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Date: Mon, 26 Mar 2007 20:05:52 +0200
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From: Marcel Block <marcelblock@googlemail.com>
Subject: [Qemu-devel] Ne2000 bug?, buffer full/empty
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To: qemu-devel@nongnu.org

Hi qemu-devers,

this is my first post here, so please be kind :-)

i believe i found a bug in qemu's ne2000 code. i'm currently working on 
a ne2000 driver for my little hobby-os and i'm having some troubles 
getting it to receive any packets when running on qemu. but it works 
fine on bochs and on _real_ ne2000-hardware (rtl8029-pci in my case).

i believe the case "current page register == boundary register" should 
be interpreted as 'buffer empty' instead of 'buffer full'. this was 
changed last year or so by dr. han zhu, i tried to send him a email but 
it looks like the email-address he used back then is no longer available.

the current code looks like this: (hw/ne2000.c):
static int ne2000_buffer_full(NE2000State *s)
{
    int avail, index, boundary;

    index = s->curpag << 8;
    boundary = s->boundary << 8;
    if (index <= boundary)
        avail = boundary - index;
    else
        avail = (s->stop - s->start) - (index - boundary);
    if (avail < (MAX_ETH_FRAME_SIZE + 4))
        return 1;
    return 0;
}

that way qemu thinks the buffer is full if index==boundary. if i change 
the "if(index <= boundary)" back to "if(index < boundary)" my driver 
appears to work fine.

all the specs i read so far tend to initialize those two registers to 
the same value. so it appears logical that index==boundary should be 
interpreted as 'buffer empty'.

i'm waiting for your feedback :-)

best regards,
marcel block