From 9496c252717dfd11d2eb762d0a52df0495996f47 Mon Sep 17 00:00:00 2001
From: Cameron Gutman <aicommander@gmail.com>
Date: Sat, 4 Apr 2026 14:47:57 -0500
Subject: [PATCH] Add a lock to synchronize DRM master ownership between Qt and
 SDL

---
 app/masterhook.c          | 19 +++++++++++++++++++
 app/masterhook_internal.c | 22 ++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/app/masterhook.c b/app/masterhook.c
index 3d7cb835..fb21793a 100644
--- a/app/masterhook.c
+++ b/app/masterhook.c
@@ -78,6 +78,9 @@ int g_QtCrtcConnectorCount;
 bool removeSdlFd(int fd);
 int takeMasterFromSdlFd(void);
 
+void lockDrmMaster();
+void unlockDrmMaster();
+
 int drmIsMaster(int fd)
 {
     // Detect master by attempting something that requires master.
@@ -136,6 +139,9 @@ int drmModePageFlip(int fd, uint32_t crtc_id, uint32_t fb_id, uint32_t flags, vo
     // Call into the real thing
     int err = fn_drmModePageFlip(fd, crtc_id, fb_id, flags, user_data);
     if (!g_DisableDrmHooks && err == -EACCES && fd == g_QtDrmMasterFd) {
+        // Don't allow DRM master ownership to change
+        lockDrmMaster();
+
         // If SDL took master from us, try to grab it back temporarily
         int oldMasterFd = takeMasterFromSdlFd();
         drmSetMaster(fd);
@@ -144,7 +150,10 @@ int drmModePageFlip(int fd, uint32_t crtc_id, uint32_t fb_id, uint32_t flags, vo
         if (oldMasterFd != -1) {
             drmSetMaster(oldMasterFd);
         }
+
+        unlockDrmMaster();
     }
+
     return err;
 }
 
@@ -172,6 +181,9 @@ int drmModeAtomicCommit(int fd, drmModeAtomicReqPtr req,
     // Call into the real thing
     int err = fn_drmModeAtomicCommit(fd, req, flags, user_data);
     if (err == -EACCES && fd == g_QtDrmMasterFd) {
+        // Don't allow DRM master ownership to change
+        lockDrmMaster();
+
         // If SDL took master from us, try to grab it back temporarily
         int oldMasterFd = takeMasterFromSdlFd();
         drmSetMaster(fd);
@@ -180,7 +192,10 @@ int drmModeAtomicCommit(int fd, drmModeAtomicReqPtr req,
         if (oldMasterFd != -1) {
             drmSetMaster(oldMasterFd);
         }
+
+        unlockDrmMaster();
     }
+
     return err;
 }
 
@@ -233,6 +248,8 @@ int close(int fd)
 
     // If we closed the last SDL FD, restore master to the Qt FD
     if (ret == 0 && lastSdlFd) {
+        lockDrmMaster();
+
         if (drmSetMaster(g_QtDrmMasterFd) < 0) {
             SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                          "Failed to restore master to Qt DRM FD: %d",
@@ -257,6 +274,8 @@ int close(int fd)
                              errno);
             }
         }
+
+        unlockDrmMaster();
     }
 
     return ret;
diff --git a/app/masterhook_internal.c b/app/masterhook_internal.c
index a8708fcb..b03be56e 100644
--- a/app/masterhook_internal.c
+++ b/app/masterhook_internal.c
@@ -38,6 +38,21 @@ int g_SdlDrmMasterFds[MAX_SDL_FD_COUNT] = {-1, -1, -1, -1, -1, -1, -1, -1};
 int g_SdlDrmMasterFdCount = 0;
 pthread_mutex_t g_FdTableLock = PTHREAD_MUTEX_INITIALIZER;
 
+// This lock protects sections of code that must run uninterrupted with DRM master
+pthread_mutex_t g_MasterLock = PTHREAD_MUTEX_INITIALIZER;
+
+// Lock order: g_MasterLock -> g_FdTableLock
+
+void lockDrmMaster()
+{
+    pthread_mutex_lock(&g_MasterLock);
+}
+
+void unlockDrmMaster()
+{
+    pthread_mutex_unlock(&g_MasterLock);
+}
+
 // Caller must hold g_FdTableLock
 int getSdlFdEntryIndex(bool unused)
 {
@@ -137,6 +152,9 @@ int openHook(typeof(open) *real_open, typeof(close) *real_close, const char *pat
                 int freeFdIndex;
                 int allocatedFdIndex;
 
+                // Prevent other threads from stealing DRM master for now
+                lockDrmMaster();
+
                 // It is our device. Time to do the magic!
                 pthread_mutex_lock(&g_FdTableLock);
 
@@ -144,6 +162,7 @@ int openHook(typeof(open) *real_open, typeof(close) *real_close, const char *pat
                 freeFdIndex = getSdlFdEntryIndex(true);
                 if (freeFdIndex < 0) {
                     pthread_mutex_unlock(&g_FdTableLock);
+                    unlockDrmMaster();
                     SDL_assert(freeFdIndex >= 0);
                     SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                                  "No unused SDL FD table entries!");
@@ -164,6 +183,7 @@ int openHook(typeof(open) *real_open, typeof(close) *real_close, const char *pat
                     // Drop master on Qt's FD so we can pick it up for SDL.
                     if (drmDropMaster(g_QtDrmMasterFd) < 0) {
                         pthread_mutex_unlock(&g_FdTableLock);
+                        unlockDrmMaster();
                         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                                      "Failed to drop master on Qt DRM FD: %d",
                                      errno);
@@ -195,6 +215,8 @@ int openHook(typeof(open) *real_open, typeof(close) *real_close, const char *pat
                 }
 
                 pthread_mutex_unlock(&g_FdTableLock);
+
+                unlockDrmMaster();
             }
         }
     }