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Re: bus_dma coalesce advice John Baldwin Tue Feb 21 14:01:54 2012

On Monday, February 20, 2012 1:05:40 pm JD Louw wrote:
> Hi,
> 
> I have a Xilinx FPGA PCIe DMA design that I'd like to get going on
> FreeBSD. I'd like some advice on the best practice of the bus_dma
> functions. Specifically, I'd like to understand how best to coalesce
> multiple DMA transactions.
> 
> Using the bus_dma_tag_create and bus_dmamem_alloc functions I create
> 256 contiguous descriptors.
> 
>       bus_dma_tag_create(NULL,                /* parent */
>               4,                              /* alignment */
>               0,                              /* bounds */
>               BUS_SPACE_MAXADDR,              /* lowaddr */
>               BUS_SPACE_MAXADDR,              /* highaddr */
>               NULL, NULL,                     /* filter, filterarg */
>               256*sizeof(descriptor),         /* maxsize */
>               1,                              /* nsegments */
>               256*sizeof(descriptor),         /* maxsegsize */
>               BUS_DMA_ALLOCNOW,               /* flags */
>               NULL, NULL,                     /* lockfunc, lockarg */
>               &desc_tag);                     /* dmat */
> 
> I then create another bus_dma_tag for the data:
> 
>       bus_dma_tag_create(NULL,                /* parent */
>               4,                              /* alignment */
>               0,                              /* bounds */
>               BUS_SPACE_MAXADDR,              /* lowaddr */
>               BUS_SPACE_MAXADDR,              /* highaddr */
>               NULL, NULL,                     /* filter, filterarg */
>               0xFFFFF,                        /* maxsize - 1MB */
>               256,                            /* nsegments */
>               0x1000,                         /* maxsegsize - 4KB*/
>               BUS_DMA_ALLOCNOW,               /* flags */
>               NULL, NULL,                     /* lockfunc, lockarg */
>               &data_tag);                     /* dmat */
> 
> Now my question: In order to batch several mbufs/uios in into the 256
> descriptors I'd like to do multiple bus_dmamap_loads on the data tag.
> But reading the bus_dmamap_load_mbuf/uio code it looks like this is
> not a good idea. Each mapping operation does not subtract its nsegment
> count from the tag maximum nsegment count, so at some point
> bus_dmamap_load will overrun my 256 descriptors.

Does your DMA engine really allow a single transaction to span more than 256
descriptors?  (The 'nsegmenets' is the maximum number of S/G entries for a
single transaction, not the number of entries in your ring.)

> Do I need to allocate a separate set of descriptors for each bus_dmamapping?
> 
> Any advice much appreciated,

Typically in a NIC driver you will use bus_dmamap_load_mbuf_sg() to populate
an array of S/G elements on the stack.  You can check the returned value for 
the number of segments and handle the case where it exceeds the number of
segments you actually have available (e.g. by calling m_collapse() or 
m_defrag() or just queueing the packet until you get a TX completion interrupt 
that frees up some descriptors).  Note that for all of those cases you will
need to do a bus_dmamap_unload() first.

-- 
John Baldwin
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