On Tue, 16 Dec 2014, Russell King - ARM Linux wrote: > On Tue, Dec 16, 2014 at 01:01:17PM -0700, Paul Walmsley wrote: > > So the reference clock and functional clock are (usually) required by the > > PLL to operate, and should therefore be required by the PLL clock driver > > code in the kernel; but one could claim that they aren't technically parent > > clocks of the PLL in a clock tree sense, since the downstream output clock > > isn't directly derived from either of those clocks. > > The reference clock is the parent clock for a PLL, and the output clock > is a derivative of the reference clock. The PLL maths show that very > clearly. The PLL's internal oscillator is the electrical source of the PLL's output clock. The reference clock is just used to discipline that internal oscillator. Even that's not necessary - the reference clock can be optional on some clock source implementations, which can run the internal oscillator in open-loop mode. The question of what the appropriate "parent" clock(s) should be is really a question of how the software chooses to model it, and has more to do with use-counts, constraints, etc. I don't believe we have documented a precise definition for what a "parent clock" is, in the Linux clock framework context. - Paul -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
