linux-kernel - [PATCH v1 RFC 0/3] meson-uart: Use "divide XTAL by 2" bit on G12A

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Message-Id: <20230222210425.626474-1-martin.blumenstingl@googlemail.com>
Date:   Wed, 22 Feb 2023 22:04:22 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     linux-amlogic@...ts.infradead.org, devicetree@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        jirislaby@...nel.org, neil.armstrong@...aro.org,
        krzysztof.kozlowski+dt@...aro.org, robh+dt@...nel.org,
        gregkh@...uxfoundation.org,
        Christian Hewitt <christianshewitt@...il.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Subject: [PATCH v1 RFC 0/3] meson-uart: Use "divide XTAL by 2" bit on G12A

This series improves support for UART attached Bluetooth modules on
Amlogic Meson G12A and newer SoCs. These SoCs also support the "divide
XTAL by 2" bit which (greatly) reduces jitter when generating baud
rates such as 1500000 (which is used by the Bluetooth part of the
RTL8822CS SDIO WiFi and UART Bluetooth combo chip).

Without this the baud rate calculation is based on the XTAL clock
(running at 24MHz) divided by 3 (meaning: 8MHz). 8MHz cannot be divided
with integer division to a 1500000 baud rate. Using the "divide XTAL
by 2" bit however means that we can achieve 1500000 cleanly, without any
jitter.

In future we should allow dynamic switching of these UART controller
internal dividers to pick the best divider automatically for the
requested baud rate. This however still requires the new compatible
string - which is added by this series - to enable the "divide XTAL
by 2" logic on SoCs that support it (G12A and newer).

Why am I sending this as RFC? The last change in this series means
that the resulting .dtbs are not compatible with old kernels anymore.
My understanding is that this is fine and only the opposite case (using
old .dtbs on new kernels) has to be supported (which is still the case
with this series). I'd like to get some confirmation for this.

[0] https://lore.kernel.org/linux-bluetooth/3B9D4DB2-D2CD-44FE-817A-F6EA8A0AD734@gmail.com/

Martin Blumenstingl (3):
  dt-bindings: serial: amlogic,meson-uart: Add compatible string for
    G12A
  tty: serial: meson: Add a new compatible string for the G12A SoC
  arm64: dts: meson-g12-common: Use the G12A UART compatible string

 .../devicetree/bindings/serial/amlogic,meson-uart.yaml |  2 ++
 arch/arm64/boot/dts/amlogic/meson-g12-common.dtsi      | 10 +++++-----
 drivers/tty/serial/meson_uart.c                        |  8 ++++++--
 3 files changed, 13 insertions(+), 7 deletions(-)

-- 
2.39.2