> -----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. 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
