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

Arnd Bergmann
2023-04-28T13:17:34Z

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

Arnd