NULL pointer dereference in radeon drm code

On one of my systems I use, display connector 0 does not have DDC. This results in the DRM code trying to use radeon_connector->ddc_bus when it is not set. This is known to have been an issue since the 3.2 series kernels.

Known Problem Kernels: uBuntu 12.04 LTS (linux-3.2.55), uBuntu-MATE 14.04 LTS (linux-3.16.7) and uBuntu-MATE 16.04 LTS (linux-4.4.8) and linux-4.6.0 (git:/
/git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux).

Trying to use the uBuntu-MATE 16.04 LTS LiveCD ubuntu-mate-16.04-desktop-amd64.iso (from uBuntu-MATE website) showed the problem.

Using a serial console I extracted the following information when trying to boot from a uBuntu-MATE 16.04 LTS LiveCD USB Stick (it had been upgraded from 4.4.0-21-generic in an attempt to get something usable):

<snipped>
[ 0.000000] Linux version 4.4.0-22-generic (buildd@lgw01-41) (gcc version 5.3.1 20160413 (Ubuntu 5.3.1-14ubuntu2) ) #40-Ubuntu SMP Thu May 12 22:03:46 UTC 2
016 (Ubuntu 4.4.0-22.40-generic 4.4.8)
[ 0.000000] Command line: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu-mate.seed boot=casper locale=en_GB console-setup/layoutcode=gb console=u
art,io,0x3f8,115200n8 earlyprintk=ttyS0,115200n8 debug ---
[ 0.000000] KERNEL supported cpus:
[ 0.000000] Intel GenuineIntel
[ 0.000000] AMD AuthenticAMD
[ 0.000000] Centaur CentaurHauls
<snipped>
[ 0.000000] efi: EFI v2.00 by Phoenix Technologies Ltd.
[ 0.000000] efi: EFI v2.00 by Phoenix Technologies Ltd.
[ 0.000000] efi: [ 0.000000] efi: ACPI=0x973c7000 ACPI=0x973c7000 ACPI 2.0=0x973c7014 ACPI 2.0=0x973c7014 SMBIOS=0x9726b000 SMBIOS=0x9726b000

[ 0.000000] SMBIOS 2.6 present.
[ 0.000000] SMBIOS 2.6 present.
[ 0.000000] DMI: AMD Corporation EBrazos Platform/Persimmon, BIOS LV-683 Version: 1.1 02/14/2012
[ 0.000000] DMI: AMD Corporation EBrazos Platform/Persimmon, BIOS LV-683 Version: 1.1 02/14/2012
<snipped>
[ 8.262990] [drm] ib test on ring 5 succeeded
[ 8.288897] [drm] Radeon Display Connectors
[ 8.293175] [drm] Connector 0:
[ 8.296252] [drm] DP-1
[ 8.298791] [drm] Encoders:
[ 8.301770] [drm] DFP1: INTERNAL_UNIPHY
[ 8.305973] [drm] Connector 1:
[ 8.309043] [drm] DP-2
[ 8.311598] [drm] HPD2
[ 8.314169] [drm] DDC: 0x6440 0x6440 0x6444 0x6444 0x6448 0x6448 0x644c 0x644c
[ 8.321609] [drm] Encoders:
[ 8.324589] [drm] DFP2: INTERNAL_UNIPHY
[ 8.328793] [drm] Connector 2:
[ 8.331856] [drm] VGA-1
[ 8.332316] scsi 0:0:0:0: Direct-Access SMI USB DISK 1100 PQ: 0 ANSI: 0 CCS
[ 8.342947] [drm] DDC: 0x64d8 0x64d8 0x64dc 0x64dc 0x64e0 0x64e0 0x64e4 0x64e4
[ 8.350341] [drm] Encoders:
[ 8.353310] [drm] CRT1: INTERNAL_KLDSCP_DAC1
[ 8.358195] BUG: unable to handle kernel NULL pointer dereference at 0000000000000409
[ 8.366104] IP:[ 8.367176] sd 0:0:0:0: Attached scsi generic sg1 type 0
[ 8.368538] sd 0:0:0:0: [sdb] 7831552 512-byte logical blocks: (4.01 GB/3.73 GiB)
[ 8.369421] sd 0:0:0:0: [sdb] Write Protect is off
[ 8.369427] sd 0:0:0:0: [sdb] Mode Sense: 43 00 00 00
[ 8.370288] sd 0:0:0:0: [sdb] No Caching mode page found
[ 8.370292] sd 0:0:0:0: [sdb] Assuming drive cache: write through
[ 8.374944] sdb: sdb1
[ 8.378398] sd 0:0:0:0: [sdb] Attached SCSI removable disk

