From c79f3765944e059bb2aef2b2b1b6a1fa69faa2c1 Mon Sep 17 00:00:00 2001
From: Oleh Nykyforchyn <oleh.nyk@gmail.com>
Date: Sat, 27 Dec 2025 23:01:28 +0200
Subject: [PATCH] xfree86/common: introduce SingleDriver server flag

It is patch 1/3 of a series that makes adding GPU screens
more controllable.

If SingleDriver option is set to "on", then only the first
successfully probed driver adds non-GPU screens, others
may add secondary GPU screens only.

Fixes github.com/X11Libre/xserver/issues/1669

Signed-off-by: Oleh Nykyforchyn <oleh.nyk@gmail.com>
---
 hw/xfree86/common/xf86Bus.c     | 13 ++++++++++---
 hw/xfree86/common/xf86Bus.h     |  2 +-
 hw/xfree86/common/xf86Config.c  | 14 ++++++++++++++
 hw/xfree86/common/xf86Globals.c |  1 +
 hw/xfree86/common/xf86Init.c    |  2 +-
 hw/xfree86/common/xf86Privstr.h |  3 +++
 6 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/hw/xfree86/common/xf86Bus.c b/hw/xfree86/common/xf86Bus.c
index 1389432984..0d19a4b570 100644
--- a/hw/xfree86/common/xf86Bus.c
+++ b/hw/xfree86/common/xf86Bus.c
@@ -144,9 +144,12 @@ xf86BusConfigMatch(ScrnInfoPtr scrnInfo, Bool is_gpu) {
 /**
  * @return TRUE if all buses are configured and set up correctly and FALSE
  * otherwise.
+ *
+ * If singleDriver is TRUE, then only the first successfully probed driver adds screens to xf86Screens,
+ * others may add GPU secondary screens only
  */
 Bool
-xf86BusConfig(void)
+xf86BusConfig(Bool singleDriver)
 {
     screenLayoutPtr layout;
     int i;
@@ -160,7 +163,9 @@ xf86BusConfig(void)
      *    xf86Screens[] list for each instance of the hardware found.
      */
     for (i = 0; i < xf86NumDrivers; i++) {
-        xf86CallDriverProbe(xf86DriverList[i], FALSE);
+        /* The order of the && operands below is essential! */
+        if (xf86CallDriverProbe(xf86DriverList[i], FALSE) && singleDriver)
+            break;
     }
 
     /*
@@ -171,7 +176,9 @@ xf86BusConfig(void)
     if (xf86NumScreens == 0) {
         xf86ProbeIgnorePrimary = TRUE;
         for (i = 0; i < xf86NumDrivers && xf86NumScreens == 0; i++) {
-            xf86CallDriverProbe(xf86DriverList[i], FALSE);
+            /* The order of the && operands below is essential! */
+            if (xf86CallDriverProbe(xf86DriverList[i], FALSE) && singleDriver)
+                break;
         }
         xf86ProbeIgnorePrimary = FALSE;
     }
diff --git a/hw/xfree86/common/xf86Bus.h b/hw/xfree86/common/xf86Bus.h
index 933bef492b..6bec9fa7d9 100644
--- a/hw/xfree86/common/xf86Bus.h
+++ b/hw/xfree86/common/xf86Bus.h
@@ -77,7 +77,7 @@ Bool xf86IsEntityPrimary(int entityIndex);
 _X_EXPORT /* only for internal int10 module - not supposed to be used by OOT drivers */
 ScrnInfoPtr xf86FindScreenForEntity(int entityIndex);
 
-Bool xf86BusConfig(void);
+Bool xf86BusConfig(Bool singleDriver);
 void xf86ClearEntityListForScreen(ScrnInfoPtr pScrn);
 void xf86RemoveDevFromEntity(int entityIndex, GDevPtr dev);
 
diff --git a/hw/xfree86/common/xf86Config.c b/hw/xfree86/common/xf86Config.c
index 0a2e3a7454..8dce643fbb 100644
--- a/hw/xfree86/common/xf86Config.c
+++ b/hw/xfree86/common/xf86Config.c
@@ -657,6 +657,7 @@ typedef enum {
     FLAG_IGLX,
     FLAG_DEBUG,
     FLAG_ALLOW_BYTE_SWAPPED_CLIENTS,
+    FLAG_SINGLE_DRIVER,
 } FlagValues;
 
 /**
@@ -718,6 +719,8 @@ static OptionInfoRec FlagOptions[] = {
      {0}, FALSE},
     {FLAG_ALLOW_BYTE_SWAPPED_CLIENTS, "AllowByteSwappedClients", OPTV_BOOLEAN,
      {0}, FALSE},
+    {FLAG_SINGLE_DRIVER, "SingleDriver", OPTV_BOOLEAN,
+     {0}, FALSE},
     {-1, NULL, OPTV_NONE,
      {0}, FALSE},
 };
@@ -815,6 +818,17 @@ configServerFlags(XF86ConfFlagsPtr flagsconf, XF86OptionPtr layoutopts)
     LogMessageVerb(from, 1, "%sutomatically binding GPU devices\n",
                    xf86Info.autoBindGPU ? "A" : "Not a");
 
+    if (xf86IsOptionSet(FlagOptions, FLAG_SINGLE_DRIVER)) {
+        xf86GetOptValBool(FlagOptions, FLAG_SINGLE_DRIVER,
+                          &xf86Info.singleDriver);
+        from = X_CONFIG;
+    }
+    else {
+        from = X_DEFAULT;
+    }
+    LogMessageVerb(from, 1, "Allowing %s one driver to add non-GPU screens\n",
+                   xf86Info.singleDriver ? "only" : "more than");
+
     /*
      * Set things up based on the config file information.  Some of these
      * settings may be overridden later when the command line options are
diff --git a/hw/xfree86/common/xf86Globals.c b/hw/xfree86/common/xf86Globals.c
index 58e20dcde3..5c7d5820bd 100644
--- a/hw/xfree86/common/xf86Globals.c
+++ b/hw/xfree86/common/xf86Globals.c
@@ -131,6 +131,7 @@ xf86InfoRec xf86Info = {
     .autoAddGPU = FALSE,
 #endif
     .autoBindGPU = TRUE,
+    .singleDriver = FALSE,
 };
 
 const char *xf86ConfigFile = NULL;
diff --git a/hw/xfree86/common/xf86Init.c b/hw/xfree86/common/xf86Init.c
index 33e2c4aad0..557f357d56 100644
--- a/hw/xfree86/common/xf86Init.c
+++ b/hw/xfree86/common/xf86Init.c
@@ -440,7 +440,7 @@ InitOutput(int argc, char **argv)
 	if (want_hw_access)
 	    xorgHWAccess = xf86EnableIO();
 
-        if (xf86BusConfig() == FALSE)
+        if (xf86BusConfig(xf86Info.singleDriver) == FALSE)
             return;
 
 
diff --git a/hw/xfree86/common/xf86Privstr.h b/hw/xfree86/common/xf86Privstr.h
index 5a1db5d1c1..38100c4db4 100644
--- a/hw/xfree86/common/xf86Privstr.h
+++ b/hw/xfree86/common/xf86Privstr.h
@@ -94,6 +94,9 @@ typedef struct {
     Bool autoAddGPU;
     const char *debug;
     Bool autoBindGPU;
+
+    Bool singleDriver;          /* Only the first successfully probed driver adds primary screens,
+                                 * others may add GPU secondary screens only */
 } xf86InfoRec, *xf86InfoPtr;
 
 /* ISC's cc can't handle ~ of UL constants, so explicitly type cast them. */