Subject: Re: Frame buffer / mmap() weirdness
From: Geert Uytterhoeven (Geert.Uytterhoeven@sonycom.com)
Date: Wed Dec 01 1999 - 00:28:26 MST
On Tue, 30 Nov 1999, Stephen Edie wrote:
> According to fbset, it's at: 0x8a008900
>
> I don't want to map /dev/mem if that's what you are talking about.
I did meant /dev/fb0, but I indeed incorrectly wrote `address & 0xfffff000',
while I meant `0' (offsets are relative to the start of the frame buffer
memory).
> So I am assuming that mmap() will start mapping from the first page that
> he frame buffer occupies... So I need to add the following to what
Yes.
> mmap() returns in order to get a valid pointer:
>
> PAGE_ROUND_UP(phys_address) - phys_address
Yes.
> (PAGE_ROUND_UP is assumed to return the beginning of the page that starts
> at or after the specified address)
>
> This doesn't entirely make sense. Just now I tested the program and it
> doesn't require re-alignment as I mention above.
>
> (I'm thinking now that it only occurs in 32 bit color mode??)
That's possible: on some hardware, the start address of the frame buffer
depends on the color depth.
> Furthermore, it makes most sense that mmap() should take care of page
> alignment issues for me so that the address returned by mmap() always
> points to the data at [lseek(h, 0, SEEK_SET)]. This is not technically
> difficult to implement, right?
I think it will make munmap() more difficult to implement. Furthermore I see no
provisions for this in the current mmap() code. And it will break backwards
compatibility.
> On Tue, 30 Nov 1999, Geert Uytterhoeven wrote:
> > Perhaps the frame buffer memory is not aligned on a page boundary? What does
> > `fbset --info' say about the address?
> >
> > If it's not aligned to a page boundary, the application has to mmap()
> > (address & 0xfffff000) and add (address & 0x00000fff) to the start address of
> > the returned region. Make sure to increase the size you pass to mmap(), so the
> > whole frame buffer memory is covered and size is a multiple of the page size.
Gr{oetje,eeting}s,
Geert
-- Geert Uytterhoeven ----------------- Sony Suprastructure Center Europe (SUPC-E) Geert.Uytterhoeven@sonycom.com ------------------- Sint-Stevens-Woluwestraat 55 Voice +32-2-7248632 Fax +32-2-7262686 ---------------- B-1130 Brussels, Belgium
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