DECStation zs driver continued...

[Karel van Houten <K.H.C.vanHouten@research.kpn.com>]

Hi all,

I did some more investigations in the DECStation zs driver, and the
resulting slowdown of init.

This is the relevant code from drivers/tc/zs.[ch]:

/* Read Register 0 */
#define Tx_BUF_EMP      0x4     /* Tx Buffer empty */

/* Read Register 1 */
#define ALL_SNT         0x1     /* All sent */

/*
 * rs_wait_until_sent() --- wait until the transmitter is empty
 */
static void rs_wait_until_sent(struct tty_struct *tty, int timeout)
{
 ....

        while ((read_zsreg(info->zs_channel, 1) & ALL_SNT) == 0) {
                current->state = TASK_INTERRUPTIBLE;
                current->counter = 0;   /* make us low-priority */
                schedule_timeout(char_time);
                if (signal_pending(current))
                        break;
                if (timeout && ((orig_jiffies + timeout) < jiffies))
                        break;
        }
        current->state = TASK_RUNNING;
}

Now, look how NetBSD handles this:

#define SCC_RR0         0       /* status register */
#define SCC_RR1         1       /* special receive conditions */

        /*
         * Wait for transmitter to be not busy.
         */
        do {
                SCC_READ_REG(regs, line, SCC_RR0, value);
                if (value & ZSRR0_TX_READY)
                        break;
                DELAY(100);
        } while (1);


So I changed the loop in rs_wait_until_sent to:

        while ((read_zsreg(info->zs_channel, 0) & Tx_BUF_EMP) == 0) {
                udelay(100);
        }
or 

        while ((read_zsreg(info->zs_channel, 1) & ALL_SNT) == 0) {
                udelay(100);
        }

Both versions result in a much faster init, and no problems with the
console for now.

Any comments from the kernel-specialists?

Regards,
-- 
Karel van Houten

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The box said "Requires Windows 95 or better."
I can't understand why it won't work on my Linux computer. 
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