scatter-gather doubt?

[munisekharrms@gmail.com (Muni Sekhar)]

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