[ 8.409733] [<ffffffffc02024da>] radeon_dp_getsinktype+0x1a/0x30 [radeon]
[ 8.416805] PGD 0
[ 8.418841] Oops: 0000 [#1] SMP
[ 8.422104] Modules linked in: hid_generic e1000e psmouse amdkfd usbhid uas amd_iommu_v2 ptp radeon(+) pps_core e1000(+) hid i2c_algo_bit ttm drm_kms_helper
 usb_storage syscopyarea sysfillrect sysimgblt ahci fb_sys_fops libahci drm video fjes
[ 8.444029] CPU: 0 PID: 131 Comm: systemd-udevd Not tainted 4.4.0-22-generic #40-Ubuntu
[ 8.452036] Hardware name: AMD Corporation EBrazos Platform/Persimmon, BIOS LV-683 Version: 1.1 02/14/2012
[ 8.461708] task: ffff88014780be80 ti: ffff880147888000 task.ti: ffff880147888000
[ 8.469194] RIP: 0010:[<ffffffffc02024da>] [<ffffffffc02024da>] radeon_dp_getsinktype+0x1a/0x30 [radeon]
[ 8.478850] RSP: 0018:ffff88014788b850 EFLAGS: 00010246
[ 8.484179] RAX: 0000000000000000 RBX: ffff88003485f000 RCX: 0000000000000000
[ 8.491321] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff88003485f000
[ 8.498469] RBP: ffff88014788b850 R08: 0000000000000000 R09: ffffffffc004aff6
[ 8.505602] R10: ffffea0000d5e540 R11: 0000000000000000 R12: 0000000000000001
[ 8.506137] e1000 0000:05:0b.0 eth1: (PCI:33MHz:32-bit) 00:03:1d:09:3b:61
[ 8.506142] e1000 0000:05:0b.0 eth1: Intel(R) PRO/1000 Network Connection
[ 8.526299] R13: ffff8800351f4080 R14: ffff880035120000 R15: ffff8801478e7000
[ 8.533441] FS: 00007f4ed97678c0(0000) GS:ffff88014ec00000(0000) knlGS:0000000000000000
[ 8.541525] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 8.547263] CR2: 0000000000000409 CR3: 0000000149f2d000 CR4: 00000000000006f0
[ 8.554396] Stack:
[ 8.556412] ffff88014788b888 ffffffffc01c27a6 ffff88003485f050 ffff88003485f000
[ 8.563867] 0000000000000003 ffff88003574e000 0000000000000001 ffff88014788b8e8
[ 8.571329] ffffffffc00ccd95 ffff88014788b920 ffffffffc01a9319 ffff8801478e6200
[ 8.578789] Call Trace:
[ 8.581289] [<ffffffffc01c27a6>] radeon_dp_detect+0x206/0x2b0 [radeon]
[ 8.587918] [<ffffffffc00ccd95>] drm_helper_probe_single_connector_modes_merge_bits+0x235/0x4d0 [drm_kms_helper]
[ 8.598217] [<ffffffffc01a9319>] ? atombios_crtc_disable+0x39/0x350 [radeon]
[ 8.605356] [<ffffffffc00cd043>] drm_helper_probe_single_connector_modes+0x13/0x20 [drm_kms_helper]
[ 8.614508] [<ffffffffc00d990e>] drm_fb_helper_initial_config+0xae/0x420 [drm_kms_helper]
