On Sun, Sep 25, 2016 at 1:02 AM, Greg Freemyer <greg.freemyer@gmail.com> wrote:
On Sat, Sep 24, 2016 at 9:33 AM, Muni Sekhar <munisekharrms@gmail.com> wrote:
Hi All,
I am working on a xilinx PCIe endpoint with DMA reference block.
The DMA reference block design has 2 Scatter-Gather engines, one for each DMA channel.
Channel 0 is for HostMemory -> DMA_REF FIFO transfers Channel 1 is for DMA_REF FIFO -> HostMemory transfers
Each scatter-gather engine works through a linked list of Descriptors from which it generates the required DMA activity.
The format of these descriptors is depicted as below:
Offset @ 0x00 - LSBs of pointer to DMA data
Offset @ 0x04 - MSBs of pointer to DMA data
Offset @ 0x08 - Number of data bytes to be transferred. (note: only 8 byte aligned transfers supported)
Offset @ 0x0C - LSBs of pointer to next Descriptor (Set this field & MSBs to zero to indicate end of descriptor list)
Offset @ 0x10 - MSBs of pointer to next Descriptor
Does the Linux kernel has any data structure to support the above mentioned scatter-gather descriptor?
Will it be possible to use the kernel scatterlist API’s for this hardware?
Muni,
Have you checked that they don't have a pre-existing driver?
http://www.wiki.xilinx.com/Linux+Drivers
There are 5 different DMA drivers there. Assuming you have and the DMA engine doesn't yet have a linux driver:
I'm not sure I've ever written the low level scatter-gather DMA logic for a DMA engine, but scatter-gather is decades old technology.
The basic scatterlist is defined in: linux/scatterlist.h
see http://lxr.free-electrons.com/source/include/linux/scatterlist.h#L10
=== 10 struct scatterlist { 11 #ifdef CONFIG_DEBUG_SG 12 unsigned long sg_magic; 13 #endif 14 unsigned long page_link; 15 unsigned int offset; 16 unsigned int length; 17 dma_addr_t dma_address; 18 #ifdef CONFIG_NEED_SG_DMA_LENGTH 19 unsigned int dma_length; 20 #endif 21 }; ===
The upper levels of the linux kernel will populate that and pass it into the DMA engine driver.
I believe your DMA engine driver will have to accept that scatterlist coming from the upper layers of the kernel and map it to a new structure to pass to your IOMMU.
Hi Greg, Thanks for the useful information, I will look at the existing xilinx DMA engine drivers and documentation for DMA-API's.
But don't trust my rather vague knowledge. Have you read:
https://www.kernel.org/doc/Documentation/DMA-API.txt https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt
Search through those for "scatter" and "sg". You will find the main kernel structs and API.
And of course, look at the existing xilinx DMA engine drivers and see if there isn't one close to what you need.
Hope that helps, Greg
-- Thanks, Sekhar