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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>
This commit is contained in:
Angel Pons 2020-12-07 02:00:41 +01:00 committed by Felix Held
parent c8ac2ccf80
commit 3aed6ac613
1 changed files with 21 additions and 0 deletions
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21
src/northbridge/intel/sandybridge/raminit_common.c
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@ -1059,6 +1059,7 @@ static void test_rcven(ramctr_timing *ctrl, int channel, int slotrank)
{
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 @@ static void fine_tune_rcven_pi(ramctr_timing *ctrl, int channel, int slotrank, i
}
}
/*
* 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 @@ static int find_roundtrip_latency(ramctr_timing *ctrl, int channel, int slotrank
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 @@ static int find_roundtrip_latency(ramctr_timing *ctrl, int channel, int slotrank
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);