Question about DMA mapping.

Question about DMA mapping.

[phind.uet at gmail.com]

Hi everyone,

Thank for your time, I'm new to linux kernel. I am reading LDD3 chapter 
15, Dirrect Memory Access section. I see that when I call function 
/dma_map_single/ and /dma_unmap_single/, I need to pass a direction as a 
parameter. I wonder what is purpose of it? Does It relate to something 
like permission? Can I call /dma_map_single /with DMA_BIDIRECTIONAL and 
then call /dma_unmap_single /with DMA_TO_DEVICE? Will it cause any problem?

Thank you very much.


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Question about DMA mapping.

[valdis.kletnieks at vt.edu]

On Fri, 31 Aug 2018 08:53:29 +0800, "phind.uet at gmail.com" said:

> Thank for your time, I'm new to linux kernel. I am reading LDD3 chapter
> 15, Dirrect Memory Access section. I see that when I call function
> /dma_map_single/ and /dma_unmap_single/, I need to pass a direction as a
> parameter.

Please note that the LDD is current as of 2.6.10, which is from 2005, and
before git was used to maintain the kernel.

[/usr/src/linux-next] git diff --shortstat v2.6.12 v4.19-rc1
 70553 files changed, 23122660 insertions(+), 4318380 deletions(-)

The *concepts* in LDD3 are still valid.  However, the *exact* APIs
are almost certainly changed by now.

[/usr/src/linux-next] ls -l Documentation/DMA*
-rw-r--r--. 1 source source 34951 Jul 18  2017 Documentation/DMA-API-HOWTO.txt
-rw-rw-r--. 1 source source 28310 Sep  7  2017 Documentation/DMA-API.txt
-rw-r--r--. 1 source source  5209 Jul 18  2017 Documentation/DMA-ISA-LPC.txt
-rw-r--r--. 1 source source  7062 Jul 18  2017 Documentation/DMA-attributes.txt

Start there.
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