[ 8.622816] [<ffffffffc01cd16e>] radeon_fbdev_init+0xee/0x110 [radeon]
[ 8.629471] [<ffffffffc01c726f>] radeon_modeset_init+0x43f/0xa10 [radeon]
[ 8.636391] [<ffffffffc019f513>] radeon_driver_load_kms+0x123/0x220 [radeon]
[ 8.643565] [<ffffffffc0035397>] drm_dev_register+0xa7/0xb0 [drm]
[ 8.649765] [<ffffffffc00379cd>] drm_get_pci_dev+0x8d/0x1e0 [drm]
[ 8.655985] [<ffffffffc019b3e0>] radeon_pci_probe+0xa0/0xb0 [radeon]
[ 8.662438] [<ffffffff81439dc5>] local_pci_probe+0x45/0xa0
[ 8.668006] [<ffffffff8143b203>] pci_device_probe+0x103/0x150
[ 8.673841] [<ffffffff8154c6e2>] driver_probe_device+0x222/0x4a0
[ 8.679939] [<ffffffff8154c9e4>] __driver_attach+0x84/0x90
[ 8.685519] [<ffffffff8154c960>] ? driver_probe_device+0x4a0/0x4a0
[ 8.691787] [<ffffffff8154a30c>] bus_for_each_dev+0x6c/0xc0
[ 8.697445] [<ffffffff8154be9e>] driver_attach+0x1e/0x20
[ 8.702845] [<ffffffff8154b9db>] bus_add_driver+0x1eb/0x280
[ 8.708504] [<ffffffff8154d2f0>] driver_register+0x60/0xe0
[ 8.714077] [<ffffffff814396ec>] __pci_register_driver+0x4c/0x50
[ 8.720191] [<ffffffffc0037c00>] drm_pci_init+0xe0/0x110 [drm]
[ 8.726104] [<ffffffffc030d000>] ? 0xffffffffc030d000
[ 8.731288] [<ffffffffc030d09d>] radeon_init+0x9d/0xb2 [radeon]
[ 8.737297] [<ffffffff81002123>] do_one_initcall+0xb3/0x200
[ 8.742955] [<ffffffff811cee51>] ? __vunmap+0x91/0xe0
[ 8.748093] [<ffffffff811eaf83>] ? kmem_cache_alloc_trace+0x183/0x1f0
[ 8.754622] [<ffffffff8118c233>] do_init_module+0x5f/0x1cf
[ 8.760193] [<ffffffff81109e67>] load_module+0x1667/0x1c00
[ 8.765764] [<ffffffff81106410>] ? __symbol_put+0x60/0x60
[ 8.771252] [<ffffffff81212820>] ? kernel_read+0x50/0x80
[ 8.776651] [<ffffffff8110a644>] SYSC_finit_module+0xb4/0xe0
[ 8.782395] [<ffffffff8110a68e>] SyS_finit_module+0xe/0x10
[ 8.787970] [<ffffffff818252f2>] entry_SYSCALL_64_fastpath+0x16/0x71
[ 8.794404] Code: 41 5d 41 5e 41 5f 5d c3 66 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 55 48 8b 87 a0 03 00 00 45 31 c0 31 d2 be 01 00 00 00 48 89 e5 <0f> b6 8
8 09 04 00 00 48 8b 07 48 8b 78 28 e8 23 f6 ff ff 5d c3
[ 8.814354] RIP [<ffffffffc02024da>] radeon_dp_getsinktype+0x1a/0x30 [radeon]
[ 8.821645] RSP <ffff88014788b850>
[ 8.825135] CR2: 0000000000000409
[ 8.828551] ---[ end trace 9fce5ace7679b4a0 ]---

