Hi, On Thu, Dec 8, 2016 at 8:11 PM, <andrei.pistir...@microchip.com> wrote: > > >> -----Original Message----- >> From: Richard Cochran [mailto:richardcoch...@gmail.com] >> Sent: Wednesday, December 07, 2016 11:04 PM >> To: Andrei Pistirica - M16132 >> Cc: netdev@vger.kernel.org; linux-ker...@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.
@ Andrei, I may have equated max ppb to max tsu frequency allowed on the system and set that. That will be wrong. > > 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. So, if I understand right, this is theoretically limited by the maximum input clock: So if the highest frequency allowed (also commonly sourced in my case) is 200MHz, then with a 5ns time period, considering the adjustment to slowest possible word, 0x4FFFF/0x50000 will be 999996948 ppb. Shouldn't this be the max_adj? I'm afraid I don't get why we are choosing the most limited max adj.. Sorry if I'm missing something - could you please help me understand? Regards, Harini
