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

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Date:   Sat, 29 Apr 2023 14:26:12 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Robin Murphy <robin.murphy@....com>
Cc:     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

Hi

Am 28.04.23 um 15:17 schrieb Arnd Bergmann:
> 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

After thinking a bit more about the requirements, I'd like to got back 
to v1, but with a different spin. We want to avoid ordering guarantees, 
so I looked at the _relaxed() helpers, but they seem to swap bytes to 
little endian.

I guess we can remove the fb_mem*() functions entirely. They are the 
same as the non-fb_ counterparts. For the fb read/write helpers, I'd 
like to add them to <asm-generic/fb.h> in a platform-neutral way. They'd 
be wrappers around __raw_(), as I wouldn't want invocations of  __raw_() 
functions in the fbdev drivers.

Best regards
Thomas

> 
>        Arnd

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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