mb/google/brya/acpi: Save/restore/clear some registers over GC6

Nvidia recommends saving and restoring the LTR Enable bit in PCIe config
space for the PCIe root port before/after GC6 entry. Also the detectable
error bit should be cleared, as there may be errors expected during the
GC6 flow.

BUG=b:214581763
TEST=no more correctable errors after GC6 entry/exit

Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Change-Id: I058ce1b3f17fb6cc59785a85efaf9ea0504cf2ee
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66808
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Anil Kumar K <anil.kumar.k@intel.corp-partner.google.com>
Reviewed-by: Ivy Jian <ivy.jian@quanta.corp-partner.google.com>

src/mainboard/google/brya/acpi/peg.asl

@@ -6,9 +6,14 @@ External (\_SB.PCI0.PEG0.PEGP.NPOF, MethodObj)
 OperationRegion (PCIC, PCI_Config, 0x00, 0x100)
 Field (PCIC, AnyAcc, NoLock, Preserve)
 {
+	Offset (0x4a),
+	CEDR,   1,		/* Correctable Error Detected, RW/1C/V */
 	Offset (0x52),
 	,	13,
 	LASX,	1,		/* Link Active Status */
+	Offset (0x69),
+	,       2,
+	LREN,   1,		/* LTR Enabled */
 	Offset (0xe0),
 	,	7,
 	NCB7,	1,		/* Scratch bit to save L2/3 state */

src/mainboard/google/brya/acpi/power.asl

@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0-or-later */
 
+#include <device/pci_ids.h>
+
 External (\_SB.PCI0.PMC.IPCS, MethodObj)
 
 /* Voltage rail control signals */
@@ -70,6 +72,8 @@ Name (GCOT, 0)
 #define PMC_RP_IDX		(1 << 27)
 #define PMC_RP_ENABLE		(1 << 27)
 #define PMC_RP_DISABLE		0x0
+/* Copy of LTR enable bit from PEG port */
+Name (SLTR, 0)
 
 /* Control the PCIe SRCCLK# for dGPU */
 Method (SRCC, 1, Serialized)
@@ -94,6 +98,9 @@ Method (GC6I, 0, Serialized)
 {
 	GC6E = GC6_STATE_TRANSITION
 
+	/* Save the PEG port's LTR setting */
+	SLTR = LREN
+
 	/* Put PCIe link into L2/3 */
 	\_SB.PCI0.PEG0.DL23 ()
 
@@ -152,6 +159,24 @@ Method (GC6O, 0, Serialized)
 	\_SB.PCI0.PEG0.LD23 ()
 
 	Printf ("dGPU exited GC6")
+	/* Wait for dGPU to reappear on the bus */
+	Local0 = 50
+	While (NVID != PCI_VID_NVIDIA)
+	{
+		Stall (100)
+		Local0--
+		If (Local0 == 0)
+		{
+			Break
+		}
+	}
+
+	/* Restore the PEG LTR enable bit */
+	LREN = SLTR
+
+	/* Clear recoverable errors detected bit */
+	CEDR = 1
+
 	GC6E = GC6_STATE_EXITED
 }