From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933093AbcLGVTs (ORCPT ); Wed, 7 Dec 2016 16:19:48 -0500 Received: from mail-wj0-f195.google.com ([209.85.210.195]:33512 "EHLO mail-wj0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933078AbcLGVTr (ORCPT ); Wed, 7 Dec 2016 16:19:47 -0500 Date: Wed, 7 Dec 2016 22:04:16 +0100 From: Richard Cochran To: Andrei Pistirica Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, davem@davemloft.net, nicolas.ferre@atmel.com, harinikatakamlinux@gmail.com, harini.katakam@xilinx.com, punnaia@xilinx.com, michals@xilinx.com, anirudh@xilinx.com, boris.brezillon@free-electrons.com, alexandre.belloni@free-electrons.com, tbultel@pixelsurmer.com, rafalo@cadence.com Subject: Re: [RFC PATCH net-next v3 1/2] macb: Add 1588 support in Cadence GEM. Message-ID: <20161207210416.GA27622@netboy> References: <1481134912-2243-1-git-send-email-andrei.pistirica@microchip.com> <20161207193908.GA13062@netboy> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161207193908.GA13062@netboy> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 07, 2016 at 08:39:09PM +0100, Richard Cochran wrote: > > +static s32 gem_ptp_max_adj(unsigned int f_nom) > > +{ > > + u64 adj; > > + > > + /* The 48 bits of seconds for the GEM overflows every: > > + * 2^48/(365.25 * 24 * 60 *60) =~ 8 925 512 years (~= 9 mil years), > > + * thus the maximum adjust frequency must not overflow CNS register: > > + * > > + * addend = 10^9/nominal_freq > > + * adj_max = +/- addend*ppb_max/10^9 > > + * max_ppb = (2^8-1)*nominal_freq-10^9 > > + */ > > + adj = f_nom; > > + adj *= 0xffff; > > + adj -= 1000000000ULL; > > What is this computation, and how does it relate to the comment? I am not sure what you meant, but it sounds like you are on the wrong track. Let me explain... The max_adj has nothing at all to do with the width of the time register. Rather, it should reflect the maximum possible change in the tuning word. For example, with a nominal 8 ns period, the tuning word is 0x80000. Looking at running the clock more slowly, the slowest possible word is 0x00001, meaning a difference of 0x7FFFF. This implies an adjustment of 0x7FFFF/0x80000 or 999998092 ppb. Running more quickly, we can already have 0x100000, twice as fast, or just under 2 billion ppb. You should consider the extreme cases to determine the most limited (smallest) max_adj value: Case 1 - high frequency ~~~~~~~~~~~~~~~~~~~~~~~ With a nominal 1 ns period, we have the nominal tuning word 0x10000. The smallest is 0x1 for a difference of 0xFFFF. This corresponds to an adjustment of 0xFFFF/0x10000 = .9999847412109375 or 999984741 ppb. Case 2 - low frequency ~~~~~~~~~~~~~~~~~~~~~~ With a nominal 255 ns period, the nominal word is 0xFF0000, the largest 0xFFFFFF, and the difference is 0xFFFF. This corresponds to and adjustment of 0xFFFF/0xFF0000 = .0039215087890625 or 3921508 ppb. Since 3921508 ppb is a huge adjustment, you can simply use that as a safe maximum, ignoring the actual input clock. Thanks, Richard