On Fri, 3 Feb 2012 13:10:28 -0700 (MST) Paul Walmsley [off-list ref] wrote:

One other comment..

On Fri, 3 Feb 2012, NeilBrown wrote:

quoted

On Thu, 2 Feb 2012 22:45:53 -0700 (MST) Paul Walmsley [off-list ref] wrote:

quoted

On Fri, 3 Feb 2012, NeilBrown wrote:

quoted

I can remove this effect with:

diff --git a/drivers/tty/serial/omap-serial.c b/drivers/tty/serial/omap-serial.c
index f809041..c7ef760 100644
--- a/drivers/tty/serial/omap-serial.c
+++ b/drivers/tty/serial/omap-serial.c

@@ -440,7 +440,8 @@ static unsigned int serial_omap_tx_empty(struct uart_port *port)
 	spin_lock_irqsave(&up->port.lock, flags);
 	ret = serial_in(up, UART_LSR) & UART_LSR_TEMT ? TIOCSER_TEMT : 0;
 	spin_unlock_irqrestore(&up->port.lock, flags);
-	pm_runtime_put(&up->pdev->dev);
+	pm_runtime_mark_last_busy(&up->pdev->dev);
+	pm_runtime_put_autosuspend(&up->pdev->dev);
 	return ret;
 }

It's a little surprising that this helps.  The pm_runtime_get*() and 
_put*() in serial_omap_tx_empty() are just intended to ensure that the 
UART's clocks are running for that LSR register read.

Considering your theory that the UART clocks are being cut while there's 
still data in the FIFO, you might consider removing this code at the end 
of transmit_chars():

	if (uart_circ_empty(xmit))
		serial_omap_stop_tx(&up->port);

I read the code and chickened out of just removing that.
serial_omap_stop_tx seem to do 2 things:
 1/ tell the uart to stop sending interrupts when the tx fifo is empty
 2/ set forceidle (really smartidle) on the uart.

I didn't feel comfortable removing '1' as I thought it might generate an
interrupt storm .. maybe not.
Instead I just removed '2'.  In fact I replaced the 'set_forceidle' call with
'set_noidle'.  So the uart should never report that it was idle.

I did this with my other patch removed so pm_runtime_put() was still being
called.

Result:  I still get corruption.
So having the UART say "no, I'm not idle" does *not* stop the clock
being turned off when we use omap_hwmod_idle() to turn off the clocks.

When we turn off a clock, if that is the last clock in the clock-domain, we
also turn off the clock-domain (I think).
Could it be that the clock-domain doesn't do any handshaking with modules,
and so turns off the clocks even though they are being used?

NeilBrown
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