Subject: Re: [PATCH] b44 1GB DMA workaround (was: b44: add 47xx support)

From: Pekka Pietikainen <[email-masked]>
Date: Sun, 8 Aug 2004 01:40:19 +0300

On Tue, Aug 03, 2004 at 06:39:19PM -0700, David S. Miller wrote:
> Changing skb->data is not legal.  Please implement this in
> such a way that skb->data does not get modified.  By modifying
> skb->data you will break things such as packet sniffers and
> netfilter, and that's just the tip of the iceberg. :-)
> 
Haven't noticed any breakage (tm) but I'm just a x86 weenie :-)

Current approach is:

        if(1 (just for testing ;) ) || mapping+len > B44_DMA_MASK) {
                /* Chip can't handle DMA to/from >1GB, use bounce buffer */
                pci_unmap_single(bp->pdev, mapping, len,PCI_DMA_TODEVICE);
                memcpy(bp->tx_bufs+entry*TX_PKT_BUF_SZ,skb->data,skb->len);
                mapping = pci_map_single(bp->pdev,bp->tx_bufs+entry*TX_PKT_BUF_SZ, len, PCI_DMA_TODEVICE);
        }

Which also works (tm). Setting the skb to a special value seems a bit tricky
as skb->len is used in b44_tx for that nop^H^H^Hpci_unmap_single. It looks
to me as the right things when the code gets changed as above (even for
archs where the unmapping is not a nop) get done, but I could easily be
missing something.

-- 
Pekka Pietikainen