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

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Date:   Fri, 28 Apr 2023 14:27:05 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>, deller@....de,
        javierm@...hat.com, daniel@...ll.ch, vgupta@...nel.org,
        chenhuacai@...nel.org, kernel@...0n.name, davem@...emloft.net,
        James.Bottomley@...senpartnership.com, arnd@...db.de,
        sam@...nborg.org, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, 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 2:18 PM Robin Murphy <robin.murphy@....com> wrote:
> On 2023-04-28 10:27, Thomas Zimmermann wrote:
> > Implement framebuffer I/O helpers, such as fb_read*() and fb_write*()
> > with Linux' regular I/O functions. Remove all ifdef cases for the
> > various architectures.
> >
> > Most of the supported architectures use __raw_() I/O functions or treat
> > framebuffer memory like regular memory. This is also implemented by the
> > architectures' I/O function, so we can use them instead.
> >
> > Sparc uses SBus to connect to framebuffer devices. It provides respective
> > implementations of the framebuffer I/O helpers. The involved sbus_()
> > I/O helpers map to the same code as Sparc's regular I/O functions. As
> > with other platforms, we can use those instead.
> >
> > We leave a TODO item to replace all fb_() functions with their regular
> > I/O counterparts throughout the fbdev drivers.
> >
> > Signed-off-by: Thomas Zimmermann <tzimmermann@...e.de>
> > ---
> >   include/linux/fb.h | 63 +++++++++++-----------------------------------
> >   1 file changed, 15 insertions(+), 48 deletions(-)
> >
> > diff --git a/include/linux/fb.h b/include/linux/fb.h
> > index 08cb47da71f8..4aa9e90edd17 100644
> > --- a/include/linux/fb.h
> > +++ b/include/linux/fb.h
> > @@ -15,7 +15,6 @@
> >   #include <linux/list.h>
> >   #include <linux/backlight.h>
> >   #include <linux/slab.h>
> > -#include <asm/io.h>
> >
> >   struct vm_area_struct;
> >   struct fb_info;
> > @@ -511,58 +510,26 @@ struct fb_info {
> >    */
> >   #define STUPID_ACCELF_TEXT_SHIT
> >
> > -// This will go away
> > -#if defined(__sparc__)
> > -
> > -/* We map all of our framebuffers such that big-endian accesses
> > - * are what we want, so the following is sufficient.
> > +/*
> > + * TODO: Update fbdev drivers to call the I/O helpers directly and
> > + *       remove the fb_() tokens.
> >    */
> > -
> > -// This will go away
> > -#define fb_readb sbus_readb
> > -#define fb_readw sbus_readw
> > -#define fb_readl sbus_readl
> > -#define fb_readq sbus_readq
> > -#define fb_writeb sbus_writeb
> > -#define fb_writew sbus_writew
> > -#define fb_writel sbus_writel
> > -#define fb_writeq sbus_writeq
> > -#define fb_memset sbus_memset_io
> > -#define fb_memcpy_fromfb sbus_memcpy_fromio
> > -#define fb_memcpy_tofb sbus_memcpy_toio
> > -
> > -#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.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds