gbm: split the DRI backend into a separate library and unify backend handling

This does a few things:
1. Applications linked against a non-matching GBM can load the right version of the backend
   from the environment, avoiding GBM/Mesa version mismatches in many cases
2. Distros that want to split off libgbm into a separate package from the actual drivers
   get to not ship 40MB of libgallium_dri
3. The loader logic becomes way less complex

See also: https://github.com/NixOS/nixpkgs/pull/338109 for original rationale.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31074>

src/gbm/backends/dri/gbm_dri.c

@@ -53,11 +53,15 @@
 #include "pipe/p_screen.h"
 #include "dri_screen.h"
 
+#include "gbm_backend_abi.h"
+
 /* For importing wl_buffer */
 #if HAVE_WAYLAND_PLATFORM
 #include "wayland-drm.h"
 #endif
 
+static const struct gbm_core *core;
+
 static GLboolean
 dri_validate_egl_image(void *image, void *data)
 {
@@ -324,7 +328,7 @@ static const struct gbm_dri_visual gbm_dri_visuals_table[] = {
 static int
 gbm_format_to_dri_format(uint32_t gbm_format)
 {
-   gbm_format = gbm_core.v0.format_canonicalize(gbm_format);
+   gbm_format = core->v0.format_canonicalize(gbm_format);
    for (size_t i = 0; i < ARRAY_SIZE(gbm_dri_visuals_table); i++) {
       if (gbm_dri_visuals_table[i].gbm_format == gbm_format)
          return gbm_dri_visuals_table[i].dri_image_format;
@@ -344,7 +348,7 @@ gbm_dri_is_format_supported(struct gbm_device *gbm,
    if ((usage & GBM_BO_USE_CURSOR) && (usage & GBM_BO_USE_RENDERING))
       return 0;
 
-   format = gbm_core.v0.format_canonicalize(format);
+   format = core->v0.format_canonicalize(format);
    if (gbm_format_to_dri_format(format) == 0)
       return 0;
 
@@ -380,7 +384,7 @@ gbm_dri_get_format_modifier_plane_count(struct gbm_device *gbm,
    if (!dri->has_dmabuf_import)
       return -1;
 
-   format = gbm_core.v0.format_canonicalize(format);
+   format = core->v0.format_canonicalize(format);
    if (gbm_format_to_dri_format(format) == 0)
       return -1;
 
@@ -702,7 +706,7 @@ gbm_dri_bo_import(struct gbm_device *gbm,
       /* GBM's GBM_FORMAT_* tokens are a strict superset of the DRI FourCC
        * tokens accepted by createImageFromDmaBufs, except for not supporting
        * the sARGB format. */
-      fourcc = gbm_core.v0.format_canonicalize(fd_data->format);
+      fourcc = core->v0.format_canonicalize(fd_data->format);
 
       image = dri2_from_dma_bufs(dri->screen,
                                  fd_data->width,
@@ -730,7 +734,7 @@ gbm_dri_bo_import(struct gbm_device *gbm,
       /* GBM's GBM_FORMAT_* tokens are a strict superset of the DRI FourCC
        * tokens accepted by createImageFromDmaBufs, except for not supporting
        * the sARGB format. */
-      fourcc = gbm_core.v0.format_canonicalize(fd_data->format);
+      fourcc = core->v0.format_canonicalize(fd_data->format);
 
       image = dri2_from_dma_bufs(dri->screen, fd_data->width,
                                                  fd_data->height, fourcc,
@@ -863,7 +867,7 @@ gbm_dri_bo_create(struct gbm_device *gbm,
    uint64_t *mods_filtered = NULL;
    unsigned int count_filtered = 0;
 
-   format = gbm_core.v0.format_canonicalize(format);
+   format = core->v0.format_canonicalize(format);
 
    if (usage & GBM_BO_USE_WRITE || !dri->has_dmabuf_export)
       return create_dumb(gbm, width, height, format, usage);
@@ -1095,7 +1099,7 @@ gbm_dri_surface_create(struct gbm_device *gbm,
    surf->base.gbm = gbm;
    surf->base.v0.width = width;
    surf->base.v0.height = height;
-   surf->base.v0.format = gbm_core.v0.format_canonicalize(format);
+   surf->base.v0.format = core->v0.format_canonicalize(format);
    surf->base.v0.flags = flags;
    if (!modifiers) {
       assert(!count);
@@ -1153,13 +1157,6 @@ dri_device_create(int fd, uint32_t gbm_backend_version)
    int ret;
    bool force_sw;
 
-   /*
-    * Since the DRI backend is built-in to the loader, the loader ABI version is
-    * guaranteed to match this backend's ABI version
-    */
-   assert(gbm_core.v0.core_version == GBM_BACKEND_ABI_VERSION);
-   assert(gbm_core.v0.core_version == gbm_backend_version);
-
    dri = calloc(1, sizeof *dri);
    if (!dri)
       return NULL;
@@ -1229,3 +1226,11 @@ struct gbm_backend gbm_dri_backend = {
    .v0.backend_name = "dri",
    .v0.create_device = dri_device_create,
 };
+
+struct gbm_backend * gbmint_get_backend(const struct gbm_core *gbm_core);
+
+PUBLIC struct gbm_backend *
+gbmint_get_backend(const struct gbm_core *gbm_core) {
+   core = gbm_core;
+   return &gbm_dri_backend;
+};

src/gbm/backends/dri/meson.build

@@ -0,0 +1,12 @@
+libgbm_dri = shared_library(
+  'dri_gbm',
+  files('gbm_dri.c', 'gbm_driint.h'),
+  include_directories : incs_gbm,
+  link_args : [ld_args_gc_sections],
+  link_with : [libloader, libgallium_dri],
+  dependencies : [deps_gbm, dep_dl, dep_thread, dep_libdrm, idep_mesautil, idep_xmlconfig],
+  gnu_symbol_visibility : 'hidden',
+  install : true,
+  install_dir: join_paths(get_option('libdir'), 'gbm'),
+  name_prefix : '',
+)

src/gbm/main/backend.c

@@ -42,22 +42,12 @@
 #define ARRAY_SIZE(a) (sizeof(a)/sizeof((a)[0]))
 #define VER_MIN(a, b) ((a) < (b) ? (a) : (b))
 
