[PATCH] drm/debugfs: fix potential NULL pointer dereference

[Marek Szyprowski <m.szyprowski@samsung.com>]

encoder->funcs entry might be NULL, so check it first before calling its
methods. This fixes NULL pointer dereference observed on Rasberry Pi
3b/4b boards.

Fixes: caf525ed45b4 ("drm/encoder: register per-encoder debugfs dir")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
This fixes the following issue observed on Raspberry Pi 4b:

vc4-drm gpu: bound fe400000.hvs (ops vc4_hvs_ops [vc4])
vc4-drm gpu: bound fef00700.hdmi (ops vc4_hdmi_ops [vc4])
vc4-drm gpu: bound fef05700.hdmi (ops vc4_hdmi_ops [vc4])
vc4-drm gpu: bound fe004000.txp (ops vc4_txp_ops [vc4])
vc4-drm gpu: bound fe206000.pixelvalve (ops vc4_crtc_ops [vc4])
vc4-drm gpu: bound fe207000.pixelvalve (ops vc4_crtc_ops [vc4])
vc4-drm gpu: bound fe20a000.pixelvalve (ops vc4_crtc_ops [vc4])
vc4-drm gpu: bound fe216000.pixelvalve (ops vc4_crtc_ops [vc4])
8<--- cut here ---
Unable to handle kernel NULL pointer dereference at virtual address 00000010 when read
[00000010] *pgd=00000000
Internal error: Oops: 5 [#1] SMP ARM
Modules linked in: sha256_arm raspberrypi_hwmon cfg80211(+) hci_uart btbcm bluetooth vc4(+) ecdh_generic ecc libaes snd_soc_hdmi_codec snd_soc_core v3d drm_shmem_helper ac97_bus snd_pcm_dmaengine snd_pcm genet(+) gpu_sched snd_timer snd bcm2711_thermal soundcore drm_dma_helper
CPU: 1 PID: 221 Comm: systemd-udevd Not tainted 6.7.0-rc4-next-20231205 #14267
Hardware name: BCM2711
PC is at drm_debugfs_encoder_add+0x6c/0x98
LR is at 0x0
...
 drm_debugfs_encoder_add from drm_encoder_register_all+0x20/0x60
 drm_encoder_register_all from drm_modeset_register_all+0x34/0x70
 drm_modeset_register_all from drm_dev_register+0x24c/0x28c
 drm_dev_register from vc4_drm_bind+0x21c/0x2d0 [vc4]
 vc4_drm_bind [vc4] from try_to_bring_up_aggregate_device+0x160/0x1bc
 try_to_bring_up_aggregate_device from component_master_add_with_match+0xc4/0xf8
 component_master_add_with_match from vc4_platform_drm_probe+0xa0/0xc0 [vc4]
 vc4_platform_drm_probe [vc4] from platform_probe+0x5c/0xb8
 platform_probe from really_probe+0xc8/0x2dc
 really_probe from __driver_probe_device+0x88/0x19c
 __driver_probe_device from driver_probe_device+0x30/0x104
 driver_probe_device from __driver_attach+0x90/0x174
 __driver_attach from bus_for_each_dev+0x6c/0xb4
 bus_for_each_dev from bus_add_driver+0xcc/0x1cc
 bus_add_driver from driver_register+0x7c/0x118
 driver_register from vc4_drm_register+0x44/0x1000 [vc4]
 vc4_drm_register [vc4] from do_one_initcall+0x40/0x1e0
 do_one_initcall from do_init_module+0x50/0x1e4
 do_init_module from init_module_from_file+0x90/0xbc
 init_module_from_file from sys_finit_module+0x144/0x258
 sys_finit_module from ret_fast_syscall+0x0/0x54
Exception stack(0xf0cf1fa8 to 0xf0cf1ff0)
...
---[ end trace 0000000000000000 ]---

---
 drivers/gpu/drm/drm_debugfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
index 02e7481758c0..f4715a67e340 100644
--- a/drivers/gpu/drm/drm_debugfs.c
+++ b/drivers/gpu/drm/drm_debugfs.c
@@ -638,7 +638,7 @@ void drm_debugfs_encoder_add(struct drm_encoder *encoder)
 	debugfs_create_file("bridges", 0444, root, encoder,
 			    &bridges_fops);
 
-	if (encoder->funcs->debugfs_init)
+	if (encoder->funcs && encoder->funcs->debugfs_init)
 		encoder->funcs->debugfs_init(encoder, root);
 }
 
-- 
2.34.1