clocksource/drivers/timer-microchip-pit64b: Add Microchip PIT64B support

Add driver for Microchip PIT64B timer. Timer could be used in continuous
mode or oneshot mode. The hardware has 2x32 bit registers for period
emulating a 64 bit timer. The LSB_PR and MSB_PR registers are used to
set the period value (compare value). TLSB and TMSB keeps the current
value of the counter. After a compare the TLSB and TMSB register resets.
The driver uses PIT64B timer for clocksource or clockevent. First
requested timer would be registered as clockevent, second one would be
registered as clocksource. Individual PIT64B hardware resources were
used for clocksource and clockevent to be able to support high resolution
timers with this hardware implementation.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1576235962-30123-3-git-send-email-claudiu.beznea@microchip.com

Author: claudiubeznea

Documentation/devicetree/bindings/arm/atmel-sysregs.txt

@@ -10,6 +10,12 @@ PIT Timer required properties:
 - interrupts: Should contain interrupt for the PIT which is the IRQ line
   shared across all System Controller members.
 
+PIT64B Timer required properties:
+- compatible: Should be "microchip,sam9x60-pit64b"
+- reg: Should contain registers location and length
+- interrupts: Should contain interrupt for PIT64B timer
+- clocks: Should contain the available clock sources for PIT64B timer.
+
 System Timer (ST) required properties:
 - compatible: Should be "atmel,at91rm9200-st", "syscon", "simple-mfd"
 - reg: Should contain registers location and length

drivers/clocksource/Kconfig

@@ -697,4 +697,14 @@ config INGENIC_TIMER
 	help
 	  Support for the timer/counter unit of the Ingenic JZ SoCs.
 
+config MICROCHIP_PIT64B
+	bool "Microchip PIT64B support"
+	depends on OF || COMPILE_TEST
+	select CLKSRC_MMIO
+	help
+	  This option enables Microchip PIT64B timer for Atmel
+	  based system. It supports the oneshot, the periodic
+	  modes and high resolution. It is used as a clocksource
+	  and a clockevent.
+
 endmenu

drivers/clocksource/Makefile

@@ -88,3 +88,4 @@ obj-$(CONFIG_RISCV_TIMER)		+= timer-riscv.o
 obj-$(CONFIG_CSKY_MP_TIMER)		+= timer-mp-csky.o
 obj-$(CONFIG_GX6605S_TIMER)		+= timer-gx6605s.o
 obj-$(CONFIG_HYPERV_TIMER)		+= hyperv_timer.o
+obj-$(CONFIG_MICROCHIP_PIT64B)		+= timer-microchip-pit64b.o