/*
 * Query the progress of a transfer on a DMA channel.
 *
 * To avoid having to interrupt a transfer in progress, we sample
 * each of the high and low databytes twice, and apply the following
 * logic to determine the correct count.
 *
 * Reads are performed with interrupts disabled, thus it is to be
 * expected that the time between reads is very small.  At most
 * one rollover in the low count byte can be expected within the
 * four reads that are performed.
 *
 * There are three gaps in which a rollover can occur :
 *
 * - read low1
 *              gap1
 * - read high1
 *              gap2
 * - read low2
 *              gap3
 * - read high2
*
 * If a rollover occurs in gap1 or gap2, the low2 value will be
 * greater than the low1 value.  In this case, low2 and high2 are a
 * corresponding pair.
 *
 * In any other case, low1 and high1 can be considered to be correct.
 *
 * The function returns the number of bytes remaining in the transfer,
 * or -1 if the channel requested is not active.
 *
 */
int
isa_dmastatus(int chan)
{
        u_long  cnt = 0;
        int     ffport, waport, s;
        u_long  low, high, low2, high2;

        /* channel active? */
        if (dma_inuse & (1 << chan) == 0) {
                printf("isa_dmastatus: channel %d not active\n", chan);
                return(-1);
        }
        /* still busy? */
        if (dma_busy & (1 << chan) == 0) {
                return(0);
        }

        if (chan < 4) {                 /* low DMA controller */
                ffport = DMA1_FFC;
                waport = DMA1_CHN(chan) + 1;
        } else {                        /* high DMA controller */
                ffport = DMA2_FFC;
                waport = DMA2_CHN(chan - 4) + 2;
        }

        s = splhigh();                  /* no interrupts Mr Jones! */
        outb(ffport, 0);                /* clear register LSB flipflop */
        low = inb(waport);
        high = inb(waport);
        outb(ffport, 0);                /* clear again (paranoia? */
        low2 = inb(waport);
        high2 = inb(waport);
        splx(s);

        /* now decide if a wrap has tried to skew our results */
        if (low >= low2)
                cnt = low + (high << 8) ;
        else
                cnt = low2 + (high2 << 8) ;

        cnt = (cnt + 1) & 0xffff ;

        if (chan >=4)   /* 16-bit chans transfer words */
                cnt += cnt ;
        return(cnt);
}