skylake: provide more clarity for PCR access

The current primary to sideband (P2SB) code for private configuration
register (PCR) access weren't very clear with the naming or
reasoning for some of the code. Provide more verbiage surrounding
this interface.

BUG=None
BRANCH=None
TEST=Built and booted glados.

Change-Id: I5b2e84444a29b2fc2f527502e8c9f26eb60e687a
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 06345ba1abd893059a6584856851f92f43289247
Original-Change-Id: If57a4bbc90365c1135b4986dce328b5dbabe483b
Original-Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/286900
Original-Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: http://review.coreboot.org/11029
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

2 files changed, 19 insertions, 2 deletions

diff --git a/src/soc/intel/skylake/include/soc/pcr.h b/src/soc/intel/skylake/include/soc/pcr.h
index 9ac9eddd48..a6987cb33a 100644
--- a/src/soc/intel/skylake/include/soc/pcr.h
+++ b/src/soc/intel/skylake/include/soc/pcr.h
@@ -16,9 +16,12 @@
 #ifndef _SOC_PCR_H_
 #define _SOC_PCR_H_
 
+/*
+ * Primary to sideband (P2SB) for private configuration registers (PCR).
+ */
+
 /* PCH (SunRisePoint LP) */
 #define PCH_PCR_BASE_ADDRESS	0xFD000000
-#define R_PCH_PCR_LPC_GCFD	0x3418
 
 /* DMI Control Register */
 #define R_PCH_PCR_DMI_DMIC	0x2234
diff --git a/src/soc/intel/skylake/pcr.c b/src/soc/intel/skylake/pcr.c
index 33d6cd12f5..79c32f90a7 100644
--- a/src/soc/intel/skylake/pcr.c
+++ b/src/soc/intel/skylake/pcr.c
@@ -70,6 +70,19 @@ u8 pcr_read8(PCH_SBI_PID pid, u16 offset, u8 *outdata)
 }
 
 /*
+ * After every write one needs to perform a read an innocuous register to
+ * ensure the writes are completed for certain ports. This is done for
+ * all ports so that the callers don't need the per-port knowledge for
+ * each transaction.
+ */
+static inline void complete_write(void)
+{
+	/* Read the general control and function disable register. */
+	const size_t R_PCH_PCR_LPC_GCFD = 0x3418;
+	read32(PCH_PCR_ADDRESS(PID_LPC, R_PCH_PCR_LPC_GCFD));
+}
+
+/*
  * Write PCR register. (This is internal function)
  * It returns PCR register and size in 1/2/4 bytes.
  * The offset should not exceed 0xFFFF and must be aligned with size
@@ -101,7 +114,8 @@ static u8 pch_pcr_write(PCH_SBI_PID pid, u16 offset, u32 size, u32 data)
 	default:
 		break;
 	}
-	read32(PCH_PCR_ADDRESS(PID_LPC, R_PCH_PCR_LPC_GCFD));
+	/* Ensure the writes complete. */
+	complete_write();
 
 	return 0;
 }