linux-kernel - Re: [PATCH] arm64: dts: rockchip: nanopi4: decrease Bluetooth UART baud rate

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Date:   Wed, 6 Oct 2021 11:49:20 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Chen-Yu Tsai <wens@...nel.org>, Heiko Stuebner <heiko@...ech.de>
Cc:     Chen-Yu Tsai <wens@...e.org>, linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: rockchip: nanopi4: decrease Bluetooth UART
 baud rate

On 2021-09-20 18:56, Chen-Yu Tsai wrote:
> From: Chen-Yu Tsai <wens@...e.org>
> 
> The RK3399 does not seem to be able to properly generate the required
> 64 MHz clock for the UART to operate at 4MBd.
> 
> Drop the baud rate down to 3MBd, which can be used as the clock
> controller is able to produce a 48 MHz clock.

Hmm, I've been running mine this way (with DMA) for ages now :/

Looking at clk_summary, clk_uart0_src ends up at 800MHz off CPLL (same 
as several other significant clocks), with clk_uart0 at an exact 64MHz 
as a division of that. I stuck a scope on the UART pins of the module 
and all the edges look nicely lined up to 250ns intervals.

This is with a 5.11.4 kernel, though - I wonder if the recent fractional 
divider changes in the clock driver have changed anything?

Robin.

> Fixes: 3e2f0bb72be3 ("arm64: dts: rockchip: Add nanopi4 bluetooth")
> Signed-off-by: Chen-Yu Tsai <wens@...e.org>
> ---
>   arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi b/arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi
> index 8c0ff6c96e03..45ff053b119d 100644
> --- a/arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi
> +++ b/arch/arm64/boot/dts/rockchip/rk3399-nanopi4.dtsi
> @@ -699,7 +699,7 @@ bluetooth {
>   		device-wakeup-gpios = <&gpio2 RK_PD2 GPIO_ACTIVE_HIGH>;
>   		host-wakeup-gpios = <&gpio0 RK_PA4 GPIO_ACTIVE_HIGH>;
>   		shutdown-gpios = <&gpio0 RK_PB1 GPIO_ACTIVE_HIGH>;
> -		max-speed = <4000000>;
> +		max-speed = <3000000>;
>   		pinctrl-names = "default";
>   		pinctrl-0 = <&bt_reg_on_h &bt_host_wake_l &bt_wake_l>;
>   		vbat-supply = <&vcc3v3_sys>;
>