nb/intel/sandybridge: Add support for DPR

Include DPR in the memory map calculations if enabled. DPR is required
for Intel TXT support.

TEST=Boot Debian 10 and see the DPR memory being reserved in E820 and
cbmem logs:
"BIOS-e820: [mem 0x000000007fc09000-0x00000000829fffff] reserved"
"TSEG base 0x80000000 size 8M"
"DPR base 0x7fd00000 size 3M"

Signed-off-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Change-Id: Ia22e49ba58709acfa0afe0921aa71d83cc06c129
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59512
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/northbridge/intel/sandybridge/memmap.c

@@ -10,6 +10,7 @@
 #include <cpu/x86/smm.h>
 #include <program_loading.h>
 #include "sandybridge.h"
+#include <security/intel/txt/txt_platform.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ -24,12 +25,38 @@ static size_t northbridge_get_tseg_size(void)
 	return CONFIG_SMM_TSEG_SIZE;
 }
 
+union dpr_register txt_get_chipset_dpr(void)
+{
+	return (union dpr_register) { .raw = pci_read_config32(HOST_BRIDGE, DPR) };
+}
+
+/*
+ * Return the topmost memory address below 4 GiB available for general
+ * use, from software's view of memory. Do not confuse this with TOLUD,
+ * which applies to the DRAM as viewed by the memory controller itself.
+ */
+static uintptr_t top_of_low_usable_memory(void)
+{
+	/*
+	 * Base of DPR is top of usable DRAM below 4 GiB. However, DPR
+	 * may not always be enabled. Unlike most memory map registers,
+	 * the DPR register stores top of DPR instead of its base address.
+	 * Top of DPR is R/O, and mirrored from TSEG base by hardware.
+	 */
+	uintptr_t tolum = northbridge_get_tseg_base();
+
+	const union dpr_register dpr = txt_get_chipset_dpr();
+
+	/* Subtract DMA Protected Range size if enabled */
+	if (dpr.epm)
+		tolum -= dpr.size * MiB;
+
+	return tolum;
+}
+
 void *cbmem_top_chipset(void)
 {
-	/* If DPR is disabled, base of TSEG is top of usable DRAM */
-	uintptr_t top_of_ram = northbridge_get_tseg_base();
-
-	return (void *)top_of_ram;
+	return (void *)top_of_low_usable_memory();
 }
 
 void smm_region(uintptr_t *start, size_t *size)

src/northbridge/intel/sandybridge/northbridge.c

@@ -14,6 +14,7 @@
 #include "chip.h"
 #include "sandybridge.h"
 #include <cpu/intel/smm_reloc.h>
+#include <security/intel/txt/txt_platform.h>
 
 /* IGD UMA memory */
 static uint64_t uma_memory_base = 0;
@@ -85,9 +86,11 @@ static void mc_read_resources(struct device *dev)
 {
 	uint64_t tom, me_base, touud;
 	uint32_t tseg_base, uma_size, tolud;
+	uint32_t dpr_base_k, dpr_size_k;
 	uint16_t ggc;
 	unsigned long long tomk;
 	unsigned long index = 3;
+	const union dpr_register dpr = txt_get_chipset_dpr();
 
 	pci_dev_read_resources(dev);
 
@@ -184,6 +187,15 @@ static void mc_read_resources(struct device *dev)
 	uma_memory_size += uma_size * 1024ULL;
 	printk(BIOS_DEBUG, "TSEG base 0x%08x size %uM\n", tseg_base, uma_size >> 10);
 
+	/* Calculate DMA Protected Region if enabled */
+	if (dpr.epm && dpr.size) {
+		dpr_size_k = dpr.size * MiB / KiB;
+		tomk -= dpr_size_k;
+		dpr_base_k = (tseg_base - dpr.size * MiB) / KiB;
+		reserved_ram_resource(dev, index++, dpr_base_k, dpr_size_k);
+		printk(BIOS_DEBUG, "DPR base 0x%08x size %uM\n", dpr_base_k * KiB, dpr.size);
+	}
+
 	printk(BIOS_INFO, "Available memory below 4GB: %lluM\n", tomk >> 10);
 
 	/* Report the memory regions */