How to use the DMA Engine in FreeBSD?

[Kurt Lidl]

On 12/13/18 10:53 AM, Warner Losh wrote:
> On Thu, Dec 13, 2018, 3:04 AM Rajesh Kumar <rajfbsd at gmail.com wrote:
> 
>> Hi,
>>
>> Is there any good documentation available to understand the existing
>> support, API's and how-to use the DMA Engine in FreeBSD?
>>
> 
> 
> Usually you just use pci busmastering and it just works.
> 
> I am trying to write a test driver which will use DMA Engine to do the data
>> transfer (rather than plain memcpy which involves cpu).  Can anyone point
>> to any driver implementation which has similar functions implemented?  I
>> see references to SYS_RES_DRQ to allocate DMA channels and play around. But
>> that seems to be specific to ISA. Can it be used for PCI drivers as well?
>>
> 
> No. ISA DMA is only for really old hardware without it's own DMA engine.
> 
> Look at the busdma api/man page.

For some Intel based server hardware, there is the "ioat" driver, which 
allows for user code to schedule DMA operations.  See ioat(4) for 
details, including a pointer to the test program.

-Kurt