nb/intel/sandybridge: Fix handling of clock timing

Clock is a differential signal and propagates faster than command and
control, therefore its timing needs to be offset with `pi_code_offset`.
It is also a periodic signal, so it can safely wrap around.

To avoid potential undefined behavior, make `clk_delay` signed. It makes
no difference with valid values, because the initial value can be proven
to never be negative and `pi_code_offset` is always positive. With this
change, it is possible to add an underflow check, for additional sanity.

Change-Id: I375adf84142079f341b060fba5e79ce4dcb002be
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49319
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

1 files changed, 14 insertions, 4 deletions

diff --git a/src/northbridge/intel/sandybridge/raminit_common.c b/src/northbridge/intel/sandybridge/raminit_common.c
index 092e34dd0a..f766867965 100644
--- a/src/northbridge/intel/sandybridge/raminit_common.c
+++ b/src/northbridge/intel/sandybridge/raminit_common.c
@@ -977,14 +977,24 @@ void program_timings(ramctr_timing *ctrl, int channel)
 		}
 	}
 	FOR_ALL_POPULATED_RANKS {
-		u32 clk_delay = ctrl->timings[channel][slotrank].pi_coding + cmd_delay;
+		int clk_delay = ctrl->timings[channel][slotrank].pi_coding + cmd_delay;
 
-		if (clk_delay > CCC_MAX_PI) {
-			printk(BIOS_ERR, "C%dR%d clock delay overflow: %d\n",
+		/*
+		 * Clock is a differential signal, whereas command and control are not.
+		 * This affects its timing, and it is also why it needs a magic offset.
+		 */
+		clk_delay += ctrl->pi_code_offset;
+
+		/* Can never happen with valid values */
+		if (clk_delay < 0) {
+			printk(BIOS_ERR, "C%dR%d clock delay underflow: %d\n",
 				channel, slotrank, clk_delay);
-			clk_delay = CCC_MAX_PI;
+			clk_delay = 0;
 		}
 
+		/* Clock can safely wrap around because it is a periodic signal */
+		clk_delay %= CCC_MAX_PI + 1;
+
 		clk_pi_coding |= (clk_delay % QCLK_PI) << (6 * slotrank);
 		clk_logic_dly |= (clk_delay / QCLK_PI) << slotrank;
 	}