Add runtime configuration sources to mounted dmdevfs instances

README.md

@@ -135,6 +135,52 @@ the whole string as the path - a platform that passes a secret needs this versio
 
 DMDEVFS uses configuration files to define and initialize device drivers. This is a critical mechanism that allows the filesystem to dynamically discover and configure hardware drivers at mount time.
 
+### Runtime configuration sources
+
+An existing mount accepts additional configuration directories through
+`DMDEVFS_IOCTL_ADD_CONFIG` and removes them through
+`DMDEVFS_IOCTL_REMOVE_CONFIG` (declared in `dmdevfs.h`). Pass a directory path
+as the ioctl argument, using a file handle from that mount; its built-in
+`null` node is a convenient stable control handle:
+
+```c
+void* control = Dmod_FileOpen("/dev/null", "r+");
+Dmod_Ioctl(control, DMDEVFS_IOCTL_ADD_CONFIG, "/ext/cfg/drivers");
+Dmod_Ioctl(control, DMDEVFS_IOCTL_ADD_CONFIG, "/mnt/SD/cfg/drivers");
+// The media may already be absent: removal does not read its files.
+Dmod_Ioctl(control, DMDEVFS_IOCTL_REMOVE_CONFIG, "/mnt/SD/cfg/drivers");
+Dmod_FileClose(control);
+```
+
+The directory given at mount and all added directories coexist. Each node,
+including hot-plug children and partitions, belongs to its source tree. The
+mount keeps nodes in one `dmlist`; source records store only their path,
+removal state and reference count. Withdrawn nodes stay in that list until
+their source can be reclaimed and are skipped by lookups and directory reads.
+Removal withdraws only that tree's nodes, reports their disappearance to
+libsystemd and their friends, and preserves the other trees and the mount.
+Existing handles from the removed tree fail read/write/ioctl/flush operations
+with `-ENODEV`; they can still be closed. Driver contexts and modules stay
+alive until the last retained reference is released. Open directory handles
+retain a snapshot of nodes and skip withdrawn entries; close them to release
+those references. The mount itself refuses teardown
+with `-EBUSY` while handles remain open.
+
+Commands complete synchronously, serialized with hot-plug events. Call them
+from an application thread, not synchronously from a driver or service
+callback invoked by the dmdevfs worker. An ADD failure rolls back that
+source's new nodes; it leaves existing trees registered. Boot configuration
+keeps its historical best-effort behavior for individual driver failures.
+
+Use absolute paths. Repeated slashes, `.` components and trailing slashes are
+normalized; `..` is rejected. Duplicate active sources return `-EEXIST`,
+overlapping source directories return `-EINVAL`, unknown removals return
+`-ENOENT`, and discovery/configuration failures return `-EIO`. A device-path
+collision also fails the new source. A removed source can be added again.
+The existing configuration-file path length limit still applies. This API
+manages explicitly supplied sources; media detection and deciding when to
+add/remove them remain the caller's responsibility.
+
 ### Overview
 
 When DMDEVFS is mounted with a configuration path, it:

include/dmdevfs.h

@@ -11,6 +11,21 @@
 extern "C" {
 #endif
 
+/** Runtime configuration-tree management. Pass a NUL-terminated directory
+ * path as arg to ioctl on any node of the target mount (prefer /dev/null).
+ * Calls are synchronous and run in FIFO order with hot-plug work. Do not
+ * call synchronously from a driver/service callback invoked by that worker.
+ * Paths are matched lexically, with repeated slashes and '.' normalized;
+ * '..' components are rejected. Use absolute paths in applications.
+ * ADD rejects duplicate/overlapping active trees and rolls back new nodes
+ * on failure. REMOVE needs no access to the source files, withdraws only
+ * its nodes and invalidates their handles (close remains valid). Driver
+ * contexts are reclaimed after the last file/directory reference is released. Returns 0 or a
+ * negative errno (EEXIST, EINVAL, ENOENT, EIO, ENOMEM, ENODEV, EDEADLK).
+ */
+#define DMDEVFS_IOCTL_ADD_CONFIG     0xF01
+#define DMDEVFS_IOCTL_REMOVE_CONFIG  0xF02
+
 // Module version
 #define DMDEVFS_VERSION_MAJOR 0
 #define DMDEVFS_VERSION_MINOR 2