linux-kernel - Re: [PATCH v2 5/5] fbdev: Define framebuffer I/O from Linux' I/O functions

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Message-Id: <f612c682-5767-4a58-82f6-f4a4d1b592a1@app.fastmail.com>
Date:   Fri, 28 Apr 2023 14:17:24 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Geert Uytterhoeven" <geert@...ux-m68k.org>,
        "Robin Murphy" <robin.murphy@....com>
Cc:     "Thomas Zimmermann" <tzimmermann@...e.de>,
        "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>, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-ia64@...r.kernel.org, loongarch@...ts.linux.dev,
        linux-m68k@...ts.linux-m68k.org, sparclinux@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-parisc@...r.kernel.org
Subject: Re: [PATCH v2 5/5] fbdev: Define framebuffer I/O from Linux' I/O functions

On Fri, Apr 28, 2023, at 13:27, Geert Uytterhoeven wrote:
> On Fri, Apr 28, 2023 at 2:18 PM Robin Murphy <robin.murphy@....com> wrote:
>> On 2023-04-28 10:27, Thomas Zimmermann wrote:

>> > -
>> > -#elif defined(__i386__) || defined(__alpha__) || defined(__x86_64__) ||      \
>> > -     defined(__hppa__) || defined(__sh__) || defined(__powerpc__) || \
>> > -     defined(__arm__) || defined(__aarch64__) || defined(__mips__)
>> > -
>> > -#define fb_readb __raw_readb
>> > -#define fb_readw __raw_readw
>> > -#define fb_readl __raw_readl
>> > -#define fb_readq __raw_readq
>> > -#define fb_writeb __raw_writeb
>> > -#define fb_writew __raw_writew
>> > -#define fb_writel __raw_writel
>> > -#define fb_writeq __raw_writeq
>>
>> Note that on at least some architectures, the __raw variants are
>> native-endian, whereas the regular accessors are explicitly
>> little-endian, so there is a slight risk of inadvertently changing
>> behaviour on big-endian systems (MIPS most likely, but a few old ARM
>> platforms run BE as well).
>
> Also on m68k, when ISA or PCI are enabled.
>
> In addition, the non-raw variants may do some extras to guarantee
> ordering, which you do not need on a frame buffer.
>
> So I'd go for the __raw_*() variants everywhere.

The only implementations in fbdev are

 1) sparc sbus
 2) __raw_writel
 3) direct pointer dereference

But none use the byte-swapping writel() implementations, and
the only ones that use the direct pointer dereference or sbus
are the ones on which these are defined the same as __raw_writel

      Arnd