linux-kernel - Re: [PATCH v2] arm64: defconfig: increase SERIAL_8250_NR

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Message-ID: <20231201232614.jvj4d5nu7j2xf5ia@ability>
Date:   Fri, 1 Dec 2023 17:26:14 -0600
From:   Nishanth Menon <nm@...com>
To:     Francesco Dolcini <francesco@...cini.it>
CC:     <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>,
        Tony Lindgren <tony@...mide.com>
Subject: Re: [PATCH v2] arm64: defconfig: increase SERIAL_8250_NR_UARTS

On 20:52-20231201, Francesco Dolcini wrote:
> On Fri, Dec 01, 2023 at 01:19:58PM -0600, Nishanth Menon wrote:
> > On 18:15-20231201, Francesco Dolcini wrote:
> > > 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").
> > > 
> > > The need to increase this value was noticed while working with Toradex
> > > Verdin AM62, this board has 4 serial UART instances available to the user
> > > plus an internal one that is connected to a Bluetooth module. Without this
> > > change the fifth UART connected to the BT module is not instantiated and BT
> > > is not working.
> > > 
> > > Instead of increasing the number to the bare minimum (5) that would be
> > > required to solve this specific issue, we increase this to 8 which seems a
> > > more reasonable number to have in the defconfig and should cover more valid
> > > use cases.
> > 
> > To address Arnd's concern on size increase, it will be good to add:
> 
> I can and I will add it in a v3 - it takes less time to do it than reply to
> this email and thanks for taking the time to provide the actual data.
> 
> With that said my understanding is that the goal of the arm64 defconfig is
> to enable the supported arm64 hardware and the related kernel development.
> It's not supposed to be a minimal config in size nor an optimal
> configuration from the performance point of view. It's a single
> configuration that includes support for each and every platform [1]
> supported by Linux arm64 ...
> 
> To me Arnd was just stating a fact, not raising a concern that was supposed
> to be addressed (just correct me if I'm wrong, of course).
> 
> [1] well, apart AMD Pensando and Bitmain, at the moment, but you get the
> point, I'm sure ;-).
> 

Hehe, thanks Francesco.

Arnd, my memory was a bit right on the topic though.. .. I think we
had gone down a similar road before in trying to increase the number
of 8250 UARTs [1] but without this strong a reason.. Been a while, I
think our v3 is much stronger case now.

[1] https://lore.kernel.org/all/CAK8P3a2VSBvOn1o+q1PYZaQ6LS9U4cz+DZGuDbisHkwNs2dAAw@mail.gmail.com/#t
-- 
Regards,
Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3  1A34 DDB5 849D 1736 249D