Looking at the code, there are a number of places that assume that radeon_connector->ddc_bus is not a NULL pointer.

The following workaround fixes the issue by ensuring that ddc_bus is always validated before use:

diff -rupd linux-source-4.4.0.orig/drivers/gpu/drm/radeon/atombios_dp.c linux-source-4.4.0/drivers/gpu/drm/radeon/atombios_dp.c
--- linux-source-4.4.0.orig/drivers/gpu/drm/radeon/atombios_dp.c 2016-05-12 23:03:23.000000000 +0100
+++ linux-source-4.4.0/drivers/gpu/drm/radeon/atombios_dp.c 2016-06-01 13:16:43.000000000 +0100
@@ -232,6 +232,9 @@ void radeon_dp_aux_init(struct radeon_co
        struct radeon_device *rdev = dev->dev_private;
        int ret;

+ if (!radeon_connector->ddc_bus)
+ return;
+
        radeon_connector->ddc_bus->rec.hpd = radeon_connector->hpd.hpd;
        radeon_connector->ddc_bus->aux.dev = radeon_connector->base.kdev;
        if (ASIC_IS_DCE5(rdev)) {
@@ -352,6 +355,9 @@ u8 radeon_dp_getsinktype(struct radeon_c
        struct drm_device *dev = radeon_connector->base.dev;
        struct radeon_device *rdev = dev->dev_private;

+ if (!radeon_connector->ddc_bus)
+ return 0;
+
        return radeon_dp_encoder_service(rdev, ATOM_DP_ACTION_GET_SINK_TYPE, 0,
                                         radeon_connector->ddc_bus->rec.i2c_id, 0);
 }
@@ -364,6 +370,9 @@ static void radeon_dp_probe_oui(struct r
        if (!(dig_connector->dpcd[DP_DOWN_STREAM_PORT_COUNT] & DP_OUI_SUPPORT))
                return;

+ if (!radeon_connector->ddc_bus)
+ return;
+
        if (drm_dp_dpcd_read(&radeon_connector->ddc_bus->aux, DP_SINK_OUI, buf, 3) == 3)
                DRM_DEBUG_KMS("Sink OUI: %02hx%02hx%02hx\n",
                              buf[0], buf[1], buf[2]);
@@ -379,6 +388,9 @@ bool radeon_dp_getdpcd(struct radeon_con
        u8 msg[DP_DPCD_SIZE];
        int ret, i;

+ if (!radeon_connector->ddc_bus)
+ return false;
+
        for (i = 0; i < 7; i++) {
                ret = drm_dp_dpcd_read(&radeon_connector->ddc_bus->aux, DP_DPCD_REV, msg,
                                       DP_DPCD_SIZE);
@@ -416,24 +428,26 @@ int radeon_dp_get_panel_mode(struct drm_

        dig_connector = radeon_connector->con_priv;

