linux-kernel - Re: [PATCH v2 23/25] tty: serial: samsung_tty: Add earlycon support for Apple UARTs

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Date:   Tue, 16 Feb 2021 11:18:56 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Hector Martin <marcan@...can.st>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <maz@...nel.org>, Rob Herring <robh@...nel.org>,
        Olof Johansson <olof@...om.net>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        Tony Lindgren <tony@...mide.com>,
        Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        Stan Skowronek <stan@...ellium.com>,
        Alexander Graf <graf@...zon.com>,
        Will Deacon <will@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        DTML <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 23/25] tty: serial: samsung_tty: Add earlycon support
 for Apple UARTs

On Mon, Feb 15, 2021 at 8:19 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
> On Mon, Feb 15, 2021 at 09:17:11PM +0900, Hector Martin wrote:

> > +
> > +/* Apple S5L */
> > +static int __init apple_s5l_early_console_setup(struct earlycon_device *device,
> > +                                             const char *opt)
> > +{
> > +     /* Close enough to S3C2410 for earlycon... */
> > +     device->port.private_data = &s3c2410_early_console_data;
> > +
> > +#ifdef CONFIG_ARM64
>
> if IS_ENABLED()
> (unless it cannot be used due to missing symbol?)
>
> > +     /* ... but we need to override the existing fixmap entry as nGnRnE */
> > +     __set_fixmap(FIX_EARLYCON_MEM_BASE, device->port.mapbase,
> > +                  __pgprot(PROT_DEVICE_nGnRnE));
> > +#endif

It has to be a preprocessor conditional because PROT_DEVICE_nGnRnE
is only defined on arm64. We could add a FIXMAP_PAGE_NONPOSTED
alias for it that defaults to FIXMAP_PAGE_IO, but in the end this is
really an architecture specific thing and I think leaving it guarded by
the architecture is appropriate.

> > +     return samsung_early_console_setup(device, opt);
>
> Don't you need to handle the error code - set PROT_DEFAULT() or whatever
> was there before?

__set_fixmap() has no return value, it just writes a page table entry and
does not fail.

      Arnd