DMA on GIO Devices Without Scatter/Gather Capability

DMA on GIO Devices Without Scatter/Gather Capability

If your device does not provide scatter/gather capability, it must break up a data transfer so that DMA transfer targets in physical memory are physically contiguous to the DMA engine (this assures that no transfer crosses a page boundary). The IRIX operating system provides a utility, sgset(D3X), that simulates scatter/gather registers in software. (See the IRIX Device Driver Reference Pages for details on this routine.) Your driver can use this facility 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:

/* Actual device setup for DMA, etc., if your board
 * does NOT have hardware scatter/gather DMA support.
 * Called from the gbdwrite() routine via physio().
 */
void
gbd_strategy(struct buf *bp)
{
   int unit = geteminor(bp->b_dev)&1;

   /* any checking for initial state here. */

   /* Get the kernel virtual address of the data; note
   * b_dmaaddr may be NULL if the BP_ISMAPPED(bp) macro
   * indicates false; in that case, the field bp->b_pages
   * is a pointer to a linked list of pfdat structure
   * pointers; that saves creating a virtual mapping and
   * then decoding that mapping back to physical addresses.
   * BP_ISMAPPED will never be false for character devices,
   * only block devices.
   */
   if(!BP_ISMAPPED(bp)) {
      cmn_err(CE_WARN,
         "gbd driver can't handle unmapped buffers");
      bioerror(bp, EIO);
      biodone(bp);
      return;
   }

   gbd_curbp[unit] = bp;
   /*
   * Initialize the current transfer address and count.
   * The first transfer should finish the rest of the
   * page, but do no more than the total byte count.
   */
   gbd_curaddr[unit] = bp->b_dmaaddr;
   gbd_totcount[unit] = bp->b_count;
   gbd_curcount[unit] = NBPC -
      ((unsigned int)gbd_curaddr[unit] & (NBPC-1));
   if (bp->b_count < gbd_curcount[unit])
      gbd_curcount[unit] = bp->b_count;
   /* Tell the device starting physical address, count,
   * and direction */
   gbd_device[unit]->startaddr = kvtophys(gbd_curaddr[unit]);
   gbd_device[unit]->count = gbd_curcount[unit];
   if (bp->b_flags & B_READ) == 0)
      gbd_device[unit]->direction = GBD_WRITE;
   else
      gbd_device[unit]->direction = GBD_READ;
   gbd_device[unit]->command = GBD_GO;   /* start DMA */

   /* and return; upper layers of kernel wait for iodone(bp) */
}