* Matthias Welwarsky <linux-omap@xxxxxxxxxxxx> [200617 10:58]:
> On Dienstag, 16. Juni 2020 17:27:17 CEST Tony Lindgren wrote:
> > Hi,
> >
> > * Matthias Welwarsky <linux-omap@xxxxxxxxxxxx> [200614 09:57]:
> > > Hi,
> > >
> > > while doing some timekeeping experiments with the Beaglebone Black I ran
> > > across two issues I'd like advice on before submitting patches.
> > >
> > > I'm feeding one of the dmtimers with a external clock (10MHz from a GPSDO)
> > > to improve drift behaviour of CLOCK_REALTIME. For this, I need to set the
> > > input clock multiplexer of one of the dmtimers to "tclkin_ck". I also
> > > need to set up the pin multiplexing so that the external clock is
> > > actually available.
> > OK, so presumably you want to use this for Linux system timers then.
>
> Actually, have to use one of the "normal" timers because I need the capture
> input. The driver implements a clocksource and a kpps interface, which makes
> getting exact 1PPS event timestamps very easy.
Hmm OK so a regular device driver then similar to pwm-dmtimer usage
I guess.
> > > The first issue is the framework function omap_dm_timer_set_source(). Of
> > > the available clock sources, none is a possible parent. But even when
> > > fixing them, the clk_set_parent() will fail because timer->fclk points to
> > > the "wrong" clock in the clock topology. You can only set the parent
> > > clock of the "timerN_fclk" nodes, but timer->fclk points to the actual
> > > hardware clock one level "down" in the topology. This clock only has one
> > > possible parent, which is "timerN_fck". The work-around I use in my
> > > clocksource driver is to use the clock framework directly and manually
> > > retrieve the parent clock of timer->fclk, then reparent that clock to
> > > "tclkin_ck". That works, but I'd prefer to fix the driver framework. But
> > > I'd need a hint what would be an appropriate approach for that.
>
> > You can specify the source clocks in the dts with assigned-clocks and
> > assigned-clock-parents like commit e20ef23dd693 ("ARM: dts: Configure system
> > timers for am335x") is doing for system timers starting with v5.8-rc1. That
> > should just work, if not there are bugs somewhere :)
>
> You mean, for setting the source clock of the timer to tclkin_ck through the
> dts? That would be applied during boot, right? That will probably not work for
> me as I need to switch on the external clock manually and then it'll take a
> little additional time to become stable. So I'm pretty much bound to setting
> up the clock multiplexing at runtime.
The dts configured source clocks get configured on by system timers, and on
regular device driver probe. Sounds like you additionally need to configure
and request your external clocksource. And it it could be just a regular
clock framework driver.
> > > The second issue is more of a silicon "bug". It seems that the clock
> > > multiplexer is not warm-reset sensitive but the pinmux is. In consequence,
> > > when the chip is reset (watchdog or "reboot" command), the pinmux defaults
> > > back to GPIO but the timer functional clock mux still points to
> > > "tclkin_ck"
> > > and when the kernel boots up and the dmtimer framework tries to initialize
> > > the timer, it accesses a hwmod that has no functional clock and the
> > > kernel receives an async external abort and dies.
> > >
> > > Two possible places for a fix come to mind: u-boot could reset the clock
> > > mux, or the kernel needs to do it when it boots, either in the dmtimer
> > > framework or in the clock framework, maybe based on a device tree
> > > attribute that specifies a default state of the clock mux.
> > >
> > > I'd like to hear your take on this.
> >
> > For system timers, we do not have struct device available as at least one
> > usable system timer is needed very early for a clockevent. You should add
> > necessary initialization based on the assigned-clocks and assigned-clock
> > parents to drivers/clocksource/timer-ti-dm-systimer.c.
> >
> > For non-systimer usage with normal device drivers, just configuring the
> > pictrl entries for the device in the dts file can be used.
>
> That would be too late, no? The kernel apparently dies in timer-ti-dm.c during
> probing, in particular in __omap_dm_timer_init_regs() while reading the TIDR.
> A device driver (especially when it's a module) would be probed way later.
Sounds like it will hang as it's not properly clocked. If you implement
a drivers/clk driver (or use some existing gpio clock?), then deferred
probe will prevent the dmtimer instance from probing until the dts
specified "fck" is available. Additionally you may need to configure
the assigned-clocks and assigned-clock-parents too in the dts.
You can add new clocksources and they get used based on rating.
So I'd boot up with the default system clocks, then at regular
device driver module_init() time configure the external clock
source.
Regards,
Tony
