @@ Raspberry Pi
          kernel=barebox.img
          enable_uart=1
 
+     If you want to use the mini-uart instead of the PL011, you might need to additionally set::
+
+         uart_2ndstage=1
+
+     This is useful on newer boards like the Raspberry Pi Zero W and CM3, which route the
+     more easily accessible primary UART to the mini-uart.
+
      (For more information, refer to the `documentation for config.txt`_.)
 
   5. Connect to board's UART (115200 8N1);
-- 
2.28.0