Re: virt_to_page/pci_map_page vs. pci_map_single

[Matt Porter <mporter@kernel.crashing.org>]

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?

-Matt

===== 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