nb/intel/sandybridge: Add comments about I/O and RT latency
Document the algorithm to adjust I/O and roundtrip latencies.
Change-Id: Ic8b9aed54a34bb3252c457e87e81387fd410e305
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48397
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/northbridge/intel/sandybridge/raminit_common.c b/src/northbridge/intel/sandybridge/raminit_common.c
index d24e5a8..70a856c 100644
--- a/src/northbridge/intel/sandybridge/raminit_common.c
+++ b/src/northbridge/intel/sandybridge/raminit_common.c
@@ -1059,6 +1059,7 @@
{
wait_for_iosav(channel);
+ /* Send a burst of 16 back-to-back read commands (4 DCLK apart) */
iosav_write_read_mpr_sequence(channel, slotrank, ctrl->tMOD, 1, 3, 15, ctrl->CAS + 36);
/* Execute command queue */
@@ -1193,6 +1194,12 @@
}
}
+/*
+ * Once the DQS high phase has been found (for each DRAM) the next stage
+ * is to find out the round trip latency, by locating the preamble cycle.
+ * This is achieved by trying smaller and smaller roundtrip values until
+ * the strobe sampling is done on the preamble cycle.
+ */
static int find_roundtrip_latency(ramctr_timing *ctrl, int channel, int slotrank, int *upperA)
{
int works[NUM_LANES];
@@ -1212,10 +1219,17 @@
else
all_works = 0;
}
+
+ /* If every lane is working, exit */
if (all_works)
return 0;
+ /*
+ * If all bits are one (everyone is failing), decrement
+ * the roundtrip value by two, and do another iteration.
+ */
if (!some_works) {
+ /* Guard against roundtrip latency underflow */
if (ctrl->timings[channel][slotrank].roundtrip_latency < 2) {
printk(BIOS_EMERG, "402x discovery failed (1): %d, %d\n",
channel, slotrank);
@@ -1225,9 +1239,16 @@
printram("4024 -= 2;\n");
continue;
}
+
+ /*
+ * Else (if some lanes are failing), increase the rank's
+ * I/O latency by 2, and increase rcven logic delay by 2
+ * on the working lanes, then perform another iteration.
+ */
ctrl->timings[channel][slotrank].io_latency += 2;
printram("4028 += 2;\n");
+ /* Guard against I/O latency overflow */
if (ctrl->timings[channel][slotrank].io_latency >= 0x10) {
printk(BIOS_EMERG, "402x discovery failed (2): %d, %d\n",
channel, slotrank);