DMA on A32 Devices with No Scatter/Gather Capability

DMA on A32 Devices with No Scatter/Gather Capability

If neither your device nor your workstation provides scatter/gather capability, your driver must break up a data transfer so that no transfer crosses a page boundary. The IRIX operating system provides a utility called sgset(D3X), which simulates scatter/gather registers in software. It should not be used on systems that support DMA address mapping. (See the IRIX Device Driver Reference Pages for details on this routine.) Your driver can use this utility to perform the virtual-to-physical mapping up front. Or, as the example below shows, your driver can do this mapping following the transfer of each page:

/* pointer to device registers */
volatile struct vdk_device    *vdk_device;
struct buf   *vdk_curbp      /* current buffer */
caddr_t      vdk_curaddr;    /* current address to transfer */
int          vdk_curcount;
vdkstrategy(bp)
struct buf    *bp;
{
    ...
    vdk_curbp = bp;
    bp->b_resid = bp->b_bcount;
    /*
     * Initialize the current transfer address and count.
     * The first transfer must finish the rest of the
     * page, but do no more than the total byte count.
     */
    vdk_curaddr = bp->b_un.b_addr;
    vdk_curcount = NBPC -
        ((unsigned int)vdk_curaddr & (NBPC-1));
    if (bp->b_resid < vdk_curcount)
        vdk_curcount = bp->b_resid;
    /* Tell the device starting physical address, count,
     * and direction */
    vdk_device->startaddr = kvtophys(vdk_curaddr);
    vdk_device->count = vdk_curcount;
    if (bp->b_flags & B_READ) == 0)
    vdk_device->direction = VDK_WRITE;
    else
        vdk_device->direction = VDK_READ;
    vdk_device->command = VDK_GO;
    vdk_curaddr += vdk_curcount;
    biowait(bp);
    ...
}
vdkintr(unit)
int unit;
{
    int count, error;
    register struct buf *bp = vdk_curbp;
    ...
    if(error) {
        bioerror (bp,EIO);
        biodone(bp);
        return;
    }
    /* On successful transfer of last chunk, continue
     * if necessary. */
    vdk_curaddr -= vdk_curcount;
    if (bp->b_resid > 0) {
            count =
                (bp->b_resid < NBPC ? bp->b_resid : NBPC);
            vdk_device->startaddr = kvtophys(vdk_curaddr);
            vdk_device->count = count;
            if (bp->b_flags & B_READ) == 0)
                vdk_device->direction = VDK_WRITE;
            else
                vdk_device->direction = VDK_READ;
            vdk_device->command = VDK_GO;
            vdk_curaddr += count;
    } else {
            biodone(bp);
    }
    ...
}