linux-kernel - Re: [RFC PATCH] tty: serial: core: Only invoke ->start

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Message-ID: <5411576E.204@linutronix.de>
Date:	Thu, 11 Sep 2014 10:03:58 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] tty: serial: core: Only invoke ->start_tx() if there
 is data to send

On 09/10/2014 09:52 PM, Peter Hurley wrote:
> The serial core can't assume that start_tx() does not need invoking
> because hardware that can stop_tx() with data in the transmitter
> won't restart if the ring buffer is empty but data is still in the
> transmitter. [Note that the 16C950 port type does this in the 8250 driver.]

oh, not sure how I missed this… But now that I look at this, it is also
that ->x_char that could be use for flow control which would have to be
sent even with an empty xmit buffer.
And 8250 in DMA mode does not look at x_char at all. But it would be
better if it would, right? However if the TX side does a 2 KiB transfer
not sure what should be done…

> So this has to be handled in the 8250 driver.
> 
> What is the actual issue? Are you trying not to unnecessarily wake
> the omap hardware if runtime-PM is on?

no, it is actually the extra interrupt for no reason that looked like
not needed at all. But I guess it is only during "startup" while the
init-script do fancy things and not during "normal" operations.

> Regards,
> Peter Hurley

Sebastian
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