Subject: Re: Frame buffer / mmap() weirdness
From: Gabriel Paubert (paubert@iram.es)
Date: Wed Dec 01 1999 - 06:02:17 MST
On Wed, 1 Dec 1999, Momchil Velikov wrote:
> The POSIX I have (POSIX 1003.1b-1993) allows the implementation
> to require the off parameter to be a multiple of the page size.
> But:
> "NOTE: It is expected that a later amendment of this standard will
> disallow the implementation from imposing the restriction on the
> alignment of the off and addr arguments".
> Anyway, whatever is the offset it is still required that
> pa[i] == [lseek(fildes, off+i, SEEK_SET)] for each i : 0 <= i < len
> after a successful
> pa = mmap( addr, len, prot, flags, fildes, off)
>
> Frankly speaking, I can't imagine why addresses returned by mmap()
> have to be page aligned. If the frame buffer at address range
> [a, a+off) is to be mapped at [b, b+off), just map the range
> [a & ~PAGE_MASK, (a + off + PAGE_SIZE - 1) & ~PAGE_SIZE)
> to
> [b & ~PAGE_MASK, (b + off + PAGE_SIZE - 1) & ~PAGE_SIZE)
Some PAGE_SIZE are actually PAGE_MASK in this snippet...
> and give the user the value b. Upon munmap() mask the lower
> bits of the address and you'll get the actual start address
> of the mapping. Note that this can be implemented in the
> library even if the kernel imposes restrictions on the
> alignment of the addr and off parameters.
You can do anything when mapping a file privately (although being block
aligned certainly helps), not when mapping hardware or a file with write
sharing. At least the low order bits of a and b have to be the same
(that's only a problem when using MAP_FIXED).
Regards,
Gabriel.
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