linux-kernel - Re: [PATCH 3/3] arm64: dts: rockchip: Enable UART8 on rock-3b

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Message-ID: <CA+GksrJjDPve29Vh7ZFhM+JFp058xmXZAPeuLuFth7v=JeiH2w@mail.gmail.com>
Date: Tue, 12 Nov 2024 22:04:43 +0100
From: Tamás Szűcs <tszucs@...ux.com>
To: Dragan Simic <dsimic@...jaro.org>
Cc: Tamás Szűcs <tszucs@...ux.com>, 
	Jonas Karlman <jonas@...boo.se>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Heiko Stuebner <heiko@...ech.de>, FUKAUMI Naoki <naoki@...xa.com>, Chukun Pan <amadeus@....edu.cn>, 
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] arm64: dts: rockchip: Enable UART8 on rock-3b

Hi Dragan,

On Tue, Nov 12, 2024 at 4:07 PM Dragan Simic <dsimic@...jaro.org> wrote:
> Please correct me if I'm wrong, but isn't this UART supposed to be
> used for the Bluetooth part of an SDIO WiFi + Bluetooth module, in
> form of a non-standard M.2 module that Radxa sells?

UART8 is supposed to be used for any radio module connected to the M2E
connector.
It will typically be responsible for Bluetooth or BLE but it could be
802.15.4 or whatever. In any case, all wanting to use it will need the
uart8 node enabled.

>
> With that in mind, I see very little sense in just enabling the UART,
> without defining the entire Bluetooth interface, which AFAIK produces

Defining a bluetooth node would hardwire idiosyncrasies of a given
radio module's Bluetooth core. Sure you could add a sleep clock, all
kind of sideband signals for wakeups, reset, power down, etc. But hey,
some will use them, some won't. I think it's undesirable and
unnecessary. You can hciattach from here and most will work just like
that. Tighter integration or anything special, module specific on top
should be handled individially, on a case-by-case basis. This is a dev
board after all. I say trick of all trades.

> nasty looking error messages in the kernel log when there's actually
> nothing connected to the UART.

My dmesg is clean as a whistle
root@...k-3b:~# dmesg | grep -E 'fe6c0000|ttyS0'
[    0.344818] fe6c0000.serial: ttyS0 at MMIO 0xfe6c0000 (irq = 26,
base_baud = 1500000) is a 16550A
What kind of nasty errors do you recall?

Kind regards,
Tamas