Re: [PATCH v3 5/6] fbdev: Move framebuffer I/O helpers into <asm/fb.h>

[Thomas Zimmermann]

Hi Sam

Am 02.05.23 um 22:03 schrieb Sam Ravnborg:
> Hi Thomas,
>
> On Tue, May 02, 2023 at 03:02:22PM +0200, Thomas Zimmermann wrote:
> > Implement framebuffer I/O helpers, such as fb_read*() and fb_write*(),
> > in the architecture's <asm/fb.h> header file or the generic one.
>
> In reality they are now all implemented in the generic one.
>
> > The common case has been the use of regular I/O functions, such as
> > __raw_readb() or memset_io(). A few architectures used plain system-
> > memory reads and writes. Sparc used helpers for its SBus.
> >
> > The architectures that used special cases provide the same code in
> > their __raw_*() I/O helpers. So the patch replaces this code with the
> > __raw_*() functions and moves it to <asm-generic/fb.h> for all
> > architectures.
> Which is also documented here.

The first paragraph documents the design intention, the other one the 
implementation. But I agree that it's not well phrased. I'll see if I 
can improve the text.

> > v3:
> > 	* implement all architectures with generic helpers
> > 	* support reordering and native byte order (Geert, Arnd)
> >
> > Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> > ---
> >   include/asm-generic/fb.h | 101 +++++++++++++++++++++++++++++++++++++++
> >   include/linux/fb.h       |  53 --------------------
> >   2 files changed, 101 insertions(+), 53 deletions(-)
> >
> > diff --git a/include/asm-generic/fb.h b/include/asm-generic/fb.h
> > index 6922dd248c51..0540eccdbeca 100644
> > --- a/include/asm-generic/fb.h
> > +++ b/include/asm-generic/fb.h
> > @@ -31,4 +31,105 @@ static inline int fb_is_primary_device(struct fb_info *info)
> >   }
> >   #endif
> >   
> > +/*
> > + * I/O helpers for the framebuffer. Prefer these functions over their
> > + * regular counterparts. The regular I/O functions provide in-order
> > + * access and swap bytes to/from little-endian ordering. Neither is
> > + * required for framebuffers. Instead, the helpers read and write
> > + * raw framebuffer data. Independent operations can be reordered for
> > + * improved performance.
> > + */
> > +
> > +#ifndef fb_readb
> > +static inline u8 fb_readb(const volatile void __iomem *addr)
> > +{
> > +	return __raw_readb(addr);
> > +}
> > +#define fb_readb fb_readb
> > +#endif
>
> When we need to provide an architecture specific variant the
> #ifndef foo
> ...
> #define foo foo
> can be added. Right now it is just noise as no architectures provide
> their own variants.

Given Arnd's comments, this might change. It also makes sense from a 
design POV.

> But I am missing something somewhere as I cannot see how this builds.
> asm-generic now provide the fb_read/fb_write helpers.
> But for example sparc has an architecture specifc fb.h so it will not
> use the asm-generic variant. So I wonder how sparc get hold of the
> asm-generic fb.h file?

All architecture's <asm/fb.h> files include <asm-generic/fb.h>, so that 
they all get the interfaces which they don't define themselves. For 
Sparc, this is at [1].

Best regards
Thomas


[1] 
https://cgit.freedesktop.org/drm/drm-tip/tree/arch/sparc/include/asm/fb.h#n19

> Maybe it is obvious, but I miss it.
>
> 	Sam

--
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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