linux-kernel - [PATCH v3 1/2] dt-bindings: serial: samsung: fix maxItems for gs101 & document earlycon requirements

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Message-Id: <20240710-gs101-non-essential-clocks-2-v3-1-5dcb8d040d1c@linaro.org>
Date: Wed, 10 Jul 2024 14:29:14 +0100
From: André Draszik <andre.draszik@...aro.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 Jiri Slaby <jirislaby@...nel.org>, Rob Herring <robh@...nel.org>, 
 Krzysztof Kozlowski <krzk+dt@...nel.org>, 
 Conor Dooley <conor+dt@...nel.org>, 
 Peter Griffin <peter.griffin@...aro.org>, 
 Sylwester Nawrocki <s.nawrocki@...sung.com>, 
 Chanwoo Choi <cw00.choi@...sung.com>, Alim Akhtar <alim.akhtar@...sung.com>, 
 Michael Turquette <mturquette@...libre.com>, 
 Stephen Boyd <sboyd@...nel.org>, Sam Protsenko <semen.protsenko@...aro.org>, 
 Tudor Ambarus <tudor.ambarus@...aro.org>
Cc: Will McVicker <willmcvicker@...gle.com>, kernel-team@...roid.com, 
 linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org, 
 devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
 linux-samsung-soc@...r.kernel.org, linux-clk@...r.kernel.org, 
 André Draszik <andre.draszik@...aro.org>
Subject: [PATCH v3 1/2] dt-bindings: serial: samsung: fix maxItems for
 gs101 & document earlycon requirements

While gs101 needs exactly two clocks for the UART, the schema doesn't
currently limit the maximum number to this and instead the default of
five from this schema is applied.

Update the schema accordingly.

Also, as pointed out in [1] before, the hand-over between earlycon and
serial console is fragile due to clocking issues, at least on Google
Tensor gs101. Therefore, document the clocking requirements for
earlycon in the description for posterity, so the information is not
lost.

Link: https://lore.kernel.org/all/d45de3b2bb6b48653842cf1f74e58889ed6783ae.camel@linaro.org/ [1]
Signed-off-by: André Draszik <andre.draszik@...aro.org>
---
 Documentation/devicetree/bindings/serial/samsung_uart.yaml | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Documentation/devicetree/bindings/serial/samsung_uart.yaml b/Documentation/devicetree/bindings/serial/samsung_uart.yaml
index 0f0131026911..2435c3d92158 100644
--- a/Documentation/devicetree/bindings/serial/samsung_uart.yaml
+++ b/Documentation/devicetree/bindings/serial/samsung_uart.yaml
@@ -145,6 +145,20 @@ allOf:
         - samsung,uart-fifosize
       properties:
         reg-io-width: false
+        clocks:
+          description: |
+            Note that for earlycon to work, the respective ipclk and pclk need
+            to be running! The bootloader normally leaves them enabled, but the
+            serial driver will start handling those clocks before the console
+            driver takes over from earlycon, breaking earlycon. If earlycon is
+            required, please revert the patch "clk: samsung: gs101: don't mark
+            non-essential (UART) clocks critical" locally first to mark them
+            CLK_IS_CRITICAL and avoid this problem.
+          maxItems: 2
+        clock-names:
+          items:
+            - const: uart
+            - const: clk_uart_baud0
 
 unevaluatedProperties: false
 

-- 
2.45.2.803.g4e1b14247a-goog