[PATCH] svgalib error in mmap documentation

[PATCH] svgalib error in mmap documentation

[Matan Ziv-Av]

Hi,

svgalib needs to be compiled without background support in order to run
under kernl 2.4.0 or newer. Here's a patch to Documentation/Changes that
says this.

--- /usr/src/linux.b/Documentation/Changes        Tue Dec 12 18:43:22 2000
+++ linux/Documentation/Changes     Sat Jan  6 10:28:20 2001
@@ -517,6 +517,14 @@
 Older isdn4k-utils versions don't support EXTRAVERSION into kernel version
 string. A upgrade to isdn4k-utils.v3.1beta7 or later is recomented.

+SVGAlib
+=======
+If you want svgalib programs to run with kernel 2.4.0 or newer, svgalib
+needs to be compiled without background support (BACKGROUND not defined in
+Makefile.cfg). This is relevant to any svgalib version.
+This is because svgalib uses mmap of /proc/mem to emulate vga's memory bank
+switching when in background, and kernel 2.4.0 stopped supporting this feature.
+


-- 
Matan Ziv-Av.                         matan@svgalib.org


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Re: [PATCH] svgalib error in mmap documentation

[Alan Cox]

> +If you want svgalib programs to run with kernel 2.4.0 or newer, svgalib
> +needs to be compiled without background support (BACKGROUND not defined in
> +Makefile.cfg). This is relevant to any svgalib version.
> +This is because svgalib uses mmap of /proc/mem to emulate vga's memory bank
> +switching when in background, and kernel 2.4.0 stopped supporting this feature.

2.4 has real support for shared mappings, so you can I suspect do it properly
now

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Re: [PATCH] svgalib error in mmap documentation

[Matan Ziv-Av]

On Sat, 6 Jan 2001, Alan Cox wrote:

> > +If you want svgalib programs to run with kernel 2.4.0 or newer, svgalib
> > +needs to be compiled without background support (BACKGROUND not defined in
> > +Makefile.cfg). This is relevant to any svgalib version.
> > +This is because svgalib uses mmap of/proc/mem to emulate vga's memory bank
> > +switching when in background, and kernel 2.4.0 stopped supporting this feature.
> 
> 2.4 has real support for shared mappings, so you can I suspect do it properly
> now

I hope it is reasonable to ask, how?

What I need is to allocate a big amount of memory (say 1MB, for
example), copy the video memory to it, and then have fixed 64K of
virutal address of the process point to any 64K window of the large
allocated memory. How can I do it?


-- 
Matan Ziv-Av.                         matan@svgalib.org

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Re: [PATCH] svgalib error in mmap documentation

[Alan Cox]

> I hope it is reasonable to ask, how?

POSIX shared memory calls or sys5 shm

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Re: [PATCH] svgalib error in mmap documentation

[Christoph Rohland]

Matan Ziv-Av <matan@svgalib.org> writes:

> I hope it is reasonable to ask, how?
> 
> What I need is to allocate a big amount of memory (say 1MB, for
> example), copy the video memory to it, and then have fixed 64K of
> virutal address of the process point to any 64K window of the large
> allocated memory. How can I do it?

fd = shm_open("vidmem-filename", O_CREAT,...);
ftruncate(fd, 1<<20);
ptr = mmap(0, 64 * 1<<10, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

to remap another area:

if (mmap(ptr, 64 * 1<<10, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_FIXED, 
         fd, blocknr * 64 * 1<<10) != ptr)
        error();

On exit:

munmap(ptr, 64 * 1<<10);
shm_unlink ("vidmem-filename");

Note the MAP_FIXED argument to the remap operation. You do not need to
unmap on Linux to remap an area when giving MAP_FIXED. (So MAP_FIXED
can to really bad things to your program...)

Greetings
                Christoph

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