FSL DMA engine transfer to PCI memory

David Laight David.Laight at ACULAB.COM
Tue Jan 25 19:56:09 EST 2011

> I'm trying to use FSL DMA engine to perform DMA transfer from
> memory buffer obtained by kmalloc() to PCI memory. This is on
> custom board based on P2020 running linux-2.6.35. The PCI
> device is Altera FPGA, connected directly to SoC PCI-E controller.

You'll need to use the dma engine that is part of the PCIe
interface in order to get large PCIe transfers.
I think everything else will still generate single 32bit PCIe
transfers - which are (if your measurements match mine)
exceptionally lethargic - the ISA bus is faster!

That does work provided you remember to give the dma controller
physical addresses and byteswap absolutely everything.
(Oh, and I didn't get single word transfers to work - they locked
the dma controller - not a problem since they are faster by PIO.)

Note that the PPC Linux (Linux in general??) doesn't have a
'virtual to physical' function that works for all addresses,
you'll need to remember the physical address of the PCIe slave
and use malloc'ed memory for the descriptors (on which
virt_to_phys() actually works).

I don't think there is a standard device driver for the PCIe dma,
I couldn't even find any header files that were vaugely relevent
except in the uboot sources.
I certainly wrote some code that just assumes it is on the right

These are the relevant bits of code ....

Global initialisation:
    /* Enable the read/write dma controllers */
    csb_ctrl =  in_le32(&pex->pex_csb_ctrl);
    out_le32(&pex->pex_csb_ctrl, csb_ctrl);

    /* We don't rely on the dma polling the descriptor, I have NFI
     * whether the default of 0 means 'never poll' or 'poll very
     * Set a large slow value for sanity. */
    out_le32(&pex->pex_dms_dstmr, ~0u);

Transfer setup:
    /* We only support aligned writes - caller must verify */
    dma_ctrl = PDMAD_CTRL_VALID;
    dma_ctrl |= PDMAD_CTRL_SNOOP_CSB;
    dma_ctrl |= PDMAD_CTRL_NEXT_VALID;
    dma_ctrl |= len << (PDMAD_CTRL_LEN_SHIFT - 2);

    /* Fill in DMA descriptor */
    st_le32(&desc->pdmad_ctrl, dma_ctrl);
    /* We MUST clear the status - otherwise the xfer will be skipped */
    st_le32(&desc->pdmad_stat, 0);
    st_le32(&desc->pdmad_src_address, src_phys);
    st_le32(&desc->pdmad_dst_address, dst_phys);
    st_le32(&desc->pdmad_next_desc, 0);

    /* Clear old status */
    st_le32(&pex_dma->pex_dma_stat, in_le32(&pex_dma->pex_dma_stat));

    /* Give descriptor address to dma engine */
    st_le32(&pex_dma->pex_dma_addr, virt_to_phys(desc));

    /* Wait for all above memory cycles, then start xfer */
    st_le32(&pex_dma->pex_dma_ctrl, PEX_DMA_CTRL_START |

Poll for completion:
    /* Wait for transfer to complete/fail */
    do {
        desc_stat = ld_le32(&desc->pdmad_stat);
    } while (!(desc_stat & PDMAD_STAT_DONE));

    status = ld_le32(&pex_dma->pex_dma_stat);

            && desc_stat == PDMAD_STAT_DONE)
        /* Transfer ok */
        return 0;
    /* Transfer failed */

Oh, since I couldn't find it in the documentation, the first
word of the dma descriptor is 'ctrl' and the last 'next_desc'.


More information about the Linuxppc-dev mailing list