omap dmtimer driver bug and a silicon issue on TI AM3358

[Matthias Welwarsky <linux-omap@welwarsky.de>]

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.

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.

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.

BR,
Matthias