linux-kernel - Re: [PATCH] arm64: defconfig: increase SERIAL_8250_NR

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Date:   Fri, 1 Dec 2023 00:49:19 -0600
From:   Nishanth Menon <nm@...com>
To:     Tony Lindgren <tony@...mide.com>
CC:     Francesco Dolcini <francesco@...cini.it>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Tero Kristo <kristo@...nel.org>
Subject: Re: [PATCH] arm64: defconfig: increase SERIAL_8250_NR_UARTS

On 08:03-20231201, Tony Lindgren wrote:
> * Francesco Dolcini <francesco@...cini.it> [231130 23:17]:
> > Increase CONFIG_SERIAL_8250_NR_UARTS from 4 to 8, the current legacy value
> > is not adequate for embedded systems that use SoCs where it's common to
> > have a large number of serial ports.
> > 
> > No need to change CONFIG_SERIAL_8250_RUNTIME_UARTS, see commit 9d86719f8769
> > ("serial: 8250: Allow using ports higher than SERIAL_8250_RUNTIME_UARTS").
> > 
> > This enables using the UART connected Bluetooth device on Verdin AM62
> > board.
> 
> OK makes sense for distro use.
> 
> Disabling unused ports leads into port names shifting, which we still can't
> easily tolerate until we have the DEVNAME:0.0 style addressing available for
> ports. So for now we still depend CONFIG_SERIAL_8250_NR_UARTS, eventually
> that too should become just a legacy ISA port array.. Meanwhile:
> 
> Reviewed-by: Tony Lindgren <tony@...mide.com>

I'd prefer to get Arnd's view on the topic as well (I kind of
recollect some historic discussion which I am not failing to trace
that there usage model doesn't exceed 4 and aliases could be used to
map these as required for the platform). The 8250 debate has been
popping on and off over the years.. Sigh.. memories of [1] still haunt
me.

I assume you are talking of
arch/arm64/boot/dts/ti/k3-am62-verdin-wifi.dtsi which in turn uses
arch/arm64/boot/dts/ti/k3-am62-verdin.dtsi (5 uarts) and that is where the
increase in serial port is coming from.

Now 8 or 5 (which seems to be the relevant need) is subjective :(


[1] https://lore.kernel.org/linux-arm-kernel/Y3x%2FcGYWLLB+J2zU@atomide.com/

-- 
Regards,
Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3  1A34 DDB5 849D 1736 249D