On Wed, Jan 03, 2024 at 05:25:51PM +0530, lakshmi.sowjanya.d@xxxxxxxxx wrote: > From: Lakshmi Sowjanya D <lakshmi.sowjanya.d@xxxxxxxxx> > > The goal of the PPS(Pulse Per Second) hardware/software is to generate a > signal from the system on a wire so that some third-party hardware can > observe that signal and judge how close the system's time is to another > system or piece of hardware. > > Existing methods (like parallel ports) require software to flip a bit at > just the right time to create a PPS signal. Many things can prevent > software from doing this precisely. This (Timed I/O) method is better > because software only "arms" the hardware in advance and then depends on > the hardware to "fire" and flip the signal at just the right time. > > To generate a PPS signal with this new hardware, the kernel wakes up > twice a second, once for 1->0 edge and other for the 0->1 edge. It does > this shortly (~10ms) before the actual change in the signal needs to be > made. It computes the TSC value at which edge will happen, convert to a > value hardware understands and program this value to Timed I/O hardware. > The actual edge transition happens without any further action from the > kernel. > > The result here is a signal coming out of the system that is roughly > 1,000 times more accurate than the old methods. If the system is heavily > loaded, the difference in accuracy is larger in old methods. > Facebook and Google are the customers that use this feature. > > Application Interface: > The API to use Timed I/O is very simple. It is enabled and disabled by > writing a '1' or '0' value to the sysfs enable attribute associated with > the Timed I/O PPS device. Each Timed I/O pin is represented by a PPS > device. When enabled, a pulse-per-second(PPS) synchronized with the > system clock is continuously produced on the Timed I/O pin, otherwise it > is pulled low. > > The Timed I/O signal on the motherboard is enabled in the BIOS setup. At some point you should announce v1 of the series. RFC is usually being neglected by many (busy) maintainers. -- With Best Regards, Andy Shevchenko
