hv: vhostbridge: add comments to clarify the statement

A vhostbridge can be emulated in hypervisor. Function `init_vhostbridge()` is
used to initialize a virtual host bridge and it configures the PCI configuration
space.

However, some configuration elements are not clearly described, which affects
maintainability and readability. This patch add some comments to address it.

Tracked-On: #8665

Signed-off-by: Haiwei Li <[email protected]>

hypervisor/dm/vpci/vhostbridge.c

@@ -88,26 +88,42 @@ static void init_vhostbridge(struct pci_vdev *vdev)
 {
 	union pci_bdf hostbridge_bdf = {.value = 0x0U};
 	uint32_t pciexbar_low = 0x0U, pciexbar_high = 0x0U, phys_did, i;
+	/* Refer to Section 9 C-Unit in Intel® Pentium® and Celeron® Processor N- and J- Series, Datasheet Volume 2 */
 	/* PCI config space */
 	pci_vdev_write_vcfg(vdev, PCIR_VENDOR, 2U, 0x8086U);
 	pci_vdev_write_vcfg(vdev, PCIR_DEVICE, 2U, 0x5af0U);
 	pci_vdev_write_vcfg(vdev, PCIR_REVID, 1U, 0xbU);
-	pci_vdev_write_vcfg(vdev, PCIR_HDRTYPE, 1U, (PCIM_HDRTYPE_NORMAL | PCIM_MFDEV));
-	pci_vdev_write_vcfg(vdev, PCIR_CLASS, 1U, PCIC_BRIDGE);
 	pci_vdev_write_vcfg(vdev, PCIR_SUBCLASS, 1U, PCIS_BRIDGE_HOST);
-	pci_vdev_write_vcfg(vdev, 0x34U, 1U, 0xe0U);
-	pci_vdev_write_vcfg(vdev, 0x3cU, 1U, 0xe0U);
+	pci_vdev_write_vcfg(vdev, PCIR_CLASS, 1U, PCIC_BRIDGE);
+	pci_vdev_write_vcfg(vdev, PCIR_HDRTYPE, 1U, (PCIM_HDRTYPE_NORMAL | PCIM_MFDEV));
+	/* First Capability Register is CAPID0_CAPCTRL0 */
+	pci_vdev_write_vcfg(vdev, PCIR_CAP_PTR, 1U, 0xe0U);
+	pci_vdev_write_vcfg(vdev, PCIR_INTERRUPT_LINE, 1U, 0xe0U);
 
+	/* Memory Controller Hub Base Address Register, MCHBAR_LO */
+	/* MCHBAR[38:15] is {MCHBAR_HI[6:0],MCHBAR_LO[31:15]} */
 	pci_vdev_write_vcfg(vdev, 0x48U, 4U, 0xfed10001U);
+	/* Graphics and Memory Controller Hub Graphics Control Register, GGC */
+	/* [15:8] is Graphics Memory Select (GMS), 512MB */
 	pci_vdev_write_vcfg(vdev, 0x50U, 4U, 0x000002c1U);
+	/* Device Enable Register, DEVEN */
 	pci_vdev_write_vcfg(vdev, 0x54U, 4U, 0x00000033U);
+	/* Protected Audio Video Path Control, PAVPC */
 	pci_vdev_write_vcfg(vdev, 0x58U, 4U, 0x7ff00007U);
+	/* Top of Upper Usable DRAM Low, TOUUD_LO */
 	pci_vdev_write_vcfg(vdev, 0xa8U, 4U, 0x80000000U);
+	/* Top of Upper Usable DRAM High, TOUUD_HI */
 	pci_vdev_write_vcfg(vdev, 0xacU, 4U, 0x00000002U);
+	/* Base of Data Stolen Memory, BDSM */
 	pci_vdev_write_vcfg(vdev, 0xb0U, 4U, 0x7c000001U);
+	/* Base of Graphics Stolen Memory, BGSM */
 	pci_vdev_write_vcfg(vdev, 0xb4U, 4U, 0x7b800001U);
+	/* Top Segment Memory Base, TSEGMB */
 	pci_vdev_write_vcfg(vdev, 0xb8U, 4U, 0x7b000001U);
+	/* Top of Lower Usable DRAM, TOLUD */
 	pci_vdev_write_vcfg(vdev, 0xbcU, 4U, 0x80000001U);
+	/* Capability ID0 Capability Control, CAPID0_CAPCTRL0 */
+	/* CAP_ID: 9h, NEXT_CAP: 0h, CAPIDLEN: Ch, CAPID_VER: 1h */
 	pci_vdev_write_vcfg(vdev, 0xe0U, 4U, 0x010c0009U);
 	pci_vdev_write_vcfg(vdev, 0xf4U, 4U, 0x011c0f00U);
 
@@ -117,7 +133,7 @@ static void init_vhostbridge(struct pci_vdev *vdev)
 		 */
 		pciexbar_low = USER_VM_VIRT_PCI_MMCFG_BASE | 0x1U;
 	} else {
-		/*Inject physical ECAM value to Service VM vhostbridge since Service VM may check PCIe-MMIO Base Address with it */
+		/* Inject physical ECAM value to Service VM vhostbridge since Service VM may check PCIe-MMIO Base Address with it */
 		phys_did = pci_pdev_read_cfg(hostbridge_bdf, PCIR_DEVICE, 2);
 		for (i = 0U; i < (sizeof(hostbridge_did_highbytes) / sizeof(uint32_t)); i++) {
 			if (((phys_did & 0xff00U) >> 8) == hostbridge_did_highbytes[i]) {
@@ -128,8 +144,9 @@ static void init_vhostbridge(struct pci_vdev *vdev)
 			}
 		}
 	}
-
+	/* PCI Express Enhanced Configuration Range Base Address Low, PCIEXBAR_LO */
 	pci_vdev_write_vcfg(vdev, 0x60U, 4, pciexbar_low);
+	/* PCI Express Enhanced Configuration Range Base Address High, PCIEXBAR_HI */
 	pci_vdev_write_vcfg(vdev, 0x64U, 4, pciexbar_high);
 	vdev->parent_user = NULL;
 	vdev->user = vdev;