DMA Transfers (Synchronous)

Character drivers generally use physio(9F) to do the setup work for DMA transfers in read(9E) and write(9E), as is shown in Example 15–5.

int physio(int (*strat)(struct buf *), struct buf *bp,
     dev_t dev, int rw, void (*mincnt)(struct buf *),
     struct uio *uio);

physio(9F) requires the driver to provide the address of a strategy(9E) routine. physio(9F) ensures that memory space is locked down, that is, memory cannot be paged out, for the duration of the data transfer. This lock-down is necessary for DMA transfers because DMA transfers cannot handle page faults. physio(9F) also provides an automated way of breaking a larger transfer into a series of smaller, more manageable ones. See minphys() Entry Point for more information.

Example 15–5 read(9E) and write(9E) Routines Using physio(9F)

static int
xxread(dev_t dev, struct uio *uiop, cred_t *credp)
{
     struct xxstate *xsp;
     int ret;

     xsp = ddi_get_soft_state(statep, getminor(dev));
     if (xsp == NULL)
        return (ENXIO);
     ret = physio(xxstrategy, NULL, dev, B_READ, xxminphys, uiop);
     return (ret);
}    

static int
xxwrite(dev_t dev, struct uio *uiop, cred_t *credp)
{     
     struct xxstate *xsp;
     int ret;

     xsp = ddi_get_soft_state(statep, getminor(dev));
     if (xsp == NULL)
        return (ENXIO);
     ret = physio(xxstrategy, NULL, dev, B_WRITE, xxminphys, uiop);
     return (ret);
}

In the call to physio(9F), xxstrategy is a pointer to the driver strategy() routine. Passing NULL as the buf(9S) structure pointer tells physio(9F) to allocate a buf(9S) structure. If the driver must provide physio(9F) with a buf(9S) structure, getrbuf(9F) should be used to allocate the structure. physio(9F) returns zero if the transfer completes successfully, or an error number on failure. After calling strategy(9E), physio(9F) calls biowait(9F) to block until the transfer either completes or fails. The return value of physio(9F) is determined by the error field in the buf(9S) structure set by bioerror(9F).