linux-kernel - Re: locking changes in tty broke low latency feature

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Message-ID: <20140224132334.313f2527@alan.etchedpixels.co.uk>
Date:	Mon, 24 Feb 2014 13:23:34 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Hal Murray <murray+fedora@...64-139-1-69.sjc.megapath.net>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-rt-users@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: locking changes in tty broke low latency feature

> This is a complete pointless test. Use a bog standard 8250 UART on the
> PC and connect a microcontroller on the other end which serves you an
> continous stream of data at 115200 Baud.
> 
> There is no way you can keep up with that without the low latency
> option neither on old and nor on new machines if you have enough other
> stuff going on in the system.

Sorry but having done this in the past the reverse is true. On ancient
machines with crap uarts the low_latency case would routinely overrun
while the non low_latency case did not. That was half of the point of
deferred processing - it pushed tty processing out of the IRQ handler so
bytes were not lost and a 486DX could cope with a 56Kbit modem.

Alan
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