google/rambi: add explicit pull-down for ram-id

Some variants need the internal pull resistor on GPIO_SSUS_40
set explicitly to pull down rather than disabling the pull,
in order for the ram-id to be read correctly via GPIO.

Correct this by adding a function to enable and set the internal pull
and define its use as needed in the board's variant.h.

Chromium source:
branch: firmware-gnawty-5216.239.B
/src/soc/intel/baytrail/baytrail/gpio.h#418
/src/mainboard/google/gnawty/romstage.c#60

Test: boot 4GB Candy board and observe correct RAM id, amount detected

Change-Id: I8823c27385f4422184b5afa57f6048f7ff2a25ab
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/18309
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/mainboard/google/rambi/romstage.c

@@ -32,7 +32,10 @@ static void *get_spd_pointer(char *spd_file_content, int total_spds, int *dual)
 	ssus_disable_internal_pull(GPIO_SSUS_37_PAD);
 	ssus_disable_internal_pull(GPIO_SSUS_38_PAD);
 	ssus_disable_internal_pull(GPIO_SSUS_39_PAD);
-#ifdef GPIO_SSUS_40_PAD
+#ifdef GPIO_SSUS_40_PAD_USE_PULLDOWN
+	/* To prevent floating pin on shipped systems. */
+	ssus_enable_internal_pull(GPIO_SSUS_40_PAD, PAD_PULL_DOWN | PAD_PU_20K);
+#elif defined (GPIO_SSUS_40_PAD)
 	ssus_disable_internal_pull(GPIO_SSUS_40_PAD);
 #endif
 	ram_id |= (ssus_get_gpio(GPIO_SSUS_37_PAD) << 0);

src/mainboard/google/rambi/variants/candy/include/variant/variant.h

@@ -38,5 +38,6 @@ static const uint32_t dual_channel_config =
 #define GPIO_SSUS_38_PAD 50
 #define GPIO_SSUS_39_PAD 58
 #define GPIO_SSUS_40_PAD 52
+#define GPIO_SSUS_40_PAD_USE_PULLDOWN
 
 #endif

src/mainboard/google/rambi/variants/glimmer/include/variant/variant.h

@@ -38,5 +38,6 @@ static const uint32_t dual_channel_config =
 #define GPIO_SSUS_38_PAD 50
 #define GPIO_SSUS_39_PAD 58
 #define GPIO_SSUS_40_PAD 52
+#define GPIO_SSUS_40_PAD_USE_PULLDOWN
 
 #endif

src/mainboard/google/rambi/variants/gnawty/include/variant/variant.h

@@ -33,5 +33,7 @@ static const uint32_t dual_channel_config =
 #define GPIO_SSUS_37_PAD 57
 #define GPIO_SSUS_38_PAD 50
 #define GPIO_SSUS_39_PAD 58
+#define GPIO_SSUS_40_PAD 52
+#define GPIO_SSUS_40_PAD_USE_PULLDOWN
 
 #endif

src/soc/intel/baytrail/include/soc/gpio.h

@@ -454,4 +454,11 @@ static inline void ssus_disable_internal_pull(int pad)
 	write32(ssus_pconf0(pad), read32(ssus_pconf0(pad)) & pull_mask);
 }
 
+static inline void ssus_enable_internal_pull(int pad, int mask)
+{
+	const int pull_mask = ~(0xf << 7);
+	write32(ssus_pconf0(pad),
+		(read32(ssus_pconf0(pad)) & pull_mask) | mask);
+}
+
 #endif /* _BAYTRAIL_GPIO_H_ */