- if (dp_bridge != ENCODER_OBJECT_ID_NONE) {
- /* DP bridge chips */
- if (drm_dp_dpcd_readb(&radeon_connector->ddc_bus->aux,
- DP_EDP_CONFIGURATION_CAP, &tmp) == 1) {
- if (tmp & 1)
- panel_mode = DP_PANEL_MODE_INTERNAL_DP2_MODE;
- else if ((dp_bridge == ENCODER_OBJECT_ID_NUTMEG) ||
- (dp_bridge == ENCODER_OBJECT_ID_TRAVIS))
- panel_mode = DP_PANEL_MODE_INTERNAL_DP1_MODE;
- else
- panel_mode = DP_PANEL_MODE_EXTERNAL_DP_MODE;
- }
- } else if (connector->connector_type == DRM_MODE_CONNECTOR_eDP) {
- /* eDP */
- if (drm_dp_dpcd_readb(&radeon_connector->ddc_bus->aux,
- DP_EDP_CONFIGURATION_CAP, &tmp) == 1) {
- if (tmp & 1)
- panel_mode = DP_PANEL_MODE_INTERNAL_DP2_MODE;
+ if (radeon_connector->ddc_bus) {
+ if (dp_bridge != ENCODER_OBJECT_ID_NONE) {
+ /* DP bridge chips */
+ if (drm_dp_dpcd_readb(&radeon_connector->ddc_bus->aux,
+ DP_EDP_CONFIGURATION_CAP, &tmp) == 1) {
+ if (tmp & 1)
+ panel_mode = DP_PANEL_MODE_INTERNAL_DP2_MODE;
+ else if ((dp_bridge == ENCODER_OBJECT_ID_NUTMEG) ||
+ (dp_bridge == ENCODER_OBJECT_ID_TRAVIS))
+ panel_mode = DP_PANEL_MODE_INTERNAL_DP1_MODE;
+ else
+ panel_mode = DP_PANEL_MODE_EXTERNAL_DP_MODE;
+ }
+ } else if (connector->connector_type == DRM_MODE_CONNECTOR_eDP) {
+ /* eDP */
+ if (drm_dp_dpcd_readb(&radeon_connector->ddc_bus->aux,
+ DP_EDP_CONFIGURATION_CAP, &tmp) == 1) {
+ if (tmp & 1)
+ panel_mode = DP_PANEL_MODE_INTERNAL_DP2_MODE;
+ }
                }
        }

@@ -499,6 +513,9 @@ bool radeon_dp_needs_link_train(struct r
        u8 link_status[DP_LINK_STATUS_SIZE];
        struct radeon_connector_atom_dig *dig = radeon_connector->con_priv;

+ if (!radeon_connector->ddc_bus)
+ return false;
+
        if (drm_dp_dpcd_read_link_status(&radeon_connector->ddc_bus->aux, link_status)
            <= 0)
                return false;
@@ -519,7 +536,7 @@ void radeon_dp_set_rx_power_state(struct
        dig_connector = radeon_connector->con_priv;

        /* power up/down the sink */
- if (dig_connector->dpcd[0] >= 0x11) {
+ if (radeon_connector->ddc_bus && dig_connector->dpcd[0] >= 0x11) {
                drm_dp_dpcd_writeb(&radeon_connector->ddc_bus->aux,
                                   DP_SET_POWER, power_state);
                usleep_range(1000, 2000);
@@ -822,7 +839,8 @@ void radeon_dp_link_train(struct drm_enc
        else
                dp_info.enc_id |= ATOM_DP_CONFIG_LINK_A;

- if (drm_dp_dpcd_readb(&radeon_connector->ddc_bus->aux, DP_MAX_LANE_COUNT, &tmp)
+ if (radeon_connector->ddc_bus &&
+ drm_dp_dpcd_readb(&radeon_connector->ddc_bus->aux, DP_MAX_LANE_COUNT, &tmp)
            == 1) {
                if (ASIC_IS_DCE5(rdev) && (tmp & DP_TPS3_SUPPORTED))
                        dp_info.tp3_supported = true;
@@ -838,7 +856,7 @@ void radeon_dp_link_train(struct drm_enc
        dp_info.connector = connector;
        dp_info.dp_lane_count = dig_connector->dp_lane_count;
        dp_info.dp_clock = dig_connector->dp_clock;
- dp_info.aux = &radeon_connector->ddc_bus->aux;
+ dp_info.aux = radeon_connector->ddc_bus ? &radeon_connector->ddc_bus->aux : 0;

        if (radeon_dp_link_train_init(&dp_info))
                goto done;
diff -rupd linux-source-4.4.0.orig/drivers/gpu/drm/radeon/radeon_connectors.c linux-source-4.4.0/drivers/gpu/drm/radeon/radeon_connectors.c
--- linux-source-4.4.0.orig/drivers/gpu/drm/radeon/radeon_connectors.c 2016-01-10 23:01:32.000000000 +0000
+++ linux-source-4.4.0/drivers/gpu/drm/radeon/radeon_connectors.c 2016-06-01 13:21:24.000000000 +0100
@@ -327,26 +327,27 @@ static void radeon_connector_get_edid(st
        if (radeon_connector->router.ddc_valid)
                radeon_router_select_ddc_port(radeon_connector);

