On Thu, Nov 08, 2018 at 01:50:23PM -0800, Greg Kroah-Hartman wrote: > 3.18-stable review patch. If anyone has any objections, please let me know. > > ------------------ > > [ Upstream commit afc9d590b8a150cfeaac0078ef5de6fb21a5ea6a ] > > Errata i856 for the AM572x (DRA7xx) points out that the 32.768KHz external > crystal is not enabled at power up. Instead the CPU falls back to using > an emulation for the 32KHz clock which is SYSCLK1/610. SYSCLK1 is usually > 20MHz on boards so far (which gives an emulated frequency of 32.786KHz), > but can also be 19.2 or 27MHz which result in much larger drift. > > Since this is used to drive the master counter at 32.768KHz * 375 / > 2 = 6.144MHz, the emulated speed for 20MHz is of by 570ppm, or about 43 > seconds per day, and more than the 500ppm NTP is able to tolerate. > > Checking the CTRL_CORE_BOOTSTRAP register can determine if the CPU > is using the real 32.768KHz crystal or the emulated SYSCLK1/610, and > by known that the real counter frequency can be determined and used. > The real speed is then SYSCLK1 / 610 * 375 / 2 or SYSCLK1 * 75 / 244. I certainly can't see a problem including it. Of course I doubt there really are any DRA7 systems running that old a kernel (certainly the beaglebone X15 seems to require a newer kernel to even have a DTB included). The RuggedCom RX1400 might still be running 3.14 kernel, but already has the patch applied (because I did it). So certainly nothing wrong with adding the patch, but I doubt anyone will notice. -- Len Sorensen
