chiptod: Keep boot timestamps contiguous

Currently we reset the timebase value to (almost) zero when
synchronising the timebase of each chip to the Chip TOD network which
results in this:

[   42.374813167,5] CPU: All 80 processors called in...
[    2.222791151,5] FLASH: Found system flash: Macronix MXxxL51235F id:0
[    2.222977933,5] BT: Interface initialized, IO 0x00e4

This patch modifies the chiptod_init() process to use the current
timebase value rather than resetting it to zero. This results in the
timestamps remaining contigious from the start of hostboot until
the petikernel starts. e.g.

[   70.188811484,5] CPU: All 144 processors called in...
[   72.458004252,5] FLASH: Found system flash:  id:0
[   72.458147358,5] BT: Interface initialized, IO 0x00e4

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

core/init.c

@@ -972,9 +972,11 @@ void __noreturn __nomcount main_cpu_entry(const void *fdt)
 	cpu_set_sreset_enable(true);
 
 	/*
-	 * Synchronize time bases. Thi resets all the TB values to a small
-	 * value (so they appear to go backward at this point), and synchronize
-	 * all core timebases to the global ChipTOD network
+	 * Synchronize time bases. Prior to chiptod_init() the timebase
+	 * is free-running at a frequency based on the core clock rather
+	 * than being synchronised to the ChipTOD network. This means
+	 * that the timestamps in early boot might be a little off compared
+	 * to wall clock time.
 	 */
 	chiptod_init();
 

hw/chiptod.c

@@ -120,9 +120,6 @@
 #define LOCAL_CORE_FIR		0x0104000C
 #define LFIR_SWITCH_COMPLETE	PPC_BIT(18)
 
-/* Magic TB value. One step cycle ahead of sync */
-#define INIT_TB	0x000000000001ff0
-
 /* Number of iterations for the various timeouts */
 #define TIMEOUT_LOOPS		20000000
 
@@ -775,6 +772,7 @@ static void chiptod_reset_tod_errors(void)
 
 static void chiptod_sync_master(void *data)
 {
+	uint64_t initial_tb_value;
 	bool *result = data;
 
 	prlog(PR_DEBUG, "Master sync on CPU PIR 0x%04x...\n",
@@ -824,8 +822,17 @@ static void chiptod_sync_master(void *data)
 		goto error;
 	}
 
+	/*
+	 * Load the master's current timebase value into the Chip TOD
+	 * network. This is so we have sane timestamps across the whole
+	 * IPL process. The Chip TOD documentation says that the loaded
+	 * value needs to be one STEP before a SYNC. In other words,
+	 * set the low bits to 0x1ff0.
+	 */
+	initial_tb_value = (mftb() & ~0x1fff) | 0x1ff0;
+
 	/* Chip TOD load initial value */
-	if (xscom_writeme(TOD_CHIPTOD_LOAD_TB, INIT_TB)) {
+	if (xscom_writeme(TOD_CHIPTOD_LOAD_TB, initial_tb_value)) {
 		prerror("XSCOM error setting init TB\n");
 		goto error;
 	}