- if ((radeon_connector_encoder_get_dp_bridge_encoder_id(connector) !=
- ENCODER_OBJECT_ID_NONE) &&
- radeon_connector->ddc_bus->has_aux) {
- radeon_connector->edid = drm_get_edid(connector,
- &radeon_connector->ddc_bus->aux.ddc);
- } else if ((connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) ||
- (connector->connector_type == DRM_MODE_CONNECTOR_eDP)) {
- struct radeon_connector_atom_dig *dig = radeon_connector->con_priv;
-
- if ((dig->dp_sink_type == CONNECTOR_OBJECT_ID_DISPLAYPORT ||
- dig->dp_sink_type == CONNECTOR_OBJECT_ID_eDP) &&
- radeon_connector->ddc_bus->has_aux)
- radeon_connector->edid = drm_get_edid(&radeon_connector->base,
+ if (radeon_connector->ddc_bus) {
+ if ((radeon_connector_encoder_get_dp_bridge_encoder_id(connector) !=
+ ENCODER_OBJECT_ID_NONE) &&
+ radeon_connector->ddc_bus->has_aux) {
+ radeon_connector->edid = drm_get_edid(connector,
                                                              &radeon_connector->ddc_bus->aux.ddc);
- else if (radeon_connector->ddc_bus)
+ } else if ((connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort) ||
+ (connector->connector_type == DRM_MODE_CONNECTOR_eDP)) {
+ struct radeon_connector_atom_dig *dig = radeon_connector->con_priv;
+
+ if ((dig->dp_sink_type == CONNECTOR_OBJECT_ID_DISPLAYPORT ||
+ dig->dp_sink_type == CONNECTOR_OBJECT_ID_eDP) &&
+ radeon_connector->ddc_bus->has_aux)
+ radeon_connector->edid = drm_get_edid(&radeon_connector->base,
+ &radeon_connector->ddc_bus->aux.ddc);
+ else
+ radeon_connector->edid = drm_get_edid(&radeon_connector->base,
+ &radeon_connector->ddc_bus->adapter);
+ } else
                        radeon_connector->edid = drm_get_edid(&radeon_connector->base,
                                                              &radeon_connector->ddc_bus->adapter);
- } else if (radeon_connector->ddc_bus) {
- radeon_connector->edid = drm_get_edid(&radeon_connector->base,
- &radeon_connector->ddc_bus->adapter);
        }

        if (!radeon_connector->edid) {
@@ -1306,6 +1307,7 @@ radeon_dvi_detect(struct drm_connector *
                                                continue;
                                        list_radeon_connector = to_radeon_connector(list_connector);
                                        if (list_radeon_connector->shared_ddc &&
+ radeon_connector->ddc_bus &&
                                            (list_radeon_connector->ddc_bus->rec.i2c_id ==
                                             radeon_connector->ddc_bus->rec.i2c_id)) {
                                                /* cases where both connectors are digital */
diff -rupd linux-source-4.4.0.orig/drivers/gpu/drm/radeon/radeon_dp_mst.c linux-source-4.4.0/drivers/gpu/drm/radeon/radeon_dp_mst.c
--- linux-source-4.4.0.orig/drivers/gpu/drm/radeon/radeon_dp_mst.c 2016-05-12 23:03:23.000000000 +0100
+++ linux-source-4.4.0/drivers/gpu/drm/radeon/radeon_dp_mst.c 2016-06-01 13:16:43.000000000 +0100
@@ -662,7 +662,7 @@ radeon_dp_mst_init(struct radeon_connect
 {
        struct drm_device *dev = radeon_connector->base.dev;

