xf86drm: Add faux bus

Linux 6.14 included a new type of bus, the "faux" bus [1].

The next version of Linux (v6.16) will move the VKMS driver to the faux
bus. See kernel commit 5686601908d8 ("drm/vkms: convert to use
faux_device") for more details.

Add support for the faux bus so drmGetDeviceFromDevId(), drmGetDevices()
and drmGetDevices2() return the devices on it.

[1] https://lore.kernel.org/all/2025021023-sandstorm-precise-9f5d@gregkh/
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>

tests/drmdevice.c

@@ -98,6 +98,12 @@ print_device_info(drmDevicePtr device, int i, bool print_revision)
             printf("                    %s\n", *compatible);
             compatible++;
         }
+    } else if (device->bustype == DRM_BUS_FAUX) {
+        printf("|   +-> faux\n");
+
+        printf("+-> deviceinfo\n");
+        printf("    +-> faux\n");
+        printf("        +-> name  %s\n", device->deviceinfo.faux->name);
     } else {
         printf("Unknown/unhandled bustype\n");
     }

xf86drm.c

@@ -42,6 +42,7 @@
 #include <stddef.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <libgen.h>
 #include <limits.h>
 #include <signal.h>
 #include <time.h>
@@ -3592,6 +3593,7 @@ static int get_subsystem_type(const char *device_path)
         { "/spi", DRM_BUS_PLATFORM },
         { "/host1x", DRM_BUS_HOST1X },
         { "/virtio", DRM_BUS_VIRTIO },
+        { "/faux", DRM_BUS_FAUX },
     };
 
     strncpy(path, device_path, PATH_MAX);
@@ -3795,6 +3797,9 @@ drm_public int drmDevicesEqual(drmDevicePtr a, drmDevicePtr b)
     case DRM_BUS_HOST1X:
         return memcmp(a->businfo.host1x, b->businfo.host1x, sizeof(drmHost1xBusInfo)) == 0;
 
+    case DRM_BUS_FAUX:
+        return memcmp(a->deviceinfo.faux, b->deviceinfo.faux, sizeof(drmFauxDeviceInfo)) == 0;
+
     default:
         break;
     }
@@ -4015,6 +4020,12 @@ static void drmFreeHost1xDevice(drmDevicePtr device)
     }
 }
 
+static void drmFreeFauxDevice(drmDevicePtr device)
+{
+    if (device->deviceinfo.faux)
+        free(device->deviceinfo.faux->name);
+}
+
 drm_public void drmFreeDevice(drmDevicePtr *device)
 {
     if (device == NULL)
@@ -4029,6 +4040,10 @@ drm_public void drmFreeDevice(drmDevicePtr *device)
         case DRM_BUS_HOST1X:
             drmFreeHost1xDevice(*device);
             break;
+
+        case DRM_BUS_FAUX:
+            drmFreeFauxDevice(*device);
+            break;
         }
     }
 
@@ -4457,6 +4472,65 @@ free_device:
     return ret;
 }
 
+static int drmParseFauxDeviceInfo(int maj, int min, drmFauxDeviceInfoPtr info)
+{
+#ifdef __linux__
+    char path[PATH_MAX + 1] = "";
+    char real_path[PATH_MAX + 1] = "";
+    char *name;
+
+    snprintf(path, sizeof(path), "/sys/dev/char/%d:%d/device", maj, min);
+
+    if (!realpath(path, real_path))
+        return -errno;
+
+    name = basename(real_path);
+    if (!name)
+        return -ENOENT;
+
+    info->name = strdup(name);
+    if (!info->name)
+        return -errno;
+
+    return 0;
+#else
+#warning "Missing implementation of drmParseFauxDeviceInfo"
+    return -EINVAL;
+#endif
+}
+
+static int drmProcessFauxDevice(drmDevicePtr *device,
+                                const char *node, int node_type,
+                                int maj, int min, bool fetch_deviceinfo,
+                                uint32_t flags)
+{
+    drmDevicePtr dev;
+    char *ptr;
+    int ret;
+
+    dev = drmDeviceAlloc(node_type, node, 0, sizeof(drmFauxDeviceInfo), &ptr);
+    if (!dev)
+        return -ENOMEM;
+
+    dev->bustype = DRM_BUS_FAUX;
+
+    if (fetch_deviceinfo) {
+        dev->deviceinfo.faux = (drmFauxDeviceInfoPtr)ptr;
+
+        ret = drmParseFauxDeviceInfo(maj, min, dev->deviceinfo.faux);
+        if (ret < 0)
+            goto free_device;
+    }
+
+    *device = dev;
+
+    return 0;
+
+free_device:
+    free(dev);
+    return ret;
+}
+
 static int
 process_device(drmDevicePtr *device, const char *d_name,
                int req_subsystem_type,
@@ -4509,6 +4583,9 @@ process_device(drmDevicePtr *device, const char *d_name,
     case DRM_BUS_HOST1X:
         return drmProcessHost1xDevice(device, node, node_type, maj, min,
                                       fetch_deviceinfo, flags);
+    case DRM_BUS_FAUX:
+        return drmProcessFauxDevice(device, node, node_type, maj, min,
+                                    fetch_deviceinfo, flags);
     default:
         return -1;
    }

xf86drm.h

@@ -843,6 +843,7 @@ extern char *drmGetRenderDeviceNameFromFd(int fd);
 #define DRM_BUS_USB       1
 #define DRM_BUS_PLATFORM  2
 #define DRM_BUS_HOST1X    3
+#define DRM_BUS_FAUX      4
 
 typedef struct _drmPciBusInfo {
     uint16_t domain;
@@ -889,6 +890,10 @@ typedef struct _drmHost1xDeviceInfo {
     char **compatible; /* NULL terminated list of compatible strings */
 } drmHost1xDeviceInfo, *drmHost1xDeviceInfoPtr;
 
+typedef struct _drmFauxDeviceInfo {
+    char *name;
+} drmFauxDeviceInfo, *drmFauxDeviceInfoPtr;
+
 typedef struct _drmDevice {
     char **nodes; /* DRM_NODE_MAX sized array */
     int available_nodes; /* DRM_NODE_* bitmask */
@@ -904,6 +909,7 @@ typedef struct _drmDevice {
         drmUsbDeviceInfoPtr usb;
         drmPlatformDeviceInfoPtr platform;
         drmHost1xDeviceInfoPtr host1x;
+        drmFauxDeviceInfoPtr faux;
     } deviceinfo;
 } drmDevice, *drmDevicePtr;