-----Original Message----- > From: andrei.pistir...@microchip.com [mailto:andrei.pistir...@microchip.com] > Sent: 8 grudnia 2016 15:42 > To: richardcoch...@gmail.com > Cc: net...@vger.kernel.org; linux-kernel@vger.kernel.org; > linux-arm-ker...@lists.infradead.org; da...@davemloft.net; > nicolas.fe...@atmel.com; harinikatakamli...@gmail.com; > harini.kata...@xilinx.com; punn...@xilinx.com; mich...@xilinx.com; > anir...@xilinx.com; boris.brezil...@free-electrons.com; > alexandre.bell...@free-electrons.com; tbul...@pixelsurmer.com; Rafal Ozieblo > Subject: RE: [RFC PATCH net-next v3 1/2] macb: Add 1588 support in Cadence > GEM. > > > > > -----Original Message----- > > From: Richard Cochran [mailto:richardcoch...@gmail.com] > > Sent: Wednesday, December 07, 2016 11:04 PM > > To: Andrei Pistirica - M16132 > > Cc: net...@vger.kernel.org; linux-kernel@vger.kernel.org; linux-arm- > > ker...@lists.infradead.org; da...@davemloft.net; > > nicolas.fe...@atmel.com; harinikatakamli...@gmail.com; > > harini.kata...@xilinx.com; punn...@xilinx.com; mich...@xilinx.com; > > anir...@xilinx.com; boris.brezil...@free-electrons.com; > > alexandre.bell...@free-electrons.com; tbul...@pixelsurmer.com; > > raf...@cadence.com > > Subject: Re: [RFC PATCH net-next v3 1/2] macb: Add 1588 support in > > Cadence GEM. > > > > 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 considered the following simple equation: increment value at nominal > frequency (which is 10^9/nominal frequency nsecs) + the maximum drift value > (nsecs) <= maximum increment value at nominal frequency (which is > 8bit:0xffff). > If maximum drift is written as function of nominal frequency and maximum ppb, > then the equation above yields that the maximum ppb is: (2^8 - 1) > *nominal_frequency - 10^9. The equation is also simplified by the fact that > the drift is written as ppm + 16bit_fractions and the increment value is > written as nsec + 16bit_fractions. > > Rafal said that this value is hardcoded: 0x64E6, while Harini said: 250000000.
To clarify a little bit. In my reference code this value (0x64E6) was taken from our legacy code. It was used for testing only. I know it should be change to something more accurate. This is the reason why I asked how did you count it (250000000). According to our calculations this value depends on actual set period (incr_ns and incr_sub_ns) and min and max value we can set. The calculation were a little bit intricate, so we decided to leave it as it was. > > I need to dig into this... > > > > > I am not sure what you meant, but it sounds like you are on the wrong track. > > Let me explain... > > Thanks. > > > > > 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 > > > > > > Regards, > Andrei > Best regards, Rafal Ozieblo | Firmware System Engineer, phone nbr.: +48 32 5085469 www.cadence.com
