[PATCH] drm/bochs: Fix NULL dereference on atomic_disable helper

[Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>]

When the subtest basic-plain-flip from IGT project was executed on Bochs
driver, the following bug appears:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD 0 P4D 0
Oops: 0010 [#1] PREEMPT SMP PTI
CPU: 0 PID: 423 Comm: kms_flip Not tainted 5.0.0-VM+ #102
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-20181126_142135-anatol 04/01/2014
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffffb6a6c0bdfb70 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff90467940bf00 RCX: ffff90467a730318
RDX: 000000000000001f RSI: 0000000000000003 RDI: ffff90467bf94050
RBP: ffff90467bf94050 R08: ffff904679424e00 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000000
R13: ffff90467a460000 R14: ffff90467946d400 R15: ffffffffc080d460
FS:  00007f259c1990c0(0000) GS:ffff90467c800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 00000000b97ec000 CR4: 00000000000006f0
Call Trace:
 ? drm_atomic_helper_commit_modeset_disables+0x396/0x3e0 [drm_kms_helper]
 ? drm_atomic_helper_commit_tail+0x19/0x60 [drm_kms_helper]
 ? commit_tail+0x58/0x70 [drm_kms_helper]
 ? drm_atomic_helper_commit+0x103/0x110 [drm_kms_helper]
 ? drm_atomic_helper_set_config+0x80/0x90 [drm_kms_helper]
 ? drm_mode_setcrtc+0x18d/0x690 [drm]
 ? tty_insert_flip_string_fixed_flag+0x85/0xe0
 ? drm_mode_getcrtc+0x180/0x180 [drm]
 ? drm_ioctl_kernel+0xb2/0xf0 [drm]
 ? drm_ioctl+0x25f/0x3f0 [drm]
 ? drm_mode_getcrtc+0x180/0x180 [drm]
 ? do_vfs_ioctl+0xa4/0x630
 ? ksys_ioctl+0x60/0x90
 ? ksys_write+0x4f/0xb0
 ? __x64_sys_ioctl+0x16/0x20
 ? do_syscall_64+0x5b/0x170
 ? entry_SYSCALL_64_after_hwframe+0x44/0xa9

This commit adds an entry to atomic_disable in the drm_crtc_helper_funcs
to avoid the NULL pointer dereference.

Fixes: 0f0eb98e33c6 ("drm/bochs: atomic: add atomic_flush+atomic_enable callbacks")

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
---
 drivers/gpu/drm/bochs/bochs_kms.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/bochs/bochs_kms.c b/drivers/gpu/drm/bochs/bochs_kms.c
index 9e7cd6b34106..7e821776727f 100644
--- a/drivers/gpu/drm/bochs/bochs_kms.c
+++ b/drivers/gpu/drm/bochs/bochs_kms.c
@@ -35,6 +35,11 @@ static void bochs_crtc_atomic_enable(struct drm_crtc *crtc,
 {
 }
 
+static void  bochs_crtc_atomic_disable(struct drm_crtc *crtc,
+				       struct drm_crtc_state *old_state)
+{
+}
+
 static void bochs_crtc_atomic_flush(struct drm_crtc *crtc,
 				    struct drm_crtc_state *old_crtc_state)
 {
@@ -66,6 +71,7 @@ static const struct drm_crtc_funcs bochs_crtc_funcs = {
 static const struct drm_crtc_helper_funcs bochs_helper_funcs = {
 	.mode_set_nofb = bochs_crtc_mode_set_nofb,
 	.atomic_enable = bochs_crtc_atomic_enable,
+	.atomic_disable = bochs_crtc_atomic_disable,
 	.atomic_flush = bochs_crtc_atomic_flush,
 };
 
-- 
2.21.0