-#ifdef HAVE_DRI
-extern const struct gbm_backend gbm_dri_backend;
-#endif
-
 struct gbm_backend_desc {
    const char *name;
    const struct gbm_backend *backend;
    void *lib;
 };
 
-static const struct gbm_backend_desc builtin_backends[] = {
-#ifdef HAVE_DRI
-   { "dri", &gbm_dri_backend },
-#endif
-};
-
 #define BACKEND_LIB_SUFFIX "_gbm"
 static const char *backend_search_path_vars[] = {
    "GBM_BACKENDS_PATH",
@@ -116,8 +106,16 @@ backend_create_device(const struct gbm_backend_desc *bd, int fd)
 }
 
 static struct gbm_device *
-load_backend(void *lib, int fd, const char *name)
+load_backend_by_name(const char *name, int fd, bool warn_on_fail)
 {
+   void *lib = loader_open_driver_lib(name, BACKEND_LIB_SUFFIX,
+                                      backend_search_path_vars,
+                                      DEFAULT_BACKENDS_PATH,
+                                      warn_on_fail);
+
+   if (!lib)
+      return NULL;
+
    struct gbm_device *dev = NULL;
    struct gbm_backend_desc *backend_desc;
    const struct gbm_backend *gbm_backend;
@@ -146,89 +144,25 @@ fail:
    return NULL;
 }
 
-static struct gbm_device *
-find_backend(const char *name, int fd)
-{
-   struct gbm_device *dev = NULL;
-   const struct gbm_backend_desc *bd;
-   void *lib;
-   unsigned i;
-
-   for (i = 0; i < ARRAY_SIZE(builtin_backends); ++i) {
-      bd = &builtin_backends[i];
-
-      if (name && strcmp(bd->name, name))
-         continue;
-
-      dev = backend_create_device(bd, fd);
-
-      if (dev)
-         break;
-   }
-
-   if (name && !dev) {
-      lib = loader_open_driver_lib(name, BACKEND_LIB_SUFFIX,
-                                   backend_search_path_vars,
-                                   DEFAULT_BACKENDS_PATH,
-                                   true);
-
-      if (lib)
-         dev = load_backend(lib, fd, name);
-   }
-
-   return dev;
-}
-
-static struct gbm_device *
-override_backend(int fd)
-{
-   struct gbm_device *dev = NULL;
-   const char *b;
-
-   b = getenv("GBM_BACKEND");
-   if (b)
-      dev = find_backend(b, fd);
-
-   return dev;
-}
-
-static struct gbm_device *
-backend_from_driver_name(int fd)
-{
-   struct gbm_device *dev = NULL;
-   drmVersionPtr v = drmGetVersion(fd);
-   void *lib;
-
-   if (!v)
-      return NULL;
-
-   lib = loader_open_driver_lib(v->name, BACKEND_LIB_SUFFIX,
-                                backend_search_path_vars,
-                                DEFAULT_BACKENDS_PATH,
-                                false);
-
-   if (lib)
-      dev = load_backend(lib, fd, v->name);
-
-   drmFreeVersion(v);
-
-   return dev;
-}
-
 struct gbm_device *
 _gbm_create_device(int fd)
 {
-   struct gbm_device *dev;
+   struct gbm_device *dev = NULL;
 
-   dev = override_backend(fd);
+   const char *b = getenv("GBM_BACKEND");
+   if (b) {
+      dev = load_backend_by_name(b, fd, true);
+      if (dev) return dev;
+   }
 
-   if (!dev)
-      dev = backend_from_driver_name(fd);
+   drmVersionPtr v = drmGetVersion(fd);
+   if (v) {
+      dev = load_backend_by_name(v->name, fd, false);
+      drmFreeVersion(v);
+      if (dev) return dev;
+   }
 
-   if (!dev)
-      dev = find_backend(NULL, fd);
-
-   return dev;
+   return load_backend_by_name("dri", fd, true);
 }
 
 void

src/gbm/meson.build

@@ -22,14 +22,15 @@ incs_gbm = [
 # TODO: fix includes to delete this
 incs_gbm += inc_mesa
 
-if with_dri2
-  files_gbm += files('backends/dri/gbm_dri.c', 'backends/dri/gbm_driint.h')
-endif
 if with_platform_wayland
   deps_gbm += dep_wayland_server
   incs_gbm += inc_wayland_drm
 endif
 
+if with_dri2
+  subdir('backends/dri')
+endif
+
 libgbm_name = 'gbm'
 
 if with_platform_android and get_option('platform-sdk-version') >= 30
@@ -42,7 +43,7 @@ libgbm = shared_library(
   include_directories : incs_gbm,
   c_args : [args_gbm],
   link_args : [ld_args_gc_sections],
-  link_with : [libloader, libgallium_dri],
+  link_with : [libloader],
   dependencies : [deps_gbm, dep_dl, dep_thread, dep_libdrm, idep_mesautil, idep_xmlconfig],
   gnu_symbol_visibility : 'hidden',
   version : '1.0.0',