From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Hurley Subject: Re: locking changes in tty broke low latency feature Date: Thu, 20 Feb 2014 17:06:08 -0500 Message-ID: <53067C50.9010708@hurleysoftware.com> References: <20140219230623.736E8406062@ip-64-139-1-69.sjc.megapath.net> <53056E99.9070900@hurleysoftware.com> <53064672.3000807@hurleysoftware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-rt-users-owner@vger.kernel.org To: Grant Edwards Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, linux-rt-users@vger.kernel.org, Hal Murray , One Thousand Gnomes , Stanislaw Gruszka List-Id: linux-serial@vger.kernel.org On 02/20/2014 02:33 PM, Grant Edwards wrote: > On 2014-02-20, Peter Hurley wrote: >> Sender completes 2000 loops in 160ms total run time; >> that's 80us average per complete round-trip. > > If I understand correctly, that 80us _includes_ the actual time for > the bits on the wire (which means the actual "baud rate" involved is > high enough that it's negligible). Yes, 80us includes the transmit time. >> I think this shows that low_latency is unnecessary and should >> just be removed/ignored by the tty core. > > If that's the sort of latency that you get for typical kernel > configurations for typical distros, then I agree that the low_latency > flag is not needed by the tty later. Stock ubuntu kernel config but preempt and 250hz (and debugging stuff). > However, it might still be useful for the lower-level tty or > serial-core driver to control CPU usage vs. latency trade-offs (for > exaple, one of my drivers uses it to decide where to set the rx FIFO > threshold). Sure, it could be left for driver consumption. Regards, Peter Hurley