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

Thomas Zimmermann
2023-04-29T12:26:23Z

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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@arm.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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