- if (!radeon_connector->ddc_bus->has_aux)
+ if (!radeon_connector->ddc_bus || !radeon_connector->ddc_bus->has_aux)
                return 0;

        radeon_connector->mst_mgr.cbs = &mst_cbs;
@@ -689,6 +689,9 @@ radeon_dp_mst_probe(struct radeon_connec
        if (dig_connector->dpcd[DP_DPCD_REV] < 0x12)
                return 0;

+ if (!radeon_connector->ddc_bus)
+ return 0;
+
        ret = drm_dp_dpcd_read(&radeon_connector->ddc_bus->aux, DP_MSTM_CAP, msg,
                               1);
        if (ret) {
@@ -718,6 +721,9 @@ radeon_dp_mst_check_status(struct radeon
                int ret = 0;
                bool handled;

+ if (!radeon_connector->ddc_bus)
+ return -EINVAL;
+
                dret = drm_dp_dpcd_read(&radeon_connector->ddc_bus->aux,
                                       DP_SINK_COUNT_ESI, esi, 8);
 go_again:
diff -rupd linux-source-4.4.0.orig/drivers/gpu/drm/radeon/radeon_i2c.c linux-source-4.4.0/drivers/gpu/drm/radeon/radeon_i2c.c
--- linux-source-4.4.0.orig/drivers/gpu/drm/radeon/radeon_i2c.c 2016-01-10 23:01:32.000000000 +0000
+++ linux-source-4.4.0/drivers/gpu/drm/radeon/radeon_i2c.c 2016-06-01 13:16:43.000000000 +0100
@@ -63,6 +63,9 @@ bool radeon_ddc_probe(struct radeon_conn
        if (radeon_connector->router.ddc_valid)
                radeon_router_select_ddc_port(radeon_connector);

+ if (!radeon_connector->ddc_bus)
+ return false;
+
        if (use_aux) {
                ret = i2c_transfer(&radeon_connector->ddc_bus->aux.ddc, msgs, 2);
        } else {

I think the changes to these four files should be pushed back to the main line kernel and it is probably also desirable to back port these changes to earlier kernels.
---
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CasperVersion: 1.376
CurrentDesktop: MATE
DistroRelease: Ubuntu 16.04
LiveMediaBuild: Ubuntu-MATE 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
MachineType: AMD Corporation EBrazos Platform
Package: linux (not installed)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
ProcFB: 0 radeondrmfb
ProcKernelCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/username.seed boot=casper locale=en_GB console-setup/layoutcode=gb radeon.pcie_gen2=0 radeon.audio=0 ---
ProcVersionSignature: Ubuntu 4.4.0-22.40-generic 4.4.8
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-22-generic N/A
 linux-backports-modules-4.4.0-22-generic N/A
 linux-firmware 1.157
RfKill:

Tags: xenial
Uname: Linux 4.4.0-22-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo
_MarkForUpload: True
dmi.bios.date: 02/14/2012
dmi.bios.vendor: Phoenix Technologies Ltd.
dmi.bios.version: LV-683 Version: 1.1
dmi.board.asset.tag: Base Board Asset Tag Unknown
dmi.board.name: Persimmon
dmi.board.vendor: AMD Corp.
dmi.board.version: A11
dmi.chassis.asset.tag: Asset Tag
dmi.chassis.type: 9
dmi.chassis.vendor: AMD Corporation
dmi.chassis.version: 0.1
dmi.modalias: dmi:bvnPhoenixTechnologiesLtd.:bvrLV-683Version1.1:bd02/14/2012:svnAMDCorporation:pnEBrazosPlatform:pvr0.1:rvnAMDCorp.:rnPersimmon:rvrA11:cvnAMDCorporation:ct9:cvr0.1:
dmi.product.name: EBrazos Platform
dmi.product.version: 0.1
dmi.sys.vendor: AMD Corporation