Re: [PATCH] synclink_gt fix transmit race and timeout

From: Paul Fulghum
Date: Tue Jun 23 2009 - 13:29:24 EST

Andrew Morton wrote:
> I'm referring to
> Where did the "7" come from?

Oops, I did not scroll down far enough.

+ /*
+ * use worst case speed of 1200bps to calculate transmit timeout
+ * based on data in buffers (tbuf_bytes) and FIFO (128 bytes)
+ */
+ if (info->params.mode == MGSL_MODE_HDLC) {
+ int timeout = (tbuf_bytes(info) * 7) + 1000;
+ mod_timer(&info->tx_timer, jiffies + msecs_to_jiffies(timeout));
+ }

7 is roughly the number of milliseconds to send a byte at 1200bps.

The problem with externally provided data clocks is that you
don't necessarily know the data rate before hand, so a somewhat
arbitrary worst case assumption is used.

> and
> I have a suspicion that tx_stop() should use del_timer_sync(), not
> del_timer(). What happens if the timer handler is concurrently
> running?

Everything is synchronized with info->lock spin_lock,
so nothing critical runs concurrently. tx_stop() is sometimes
called in interrupt context so it can't call del_timer_sync().
If the timer has already fired but has not
run yet it does nothing more than call tx_stop() itself and wake
any transmit waiters so there are no ill effects.

Paul Fulghum
MicroGate Systems, Ltd.
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