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

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Message-Id: <855dfa50-a3a3-415a-8612-4da59f995ef0@app.fastmail.com>
Date:   Fri, 01 Dec 2023 16:31:38 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Nishanth Menon" <nm@...com>, "Tony Lindgren" <tony@...mide.com>
Cc:     "Francesco Dolcini" <francesco@...cini.it>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        "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 Fri, Dec 1, 2023, at 07:49, Nishanth Menon wrote:
> 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 don't recall any reason to have the limit set to the default
of 4, other than possibly using excessive amounts of .data in
vmlinux, but we have other serial port drivers that just hardcode
a much larger value.

    Arnd