On 01/05/2019 17:11:44+0000, Trent Piepho wrote: > > I can't believe you can possibly have more than one second between the > > check in the core and the check in the driver, it doesn't make much > > sense to check, even in the current state of the core. > > It's certainly possible to have multiple seconds pass. For an external > device over SPI or I2C, one has to wait for the bus to become free. > And on SPI that requires the kernel thread running the bus to be > scheduled. Just put in some real-time tasks and maybe a big transfer > to a flash chip and it could be a while before that happens. > > I don't think this device has that issue as I don't think it's > external. And ever for a device on an external bus, delays > 1 second > are unlikely. Possible, but unlikely. > > You can also get them when Linux is running under a hypervisor, i.e. a > Linux VM. But also something like an NMI and ACPI BIOS. If the Linux > guest is not scheduled to run for while anything that is supposed to be > based on real time, like the value returned by an RTC, will still > advance. It is possible that multiple seconds elapse from the guest > CPU executing one instruction to the next. > > But even ignoring that, does it require > 1 second to elapse. Can't it > happen when the clock ticks from one second to the next, which happens > effectively instantly? > > If the time from the check to the time when the alarm is set is 1 > microsecond, and the time this call to set the alarm is made is > randomly done and not synchronized to the RTC, then isn't there a 1 out > of 1 million chance (1 microsecond / 1 second), that the once per > second clock tick will hit our 1 us window? No, let's say you want Talarm == Tcurrent + 1, if the core check happens right before the next second, then you necessarily end up with Talarm == Tcurrent after the check. This means that you now have one second before the time read in st-lpc to avoid the alarm_secs -= now_secs; underflow. Obviously, in that case, you are likely to miss the alarm but this is as likely to happen with the check that is in the driver. This check doesn't provide anything but a false sense of security. -- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
