rambi: disable internal pullups on ram_id[2:0]

The ram_id[2:0] signals have stuffing options for pull up/down
with values of 10K. However, the default pulldown values for these
pads are 20K. Therefore, one can't read a high value because of
the high voltage threshold is 0.65 * Vref. Therefore the high
signals are marginal at best.

Fix this issue by disabling the internal pull for the pads connected
to ram_id[2:0].

BUG=chrome-os-partner:23350
BRANCH=None
TEST=Built and checked that ram_id[2:0] is properly read now.

Change-Id: Ib414d5798b472574337d1b71b87a4cf92f40c762
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/173211
Reviewed-by: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-by: Bernie Thompson <bhthompson@chromium.org>
Reviewed-on: http://review.coreboot.org/4885
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/mainboard/google/rambi/romstage.c b/src/mainboard/google/rambi/romstage.c
index d15a21dc2d..369ebad445 100644
--- a/src/mainboard/google/rambi/romstage.c
+++ b/src/mainboard/google/rambi/romstage.c
@@ -37,14 +37,29 @@
 #define GPIO_SSUS_38_PAD 50
 #define GPIO_SSUS_39_PAD 58
 
+static inline void disable_internal_pull(int pad)
+{
+	const int pull_mask = ~(0xf << 7);
+	write32(ssus_pconf0(pad), read32(ssus_pconf0(pad)) & pull_mask);
+}
+
 static void *get_spd_pointer(char *spd_file_content, int total_spds)
 {
 	int ram_id = 0;
 
+	/* The ram_id[2:0] pullups on rambi are too large for the default 20K
+	 * pulldown on the pad. Therefore, disable the internal pull resistor to
+	 * read high values correctly. */
+	disable_internal_pull(GPIO_SSUS_37_PAD);
+	disable_internal_pull(GPIO_SSUS_38_PAD);
+	disable_internal_pull(GPIO_SSUS_39_PAD);
+
 	ram_id |= (ssus_get_gpio(GPIO_SSUS_37_PAD) << 0);
 	ram_id |= (ssus_get_gpio(GPIO_SSUS_38_PAD) << 1);
 	ram_id |= (ssus_get_gpio(GPIO_SSUS_39_PAD) << 2);
 
+	printk(BIOS_DEBUG, "ram_id=%d, total_spds: %d\n", ram_id, total_spds);
+
 	if (ram_id >= total_spds)
 		return NULL;