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

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Date:	Fri, 20 Nov 2015 12:12:29 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	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

On 11/20/2015 12:05 PM, 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?
> 
> Fixing up the patch, I looked at this one. It might actually do that.

Ok.

> Without having changed anything. The doc says: "The driver should
> stop transmitting characters as soon as possible.". And the
> implementation is really not draining any FIFO, but just disabling the
> transmitter. I take your question as that this might not be this way?
> Should stop_tx drain the FIFO first?

No.

Most h/w can't actually stop the transmitter (or not without losing
data), so that's why the expectation is only for "as soon as possible".
Stopping sooner is better.

Regards,
Peter Hurley



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