linux-kernel - Re: [PATH RESEND v2 03/10] tty: xuartps: Always enable transmitter in start

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Message-ID: <20151120165804.GV32017@xsjsorenbubuntu>
Date:	Fri, 20 Nov 2015 08:58:04 -0800
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	<linux-arm-kernel@...ts.infradead.org>,
	Michal Simek <michal.simek@...inx.com>,
	<linux-serial@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATH RESEND v2 03/10] tty: xuartps: Always enable transmitter
 in start_tx

Hi Peter,

On Fri, 2015-11-20 at 11:30AM -0500, Peter Hurley wrote:
> Hi Sören,
> 
> On 11/20/2015 10:28 AM, Sören Brinkmann wrote:
> > On Fri, 2015-11-20 at 07:13AM -0500, Peter Hurley wrote:
> >> On 11/19/2015 03:02 PM, Soren Brinkmann wrote:
> >>> start_tx must start transmitting characters. Regardless of the state of
> >>> the circular buffer, always enable the transmitter hardware.
> >>
> >> Why?
> >>
> >> Does cdns_uart_stop_tx() actually stop the transmitter so that
> >> data remains in the transmitter?
> > 
> > Well, I saw my system freezing and the cause seemed to be that the UART
> > receiver and/or transmitters were disabled while the system was trying
> > to print. Hence, I started questioning all locations touching the
> > transmitter/receiver enable. I read the docs in
> > https://www.kernel.org/doc/Documentation/serial/driver, which simply
> > says "Start transmitting characters." for start_tx(). Hence, I thought,
> > this function is probably supposed to just do that and start the
> > transmitter. I'll test whether this patch can be dropped.
> 
> I don't think that patch would fix any freeze problems, but restarting
> the transmitter even if the circ buffer is empty may be necessary to
> push out remaining data when the port is restarted after being stopped.
> 
> IOW, something like
> 
> 	if (uart_tx_stopped(port))
> 		return;
> 
> 	....
> 
> 
> 	if (uart_circ_empty(&port->state->xmit)
> 		return;

Thanks! I'll change the patch accordingly.

> 
> 
> Below is a (work-in-progress) serial driver validation test for flow
> control handling (it may need some tuning for slow line speeds).
> Usual caveats apply. Takes ~40secs @ 115200.

I'll try to get that running on my system.

	Thanks,
	Sören
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