Re: deprecating fix->mmio_start and smem_start

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Tue, 22 Apr 2008, Benjamin Herrenschmidt wrote:
> We currently have a problem with those two members of struct
> fb_fix_screeninfo. The struct contains an "unsigned long" which means
> that:
> 
> - 64 bits kernels with 32 bits userspace can't pass a complete address
> - 32 bits machines with 64 bits resource_size_t can't pass a complete
> address
> - The structure isn't even properly padded to be 32/64 bits neutral in
> the first place.
> 
> We could define new versions of the struct with new get/set ioctls,
> or we could try to just deprecate those fields. What do you guys think ?

As userspace doesn't really need those fields[1], we can easily deprecate them.

> If we do the later, we need another way to convey the informations.
> 
> For smem, I'm not sure it's very useful, we should just be able to mmap
> the fbdev. The problem is more with mmio_start.
> 
> Thus the idea that we could do something to allow mmap'ing mmio via
> mmap of /dev/fb via some specific offset... what do you think ?

That's exactly what drivers/video/fbmem.c:fb_mmap() does[2].

But fb_mmap() does need the correct (resource_size_t) information.
That can be done by changing the offending fields in the kernel variant of
fb_fix_screeninfo to resource_size_t.

The user variant of fb_fix_screeninfo would stay the same, and only the
FBIOGET_FSCREENINFO case in drivers/video/fbmem.c:fb_ioctl() has to be
changed to convert from the kernel to the user variant.

> Or we can just do a fb_fix_screeninfo2, with proper padding and u64
> addresses, replace the kernel one with that, have translators for the
> old ioctl's, and new ioctl's.

I don't think the new ioctls are needed.

[1] Except when it wants to mmap /dev/mem using this info, but that can
    be done using [2].

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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

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