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SubjectLinux 2.4.29-pre1 (was: Re: [PATCH] vga16fb: Fix frame buffer bad memory mapping)
On Thu, 25 Nov 2004, Linux Kernel Mailing List wrote:
> ChangeSet 1.1543, 2004/11/25 13:16:49-02:00, vince@arm.linux.org.uk
>
> [PATCH] vga16fb: Fix frame buffer bad memory mapping
>
> The vga16fb driver uses a direct ioremap on 0xa00000 to gain access to
> the vga card. This is wrong on architectures other than x86, every
> other driver uses VGA_MAP_MEM macro from vga.h to ensure the correct
> memory mapping.
>
> All this patch does is add the mapping macro this has been tested and
> works on ARM systems The driver no longer maps parts of kernel
> workspace and modifies it.

This fix is not correct!

> diff -Nru a/drivers/video/vga16fb.c b/drivers/video/vga16fb.c
> --- a/drivers/video/vga16fb.c 2004-11-25 15:01:51 -08:00
> +++ b/drivers/video/vga16fb.c 2004-11-25 15:01:51 -08:00
> @@ -142,7 +142,7 @@
> memset(fix, 0, sizeof(struct fb_fix_screeninfo));
> strcpy(fix->id,"VGA16 VGA");
>
> - fix->smem_start = VGA_FB_PHYS;
> + fix->smem_start = VGA_MAP_MEM(VGA_FB_PHYS);
> fix->smem_len = VGA_FB_PHYS_LEN;
> fix->type = FB_TYPE_VGA_PLANES;
> fix->visual = FB_VISUAL_PSEUDOCOLOR;

fix->smem_start: Although I agree VGA_FB_PHYS is not the correct value on
machines other than PC, VGA_MAP_MEM(VGA_FB_PHYS) is not appropriate either,
because fix->smem_start is supposed to be a CPU _physical_ address, not a
virtual address.

However, this value isn't really used, except by (very rare) userspace that
wants to mmap the frame buffer through /dev/mem instead of /dev/fb*, so an
incorrect value doesn't really harm.

> @@ -896,7 +896,7 @@
>
> /* XXX share VGA_FB_PHYS region with vgacon */
>
> - vga16fb.video_vbase = ioremap(VGA_FB_PHYS, VGA_FB_PHYS_LEN);
> + vga16fb.video_vbase = ioremap(VGA_MAP_MEM(VGA_FB_PHYS), VGA_FB_PHYS_LEN);
> if (!vga16fb.video_vbase) {
> printk(KERN_ERR "vga16fb: unable to map device\n");
> return -ENOMEM;

ioremap(): VGA_MAP_MEM() already a _virtual_ address:

| include/asm-alpha/vga.h:#define VGA_MAP_MEM(x) ((unsigned long) ioremap((x), 0))
| include/asm-arm/vga.h:#define VGA_MAP_MEM(x) (PCIMEM_BASE + (x))
| include/asm-i386/vga.h:#define VGA_MAP_MEM(x) (unsigned long)phys_to_virt(x)
| include/asm-ia64/vga.h:#define VGA_MAP_MEM(x) ((unsigned long) ioremap((x), 0))
| include/asm-mips/vga.h:#define VGA_MAP_MEM(x) ((unsigned long)0xb0000000 + (unsigned long)(x))
| include/asm-ppc/vga.h:#define VGA_MAP_MEM(x) (x + vgacon_remap_base)
| include/asm-ppc64/vga.h:#define VGA_MAP_MEM(x) ((unsigned long) ioremap((x), 0))
| include/asm-sparc64/vga.h:#define VGA_MAP_MEM(x) (x)
| include/asm-x86_64/vga.h:#define VGA_MAP_MEM(x) (unsigned long)phys_to_virt(x)

Doing a double ioremap(), or ioremap(phys_to_virt()) will break for sure...

BTW, on (PReP/CHRP) PPC ioremap() on ISA memory space `just works' because
__ioremap() adds _ISA_MEM_BASE to the passed pointer if it's smaller than
16*1024*1024. And yes, vga16fb used to work fine on my CHRP LongTrail
(before the machine itself died :-(.

Yes, ISA memory space is a mess to do right in a portable way...

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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