[PATCH v2] drm/dp: Fix aux->transfer NULL pointer dereference on drm_dp_dpcd_access

[Perry Yuan <Perry.Yuan@amd.com>]

Fix below crash by adding a check in the drm_dp_dpcd_access which
ensures that aux->transfer was actually initialized earlier.

BUG: kernel NULL pointer dereference, address: 0000000000000000
PGD 0 P4D 0
Oops: 0010 [#1] SMP NOPTI
RIP: 0010:0x0
Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
RSP: 0018:ffffa8d64225bab8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000020 RCX: ffffa8d64225bb5e
RDX: ffff93151d921880 RSI: ffffa8d64225bac8 RDI: ffff931511a1a9d8
RBP: ffffa8d64225bb10 R08: 0000000000000001 R09: ffffa8d64225ba60
R10: 0000000000000002 R11: 000000000000000d R12: 0000000000000001
R13: 0000000000000000 R14: ffffa8d64225bb5e R15: ffff931511a1a9d8
FS: 00007ff8ea7fa9c0(0000) GS:ffff9317fe6c0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000010d5a4000 CR4: 0000000000750ee0
PKRU: 55555554
Call Trace:
drm_dp_dpcd_access+0x72/0x110 [drm_kms_helper]
drm_dp_dpcd_read+0xb7/0xf0 [drm_kms_helper]
drm_dp_start_crc+0x38/0xb0 [drm_kms_helper]
amdgpu_dm_crtc_set_crc_source+0x1ae/0x3e0 [amdgpu]
crtc_crc_open+0x174/0x220 [drm]
full_proxy_open+0x168/0x1f0
? open_proxy_open+0x100/0x100
do_dentry_open+0x156/0x370
vfs_open+0x2d/0x30

v2: fix some typo

Signed-off-by: Perry Yuan <Perry.Yuan@amd.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 6d0f2c447f3b..76b28396001a 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -260,6 +260,10 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 	msg.buffer = buffer;
 	msg.size = size;
 
+	/* No transfer function is set, so not an available DP connector */
+	if (!aux->transfer)
+		return -EINVAL;
+
 	mutex_lock(&aux->hw_mutex);
 
 	/*
-- 
2.25.1