drm/bridge: sii902x: Fix probing race issue

A null pointer dereference crash has been observed rarely on TI
platforms using sii9022 bridge:

[   53.271356]  sii902x_get_edid+0x34/0x70 [sii902x]
[   53.276066]  sii902x_bridge_get_edid+0x14/0x20 [sii902x]
[   53.281381]  drm_bridge_get_edid+0x20/0x34 [drm]
[   53.286305]  drm_bridge_connector_get_modes+0x8c/0xcc [drm_kms_helper]
[   53.292955]  drm_helper_probe_single_connector_modes+0x190/0x538 [drm_kms_helper]
[   53.300510]  drm_client_modeset_probe+0x1f0/0xbd4 [drm]
[   53.305958]  __drm_fb_helper_initial_config_and_unlock+0x50/0x510 [drm_kms_helper]
[   53.313611]  drm_fb_helper_initial_config+0x48/0x58 [drm_kms_helper]
[   53.320039]  drm_fbdev_dma_client_hotplug+0x84/0xd4 [drm_dma_helper]
[   53.326401]  drm_client_register+0x5c/0xa0 [drm]
[   53.331216]  drm_fbdev_dma_setup+0xc8/0x13c [drm_dma_helper]
[   53.336881]  tidss_probe+0x128/0x264 [tidss]
[   53.341174]  platform_probe+0x68/0xc4
[   53.344841]  really_probe+0x188/0x3c4
[   53.348501]  __driver_probe_device+0x7c/0x16c
[   53.352854]  driver_probe_device+0x3c/0x10c
[   53.357033]  __device_attach_driver+0xbc/0x158
[   53.361472]  bus_for_each_drv+0x88/0xe8
[   53.365303]  __device_attach+0xa0/0x1b4
[   53.369135]  device_initial_probe+0x14/0x20
[   53.373314]  bus_probe_device+0xb0/0xb4
[   53.377145]  deferred_probe_work_func+0xcc/0x124
[   53.381757]  process_one_work+0x1f0/0x518
[   53.385770]  worker_thread+0x1e8/0x3dc
[   53.389519]  kthread+0x11c/0x120
[   53.392750]  ret_from_fork+0x10/0x20

The issue here is as follows:

- tidss probes, but is deferred as sii902x is still missing.
- sii902x starts probing and enters sii902x_init().
- sii902x calls drm_bridge_add(). Now the sii902x bridge is ready from
  DRM's perspective.
- sii902x calls sii902x_audio_codec_init() and
  platform_device_register_data()
- The registration of the audio platform device causes probing of the
  deferred devices.
- tidss probes, which eventually causes sii902x_bridge_get_edid() to be
  called.
- sii902x_bridge_get_edid() tries to use the i2c to read the edid.
  However, the sii902x driver has not set up the i2c part yet, leading
  to the crash.

Fix this by moving the drm_bridge_add() to the end of the
sii902x_init(), which is also at the very end of sii902x_probe().

Signed-off-by: Tomi Valkeinen <[email protected]>
Fixes: 21d8084 ("drm/bridge/sii902x: Fix EDID readback")
Acked-by: Linus Walleij <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Neil Armstrong <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: tomba

drivers/gpu/drm/bridge/sii902x.c

@@ -1080,16 +1080,6 @@ static int sii902x_init(struct sii902x *sii902x)
 			return ret;
 	}
 
-	sii902x->bridge.funcs = &sii902x_bridge_funcs;
-	sii902x->bridge.of_node = dev->of_node;
-	sii902x->bridge.timings = &default_sii902x_timings;
-	sii902x->bridge.ops = DRM_BRIDGE_OP_DETECT | DRM_BRIDGE_OP_EDID;
-
-	if (sii902x->i2c->irq > 0)
-		sii902x->bridge.ops |= DRM_BRIDGE_OP_HPD;
-
-	drm_bridge_add(&sii902x->bridge);
-
 	sii902x_audio_codec_init(sii902x, dev);
 
 	i2c_set_clientdata(sii902x->i2c, sii902x);
@@ -1102,7 +1092,21 @@ static int sii902x_init(struct sii902x *sii902x)
 		return -ENOMEM;
 
 	sii902x->i2cmux->priv = sii902x;
-	return i2c_mux_add_adapter(sii902x->i2cmux, 0, 0, 0);
+	ret = i2c_mux_add_adapter(sii902x->i2cmux, 0, 0, 0);
+	if (ret)
+		return ret;
+
+	sii902x->bridge.funcs = &sii902x_bridge_funcs;
+	sii902x->bridge.of_node = dev->of_node;
+	sii902x->bridge.timings = &default_sii902x_timings;
+	sii902x->bridge.ops = DRM_BRIDGE_OP_DETECT | DRM_BRIDGE_OP_EDID;
+
+	if (sii902x->i2c->irq > 0)
+		sii902x->bridge.ops |= DRM_BRIDGE_OP_HPD;
+
+	drm_bridge_add(&sii902x->bridge);
+
+	return 0;
 }
 
 static int sii902x_probe(struct i2c_client *client)
@@ -1170,12 +1174,11 @@ static int sii902x_probe(struct i2c_client *client)
 }
 
 static void sii902x_remove(struct i2c_client *client)
-
 {
 	struct sii902x *sii902x = i2c_get_clientdata(client);
 
-	i2c_mux_del_adapters(sii902x->i2cmux);
 	drm_bridge_remove(&sii902x->bridge);
+	i2c_mux_del_adapters(sii902x->i2cmux);
 }
 
 static const struct of_device_id sii902x_dt_ids[] = {