soc/intel/skylake: add option to statically clock gate 8254 timer

In order to save more power by shutting down clocks add the
ability to optionally clock gate the 8254 programmable interrupt
timer.  When doing this the platforms lose their "PC"-ness which
certain payloads and OSes rely on such as SeaBIOS.

BUG=chrome-os-partner:50214
BRANCH=glados
TEST=Enabled option on chell. Noted the bit is set upon booting.

Change-Id: I01f9d177bbde417d1efec2e16656a07dcebccbde
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 662575aa6a63656dedfa0ce1f202f5fac0205477
Original-Change-Id: Ib4a613cf1c28fc96c36fa2987c4b58a05beab178
Original-Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/329411
Original-Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/331171
Original-Commit-Ready: Patrick Georgi <pgeorgi@chromium.org>
Original-Tested-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/13985
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/soc/intel/skylake/chip.h

@@ -335,6 +335,8 @@ struct soc_intel_skylake_config {
 	 * 0 - Don't Send, 1 - Send
 	*/
 	u8 SendVrMbxCmd;
+	/* Statically clock gate 8254 PIT. */
+	u8 clock_gate_8254;
 };
 
 typedef struct soc_intel_skylake_config config_t;

src/soc/intel/skylake/include/soc/pcr.h

@@ -60,6 +60,9 @@
 #define R_PCH_PCR_ITSS_PIRQG_ROUT	0x3106
 /* PIRQH Routing Control Register*/
 #define R_PCH_PCR_ITSS_PIRQH_ROUT	0x3107
+/* ITSS Power reduction control */
+#define R_PCH_PCR_ITSS_ITSSPRC		0x3300
+# define CGE8254			(1 << 2)
 
 /* IO Trap PCRs */
 /* Trap status Register */

src/soc/intel/skylake/lpc.c

@@ -154,6 +154,18 @@ static const struct reg_script pch_misc_init_script[] = {
 	REG_SCRIPT_END
 };
 
+static void clock_gate_8254(struct device *dev)
+{
+	config_t *config = dev->chip_info;
+	const uint32_t cge8254_mask = CGE8254;
+
+	if (!config->clock_gate_8254)
+		return;
+
+	pcr_andthenor32(PID_ITSS, R_PCH_PCR_ITSS_ITSSPRC,
+			~cge8254_mask, cge8254_mask);
+}
+
 static void lpc_init(struct device *dev)
 {
 	/* Legacy initialization */
@@ -165,6 +177,7 @@ static void lpc_init(struct device *dev)
 	pch_pirq_init(dev);
 	setup_i8259();
 	i8259_configure_irq_trigger(9, 1);
+	clock_gate_8254(dev);
 }
 
 static void pch_lpc_add_mmio_resources(device_t dev)