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SubjectRe: [patch] zero-bounce highmem I/O
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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
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