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

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Message-ID: <0eb19e4daf2cdf3d4a04935876c3d3b0@manjaro.org>
Date: Wed, 13 Nov 2024 00:25:28 +0100
From: Dragan Simic <dsimic@...jaro.org>
To: Tamás Szűcs <tszucs@...ux.com>
Cc: 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

Hello Tamas,

On 2024-11-12 22:04, Tamás Szűcs wrote:
> 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.

I see, but I'm still guessing what's the actual use of enabling the
UART8 when it will remain pretty much useless without the additional
DT configuration, such as in the WiFi+Bluetooth DT overlay that Jonas
sent a bit earlier?

I think that the UART8 should be enabled together with something that
actually makes use of it, which in this case unfortunately cannot be
automatically detected and configured, so it belongs to a DT overlay.
I'll get back to this in my next response.

>> 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?

Those would be the kernel error messages produced with the Bluetooth
DT configuration in place, but with no SDIO module installed.