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Date: Wed, 15 Aug 2001 05:35:24 -0700 (PDT)
Message-Id: <20010815.053524.48804759.davem@redhat.com>
To: axboe@suse.de
Cc: linux-kernel@vger.kernel.org, andrea@suse.de
Subject: Re: [patch] zero-bounce highmem I/O
From: "David S. Miller" <davem@redhat.com>
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   From: Jens Axboe <axboe@suse.de>
   Date: Wed, 15 Aug 2001 14:07:40 +0200

   Ok so you just want to turn scatterlist into what I call sg_list in 2.5
   time, fine with me too. Depends on whether we want to keep the
   pci_map_sg and struct scatterlist interface intact, or just break it and
   tell driver authors they must fix their stuff regardless of whether they
   want to support highmem. As I write this sentence, it's clear to me
   which way is the superior :-)
   
pci_map_sg is pci_map_sg, if the internal representation of
scatterlist is changed such that address/alt_address no longer exist,
it will work on pages only.  Right?  The compatibility mode in
2.4.x is the "if (address != NULL) virt_to_bus(address);" stuff.

Understand that the pci64_{map,unmap}_sg is created for a seperate
purpose, independant of whether scatterlist has the backwards
compatability stuff or not.  (There have been threads here about this,
I can describe it quickly for you in quiet if you want to know).

Two more things to consider:

1) There is nobody who cannot be search&replace converted from
   	sg->address = ptr
   into
	sg->page = virt_to_page(ptr)
	sg->offset = ((unsigned long)ptr & ~PAGE_MASK);

   The only truly problematic area is the alt_address thing.
   It is would be a nice thing to rip this eyesore out of the scsi
   layer anyways.

2) I want to put scatterlist in to replace skb_frag_struct in skbuff.h
   and then have a zerocopy network driver do something like:

   	header_dma = pci_map_single(pdev, skb->data, skb->len, PCI_DMA_TODEVICE);
	data_nents = pci_map_sg(pdev, skb_shinfo(skb)->frag_list,
				skb_shinfo(skb)->nr_frags,
				PCI_DMA_TODEVICE);

See? :-)

   Yep. Want me to add in the x86 parts of your patch?

Please let me finish up my prototype with sparc64 building and
working, then I'll send you what I have ok?

Later,
David S. Miller
davem@redhat.com