From 860df4b2b627f025b963920d085316e694135564 Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh@NetBSD.org>
Date: Mon, 5 Sep 2022 12:48:53 +0000
Subject: [PATCH] x86: Fix interaction between consinit, device_pci_register,
 and drm.

Leave an essay on what's going on here in both places with
cross-references.

PR kern/56996
---
 sys/arch/x86/pci/pci_machdep.c | 31 +++++++++++++++++++++++--------
 sys/arch/x86/x86/consinit.c    |  9 +++++++++
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/sys/arch/x86/pci/pci_machdep.c b/sys/arch/x86/pci/pci_machdep.c
index 1029001b438e..8e2cdd5a6020 100644
--- a/sys/arch/x86/pci/pci_machdep.c
+++ b/sys/arch/x86/pci/pci_machdep.c
@@ -1228,14 +1228,29 @@ device_pci_register(device_t dev, void *aux)
 			 */
 			populate_fbinfo(dev, dict);
 
-#if 1 && NWSDISPLAY > 0 && NGENFB > 0
-			/* XXX */
-			if (device_is_a(dev, "genfb")) {
-				prop_dictionary_set_bool(dict, "is_console",
-				    genfb_is_console());
-			} else
-#endif
-			prop_dictionary_set_bool(dict, "is_console", true);
+			/*
+			 * If the bootloader requested console=pc and
+			 * specified a framebuffer, and if
+			 * x86_genfb_cnattach succeeded in setting it
+			 * up during consinit, then consinit will call
+			 * genfb_cnattach which makes genfb_is_console
+			 * return true.  In this case, if it's the
+			 * first genfb we've seen, we will instruct the
+			 * genfb driver via the is_console property
+			 * that it has been selected as the console.
+			 *
+			 * If not all of that happened, then consinit
+			 * can't have selected a genfb console, so this
+			 * device is definitely not the console.
+			 *
+			 * XXX What happens if there's more than one
+			 * PCI display device, and the bootloader picks
+			 * the second one's framebuffer as the console
+			 * framebuffer address?  Tough...but this has
+			 * probably never worked.
+			 */
+			prop_dictionary_set_bool(dict, "is_console",
+			    genfb_is_console());
 
 			prop_dictionary_set_bool(dict, "clear-screen", false);
 #if NWSDISPLAY > 0 && NGENFB > 0
diff --git a/sys/arch/x86/x86/consinit.c b/sys/arch/x86/x86/consinit.c
index ea34d968d01a..a619c4ba4132 100644
--- a/sys/arch/x86/x86/consinit.c
+++ b/sys/arch/x86/x86/consinit.c
@@ -197,6 +197,15 @@ consinit(void)
 		int error;
 #if (NGENFB > 0)
 		if (fbinfo && fbinfo->physaddr > 0) {
+			/*
+			 * If we have a framebuffer address, and
+			 * x86_genfb_cnattach can map it, then
+			 * genfb_cnattach causes genfb_is_console to
+			 * later return true.  device_pci_register will
+			 * use this to set up the device properties for
+			 * a PCI display-class device to notify it that
+			 * it has been selected as the console.
+			 */
 			if (x86_genfb_cnattach() == -1) {
 				initted = 0;	/* defer */
 				return;