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

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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@xxxxxxx> 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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