fbdev-emulation.md

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fbdev emulation bootstrap: drm_client_setup() → /dev/fb0

drm_client_setup() registers an in-kernel fbdev client that probes the connector, creates a GEM framebuffer, wraps it in a shadow buffer with deferred I/O, and calls register_framebuffer(). That is where /dev/fb0 appears and where the first atomic commit happens.

TL;DR

• drm_fbdev_client_setup() reads color_mode from mode_config.preferred_depth (16 → RGB565) and sets up the fb helper.
• drm_client_modeset_probe() runs fill_modes() → .get_modes → validation, then picks a CRTC with drm_client_pick_crtcs().
• drm_fb_helper_single_fb_probe() calls the driver's .fbdev_probe = drm_fbdev_dma_driver_fbdev_probe, which creates the client framebuffer through drm_client_framebuffer_create() and vmaps it.
• The shadowed path is taken only because fb->funcs->dirty is set (the driver uses drm_gem_fb_create_with_dirty): a vzalloc'ed shadow buffer becomes info->screen_buffer and deferred I/O is installed.
• register_framebuffer(info) creates /dev/fb0; fbcon's fb_set_par then triggers the first modeset through drm_client_modeset_commit() → drm_atomic_commit().

Call tree

drm_client_setup(dev, NULL)                     [drm_client_setup.c]
  └─ drm_fbdev_client_setup(dev, NULL)          [clients/drm_fbdev_client.c]
       │  color_mode = mode_config.preferred_depth = 16  → RGB565
       │  kzalloc(drm_fb_helper)
       │  drm_fb_helper_prepare()               [drm_fb_helper.c]
       │     INIT_WORK(damage_work, drm_fb_helper_damage_work)
       │     preferred_bpp = 16
       ├─ drm_client_init(client, "fbdev", &drm_fbdev_client_funcs)
       └─ drm_client_register()
            └─ client->funcs->hotplug = drm_fbdev_client_hotplug
                 ├─ drm_fb_helper_init(dev, fb_helper)
                 └─ drm_fb_helper_initial_config()
                      └─ __drm_fb_helper_initial_config_and_unlock()
                           ├─ drm_client_modeset_probe(client, 128, 160)
                           │    └─ for each connector:
                           │         connector->funcs->fill_modes()
                           │         │   = drm_helper_probe_single_connector_modes [drm_probe_helper.c]
                           │         │     ├─ .get_modes
                           │         │     │    = drm_tutorial_connector_get_modes
                           │         │     │      └─ drm_connector_helper_get_modes_fixed()
                           │         │     │           → 128x160, DRM_MODE_TYPE_PREFERRED
                           │         │     └─ __drm_helper_update_and_validate()
                           │         │          └─ per mode: validate_driver → validate_size
                           │         │             → validate_flag → validate_pipeline
                           │         │                → crtc->helper_private->mode_valid
                           │         │                   = drm_tutorial_crtc_helper_mode_valid
                           │         └─ drm_client_firmware_config() / target_preferred()
                           │            → drm_client_pick_crtcs()   // pick CRTC for the connector
                           │            → store drm_mode_set {crtc, mode, connector}
                           └─ drm_fb_helper_single_fb_probe()
                                ├─ drm_fb_helper_find_sizes()   // 128x160, bpp 16
                                └─ dev->driver->fbdev_probe(fb_helper, &sizes)
                                     = drm_fbdev_dma_driver_fbdev_probe() [drm_fbdev_dma.c]
                                       ├─ drm_client_framebuffer_create()
                                       │    └─ drm_gem_dma_create() + drm_mode_addfb2()
                                       │         → .fb_create = drm_gem_fb_create_with_dirty
                                       │           so fb->funcs->dirty = drm_atomic_helper_dirtyfb
                                       ├─ drm_client_buffer_vmap()  → dma_obj->vaddr
                                       ├─ drm_fb_helper_alloc_info() + drm_fb_helper_fill_info()
                                       └─ fb->funcs->dirty is set → *shadowed* path:
                                            vzalloc(shadow);  info->screen_buffer = shadow
                                            fbops = drm_fbdev_dma_shadowed_fb_ops
                                            fbdefio.deferred_io = drm_fb_helper_deferred_io
                                            fb_deferred_io_init(info)
                                       └─ (back in initial_config) register_framebuffer(info)
                                            → /dev/fb0 appears
                                            → fbcon takes over the console
                                              └─ fb_set_par → drm_fb_helper_set_par()
                                                   └─ __drm_fb_helper_restore_fbdev_mode_unlocked()
                                                        └─ drm_client_modeset_commit()
                                                             └─ drm_client_modeset_commit_atomic()
                                                                  └─ __drm_atomic_helper_set_config()
                                                                       └─ drm_atomic_commit()
                                                                            → first atomic_update()

This last call is what produces the probe dmesg sequence in the root README: drm_tutorial_crtc_helper_atomic_enable, then repeated drm_tutorial_plane_helper_atomic_update with the full-screen damage x1:0 y1:0 x2:128 y2:160, followed by Console: switching to colour frame buffer device 16x20.

The shadowed fbdev path is taken only because the framebuffer was created through drm_gem_fb_create_with_dirty (i.e. fb->funcs->dirty is set). That is why the driver deliberately registers .fb_create = drm_gem_fb_create_with_dirty in its mode config.

Caveats

• The call-chain symbol and file names above (drm_fbdev_dma.c, drm_fb_helper.c, clients/drm_fbdev_client.c, ...) are specific to the WSL2 kernel documented in the root README. They were not re-verified against another kernel version; on a different kernel the shadow path may be split across differently named functions.

Related

• fbdev-write-path.md - what happens after /dev/fb0 exists
• module-load-and-probe.md - step 10 that calls this
• gem-dma-framebuffers.md - the framebuffer being created