Re: dma_sync_sg_for_cpu applied to a single scatterlist element

[FUJITA Tomonori]

On Wed, 17 Mar 2010 11:22:01 -0400 (EDT)
Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:

> Here's the situation.  The USB controller drivers don't all support
> scatter-gather operation.  So there's a library routine in the USB core
> which calls dma_map_sg() and then creates a separate I/O request for
> each scatterlist element.  The driver can process these requests one at
> a time, and when they are all finished the library routine calls
> dma_unmap_sg().
> 
> However...  For tracing purposes (usbmon -- like tcpdump but for USB),
> we may need to copy the data from each I/O request's transfer buffer.  
> Unfortunately, this copying is done as each request is submitted (for 
> output) or as it completes (for input), at which times the buffers are 
> all mapped for DMA.  That's the problem.
> 
> Would we be better off not using dma_map_sg() at all in this situation?
> We could map each scatterlist buffer individually with dma_map_single()
> as the request is submitted and then unmap the buffer when the request
> completes, just like with non-sg transfers; then the problem wouldn't
> arise.
> 
> Would there be any significant penalty for doing this?  I realize it
> would prevent adjacent buffers from getting coalesced, but that's
> probably okay.  Any other reason not to?

No reason. About merging adjacent buffers, there are few IOMMU
implementations that do. The recent IOMMU implementations such as VT-d
and AMD IOMMU don't.

If drivers don't support scatter-gather operation, they had better use
dma_map_page() instead of forging scatter-gather lists and playing
with dma_map_sg().
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/