Re: virt_to_page/pci_map_page vs. pci_map_single

From: Matt Porter
Date: Tue Nov 04 2003 - 12:16:03 EST

On Tue, Nov 04, 2003 at 10:47:25AM -0600, James Bottomley wrote:
> On Tue, 2003-11-04 at 10:35, Matt Porter wrote:
> > This raises a question for me regarding these rules in 2.4 versus
> > 2.6. While fixing a bug in PPC's 2.4 pci_map_page/pci_map_sg
> > implementations I noticed that a scatterlist created by the IDE
> > subsystem will pass nents by page struct reference with a
> > size > PAGE_SIZE. Is this a 2.4ism resulting from allowing both
> > address and page reference scatterlist entries? This isn't
> > explicitly mentioned in the DMA docs AFAICT. I'm wondering
> > if this is the same expected behavior in 2.6 as well. If
> > pci_map_page() is limited to size <= PAGE_SIZE then I would
> > expect pci_map_sg() to be limited as well (and vice versa).
> Not really. By design, the SG interface can handle entries that are
> physically contiguous.
> If you have a limit on the length of your SG elements (because of the
> device hardware, say), you can express this to the block layer with the
> blk_queue_max_segment_size() API.

Ok, then to make this clear to implementers of the
pci_map_page()/pci_map_sg() APIs would the following
addition to the docs be correct?


===== Documentation/DMA-mapping.txt 1.17 vs edited =====
--- 1.17/Documentation/DMA-mapping.txt Sat Aug 16 11:46:50 2003
+++ edited/Documentation/DMA-mapping.txt Tue Nov 4 09:59:53 2003
@@ -499,7 +499,8 @@

pci_unmap_page(dev, dma_handle, size, direction);

-Here, "offset" means byte offset within the given page.
+Here, "offset" means byte offset within the given page. In addition,
+"size" must be <= PAGE_SIZE - offset.

With scatterlists, you map a region gathered from several regions by:

@@ -520,6 +521,9 @@
advantage for cards which either cannot do scatter-gather or have very
limited number of scatter-gather entries) and returns the actual number
of sg entries it mapped them to.
+Note: sglist entries are not limited to PAGE_SIZE, the pci_map_sg()
+implementation must handle entries with size > PAGE_SIZE.

Then you should loop count times (note: this can be less than nents times)
and use sg_dma_address() and sg_dma_len() macros where you previously
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