clocksource/drivers/nxp-timer: Add the System Timer Module for the s32gx platforms

STM supports commonly required system and application software timing
functions. STM includes a 32-bit count-up timer and four 32-bit
compare channels with a separate interrupt source for each
channel. The timer is driven by the STM module clock divided by an
8-bit prescale value (1 to 256).

STM has the following features:
    • One 32-bit count-up timer with an 8-bit prescaler
    • Four 32-bit compare channels
    • An independent interrupt source for each channel
    • Ability to stop the timer in Debug mode

The s32g platform is declined into two versions, the s32g2 and the
s32g3. The former has a STM block with 8 timers and the latter has 12
timers.

The platform is designed to have one usable STM instance per core on
the system which is composed of 3 x Cortex-M3 + 4 Cortex-A53 for the
s32g2 and 3 x Cortex-M3 + 8 Cortex-A53 for the s32g3.

There is a special STM instance called STM_TS which is dedicated to
the timestamp. The 7th STM instance STM_07 is directly tied to the
STM_TS which means it is not usable as a clockevent.

The driver instantiate each STM described in the device tree as a
clocksource and a clockevent conforming to the reference manual even
if the Linux system does not use all of the clocksource. Each
clockevent will have a cpumask set for a specific CPU.

Given the counter is shared between the clocksource and the
clockevent, the STM module can not be disabled by one or another so
the refcounting mechanism is used to stop the counter when it reaches
zero and to start it when it is one. The suspend and resume relies on
the refcount to stop the module.

As the device tree will have multiple STM entries, the driver can be
probed in parallel with the async option but it is not enabled
yet. However, the driver code takes care of preventing a race by
putting a lock to protect the number of STM instances global variable
which means it is ready to support the option when enough testing will
be done with the underlying time framework.

Cc: Ghennadi Procopciuc <[email protected]>
Cc: Krzysztof Kozlowski <[email protected]>
Cc: Thomas Fossati <[email protected]>
Suggested-by: Ghennadi Procopciuc <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Daniel Lezcano <[email protected]>

Author: dlezcano

drivers/clocksource/Kconfig

@@ -763,4 +763,12 @@ config RALINK_TIMER
 	  Enables support for system tick counter present on
 	  Ralink SoCs RT3352 and MT7620.
 
+config NXP_STM_TIMER
+	bool "NXP System Timer Module driver"
+	depends on ARCH_S32 || COMPILE_TEST
+	select CLKSRC_MMIO
+	help
+          Enables the support for NXP System Timer Module found in the
+          s32g NXP platform series.
+
 endmenu

drivers/clocksource/Makefile

@@ -92,3 +92,4 @@ obj-$(CONFIG_GXP_TIMER)			+= timer-gxp.o
 obj-$(CONFIG_CLKSRC_LOONGSON1_PWM)	+= timer-loongson1-pwm.o
 obj-$(CONFIG_EP93XX_TIMER)		+= timer-ep93xx.o
 obj-$(CONFIG_RALINK_TIMER)		+= timer-ralink.o
+obj-$(CONFIG_NXP_STM_TIMER)		+= timer-nxp-stm.o