Hi Thomas, On Friday 14 Feb 2020 at 01:35:58 (+0100), Thomas Gleixner wrote: > Ionela Voinescu <ionela.voinescu@xxxxxxx> writes: > > > From: Valentin Schneider <valentin.schneider@xxxxxxx> > > > > Using an arch timer with a frequency of less than 1MHz can result in an > > incorrect functionality of the system which assumes a reasonable rate. > > > > One example is the use of activity monitors for frequency invariance > > which uses the rate of the arch timer as the known rate of the constant > > cycle counter in computing its ratio compared to the maximum frequency > > of a CPU. For arch timer frequencies less than 1MHz this ratio could > > end up being 0 which is an invalid value for its use. > > > > Therefore, warn if the arch timer rate is below 1MHz which contravenes > > the recommended architecture interval of 1 to 50MHz. > > > > Signed-off-by: Ionela Voinescu <ionela.voinescu@xxxxxxx> > > So this patch is from Valentin. Where is his Signed-off-by? > Yes, sorry about this. This was based on a diff that Valentin provided in v2. I'll change the author as agreed at: https://lore.kernel.org/lkml/20200212103249.GA19041@xxxxxxx/ > > > > +static int validate_timer_rate(void) > > +{ > > + if (!arch_timer_rate) > > + return -EINVAL; > > + > > + /* Arch timer frequency < 1MHz can cause trouble */ > > + WARN_ON(arch_timer_rate < 1000000); > > This does not make sense to me. If the rate is out of bounds then why > warn an just continue instead of making it fail? > Because it's not a hard restriction, it's just atypical for the rate to be below 1Mhz. The spec only mentions a typical range of 1 to 50MHz and the warning is only here to flag a potentially problematic rate, below what is assumed typical in the spec. In [1], where I'm actually relying on arch_timer_rate being higher than than 1/SCHED_CAPACITY_SCALE² of the maximum frequency, I am making it fail, as, for that scenario, it is a hard restriction. + * We use a factor of 2 * SCHED_CAPACITY_SHIFT -> SCHED_CAPACITY_SCALE² + * in order to ensure a good resolution for arch_max_freq_scale for + * very low arch timer frequencies (up to the KHz range which should be + * unlikely). + */ + ratio = (u64)arch_timer_get_rate() << (2 * SCHED_CAPACITY_SHIFT); + ratio = div64_u64(ratio, max_freq_hz); + if (!ratio) { + pr_err("System timer frequency too low.\n"); + return -EINVAL; + } + [1] https://lore.kernel.org/lkml/89339501-5ee4-e871-3076-c8b02c6fbf6e@xxxxxxx/ Thanks, Ionela. > Thanks, > > tglx
