linux-kernel - Re: [PATCH v6 5/6] fbdev: Move framebuffer I/O helpers into <asm/fb.h>

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Message-Id: <4976d32d-b73f-49e5-9e15-78786d77dc8f@app.fastmail.com>
Date:   Thu, 11 May 2023 14:48:58 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Geert Uytterhoeven" <geert@...ux-m68k.org>
Cc:     "Thomas Zimmermann" <tzimmermann@...e.de>,
        "kernel test robot" <lkp@...el.com>,
        "Helge Deller" <deller@....de>,
        "Javier Martinez Canillas" <javierm@...hat.com>,
        "Daniel Vetter" <daniel@...ll.ch>,
        "Vineet Gupta" <vgupta@...nel.org>,
        "Huacai Chen" <chenhuacai@...nel.org>,
        "WANG Xuerui" <kernel@...0n.name>,
        "David S . Miller" <davem@...emloft.net>,
        "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
        "Sam Ravnborg" <sam@...nborg.org>, suijingfeng@...ngson.cn,
        oe-kbuild-all@...ts.linux.dev,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-fbdev@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-m68k@...ts.linux-m68k.org,
        loongarch@...ts.linux.dev, sparclinux@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        "Artur Rojek" <contact@...ur-rojek.eu>
Subject: Re: [PATCH v6 5/6] fbdev: Move framebuffer I/O helpers into <asm/fb.h>

On Thu, May 11, 2023, at 14:35, Geert Uytterhoeven wrote:
> CC Artur, who's working on HP Jornada 680.
>
> On Wed, May 10, 2023 at 5:55 PM Arnd Bergmann <arnd@...db.de> wrote:
>> On Wed, May 10, 2023, at 16:27, Thomas Zimmermann wrote:
>> > Am 10.05.23 um 16:15 schrieb Arnd Bergmann:
>> >> On Wed, May 10, 2023, at 16:03, kernel test robot wrote:
>
> See also commit 4aafae27d0ce73f8 ("sh: hd64461 tidying."), which
> claims they are no longer needed.
>
> Don't the I/O port macros just treat the port as an absolute base address
> when sh_io_port_base isn't set?

As far as I can tell, sh_io_port_base gets initialized to '-1'
specifically to prevent that from working by accident. So it's
almost treated as an absolute base address, but the off-by-one
offset ensures this never actually works unless it was first
set to the correct value.

      Arnd