Message-Id: <199911011050.SAA01756@eastgate.cyberway.com.sg>
From: "Rob Kramer" <robk AT cyberway DOT com DOT sg>
To: djgpp AT delorie DOT com
Date: Mon, 1 Nov 1999 18:50:44 +0800
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Subject: DMA despair.
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Reply-To: djgpp AT delorie DOT com

Hi all,

I'm trying to get DMA to work for a hardware MPEG decoder. A rather
large buffer is needed (256k) to allow some slack in data feeding. The
MPEG chip has it's own DMA controller that expects a 32bit physical
address of the DMA buffer in one of its registers. Page boundaries are
supposedly not a problem, all it requires is that bits 0 and 1 of the
address are zero.

Now I've tried just about everything, to no avail. I'm afraid I'm
missing something completely trivial. I tried:

- CR3 trick by Neil Jarvis (18-09-96),
- XMS trick by Neil Jarvis (13-12-95),
- DOS buffer with mapping to linear address space,
- Plain DOS buffer.

I'm not using swapping at all, cwsdpmi is configured to 0kb swap. For
the cr3 trick I use cwsdpr0. For the easiest method (DOS buffer), I've
go the following code:

ulong dma::allocateBuffer (ulong size)
{
    ulong               pages       = (size / 0x1000) + 2;
    ulong               paragraphs  = pages * (0x1000 / 0x10);
    ulong               paddress;
    _go32_dpmi_seginfo  seginfo;

    /* Size must be page aligned */
    if (size & 0xfff)
        return 0;

    if (allocated)
        freeBuffer();

    seginfo.size = paragraphs;

    if (_go32_dpmi_allocate_dos_memory (&seginfo) != -1) {
        paddress = seginfo.rm_segment << 4;

        return paddress;
    }

    return 0;
}


After that, I use the buffer with dosmemput(), and give 'paddress' to
the MPEG DMA controller.  I take it I can completely ignore the
selector returned in seginfo.pm_selector?

Can anyone see something wrong with this? My hardware totally ignores
whatever I copy into the buffer. Physical address equals linear address
in this case, right?

Thanks,

        Rob Kramer
        robk AT cyberway DOT com DOT sg