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

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Date:   Fri, 1 Dec 2023 09:48:09 +0100
From:   Francesco Dolcini <francesco@...cini.it>
To:     Nishanth Menon <nm@...com>
Cc:     Tony Lindgren <tony@...mide.com>,
        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 Fri, Dec 01, 2023 at 12:49:19AM -0600, 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 usage model that triggered this change exceed 4, we >4 uart in
use at the same time.

And the issue is not really "distro use", and I do not think anything
about aliases matter (the BT device is described as a child of the uart
in the DT).

  &main_uart5 {
    bluetooth {
      compatible = "nxp,88w8987-bt";
    };
  };

The change here is required to have basic hardware functionalities
working for kernel development and debugging using the in-tree
defconfig.


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

Correct.

Francesco