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[Bug 1587885] Re: NULL pointer dereference in radeon drm code

 

** Changed in: linux (Ubuntu)
   Importance: Undecided => High

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https://bugs.launchpad.net/bugs/1587885

Title:
  NULL pointer dereference in radeon drm code

Status in linux package in Ubuntu:
  Confirmed

Bug description:
  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

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References