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

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Message-ID: <5652BAD2.4090000@xilinx.com>
Date:	Mon, 23 Nov 2015 08:05:54 +0100
From:	Michal Simek <michal.simek@...inx.com>
To:	Peter Hurley <peter@...leysoftware.com>,
	Sören Brinkmann <soren.brinkmann@...inx.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 20.11.2015 18:16, Peter Hurley wrote:
> On 11/20/2015 11:58 AM, Sören Brinkmann wrote:
>> On Fri, 2015-11-20 at 11:30AM -0500, Peter Hurley wrote:
>>> 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.
> 
> The test below should pass too, but I know it won't because this xilinx
> driver isn't handling x_char at all.
> 
> Aside: does this h/w have rts driver/cts receiver?
> 
> --- >% ---
> --- /dev/null	2015-11-20 07:19:13.265468435 -0500
> +++ xchar.c	2015-11-20 11:55:26.210233102 -0500
> @@ -0,0 +1,354 @@
> +/*
> + * x_char unit test for tty drivers

All these tests looks very interesting. Do you have any any
work-in-progress repo with other tests? It will be good to run all of
them to validate our drivers.

Thanks,
Michal

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