[PATCH v3] Fix divide-by-zero regression on DP MST unplug with nouveau

[Chris Bainbridge <chris.bainbridge@gmail.com>]

Fix a regression when using nouveau and unplugging a StarTech MSTDP122DP
DisplayPort 1.2 MST hub (the same regression does not appear when using
a Cable Matters DisplayPort 1.4 MST hub). Trace:

 divide error: 0000 [#1] PREEMPT SMP PTI
 CPU: 7 PID: 2962 Comm: Xorg Not tainted 6.8.0-rc3+ #744
 Hardware name: Razer Blade/DANA_MB, BIOS 01.01 08/31/2018
 RIP: 0010:drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
 Code: c6 b8 01 00 00 00 75 61 01 c6 41 0f af f3 41 0f af f1 c1 e1 04 48 63 c7 31 d2 89 ff 48 8b 5d f8 c9 48 0f af f1 48 8d 44 06 ff <48> f7 f7 31 d2 31 c9 31 f6 31 ff 45 31 c0 45 31 c9 45 31 d2 45 31
 RSP: 0018:ffffb2c5c211fa30 EFLAGS: 00010206
 RAX: ffffffffffffffff RBX: 0000000000000000 RCX: 0000000000f59b00
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
 RBP: ffffb2c5c211fa48 R08: 0000000000000001 R09: 0000000000000020
 R10: 0000000000000004 R11: 0000000000000000 R12: 0000000000023b4a
 R13: ffff91d37d165800 R14: ffff91d36fac6d80 R15: ffff91d34a764010
 FS:  00007f4a1ca3fa80(0000) GS:ffff91d6edbc0000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000559491d49000 CR3: 000000011d180002 CR4: 00000000003706f0
 Call Trace:
  <TASK>
  ? show_regs+0x6d/0x80
  ? die+0x37/0xa0
  ? do_trap+0xd4/0xf0
  ? do_error_trap+0x71/0xb0
  ? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
  ? exc_divide_error+0x3a/0x70
  ? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
  ? asm_exc_divide_error+0x1b/0x20
  ? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
  ? drm_dp_calc_pbn_mode+0x2e/0x70 [drm_display_helper]
  nv50_msto_atomic_check+0xda/0x120 [nouveau]
  drm_atomic_helper_check_modeset+0xa87/0xdf0 [drm_kms_helper]
  drm_atomic_helper_check+0x19/0xa0 [drm_kms_helper]
  nv50_disp_atomic_check+0x13f/0x2f0 [nouveau]
  drm_atomic_check_only+0x668/0xb20 [drm]
  ? drm_connector_list_iter_next+0x86/0xc0 [drm]
  drm_atomic_commit+0x58/0xd0 [drm]
  ? __pfx___drm_printfn_info+0x10/0x10 [drm]
  drm_atomic_connector_commit_dpms+0xd7/0x100 [drm]
  drm_mode_obj_set_property_ioctl+0x1c5/0x450 [drm]
  ? __pfx_drm_connector_property_set_ioctl+0x10/0x10 [drm]
  drm_connector_property_set_ioctl+0x3b/0x60 [drm]
  drm_ioctl_kernel+0xb9/0x120 [drm]
  drm_ioctl+0x2d0/0x550 [drm]
  ? __pfx_drm_connector_property_set_ioctl+0x10/0x10 [drm]
  nouveau_drm_ioctl+0x61/0xc0 [nouveau]
  __x64_sys_ioctl+0xa0/0xf0
  do_syscall_64+0x76/0x140
  ? do_syscall_64+0x85/0x140
  ? do_syscall_64+0x85/0x140
  entry_SYSCALL_64_after_hwframe+0x6e/0x76
 RIP: 0033:0x7f4a1cd1a94f
 Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
 RSP: 002b:00007ffd2f1df520 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
 RAX: ffffffffffffffda RBX: 00007ffd2f1df5b0 RCX: 00007f4a1cd1a94f
 RDX: 00007ffd2f1df5b0 RSI: 00000000c01064ab RDI: 000000000000000f
 RBP: 00000000c01064ab R08: 000056347932deb8 R09: 000056347a7d99c0
 R10: 0000000000000000 R11: 0000000000000246 R12: 000056347938a220
 R13: 000000000000000f R14: 0000563479d9f3f0 R15: 0000000000000000
  </TASK>
 Modules linked in: rfcomm xt_conntrack nft_chain_nat xt_MASQUERADE nf_nat nf_conntrack_netlink nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 xfrm_user xfrm_algo xt_addrtype nft_compat nf_tables nfnetlink br_netfilter bridge stp llc ccm cmac algif_hash overlay algif_skcipher af_alg bnep binfmt_misc snd_sof_pci_intel_cnl snd_sof_intel_hda_common snd_soc_hdac_hda snd_sof_pci snd_sof_xtensa_dsp snd_sof_intel_hda snd_sof snd_sof_utils snd_soc_acpi_intel_match snd_soc_acpi snd_soc_core snd_compress snd_sof_intel_hda_mlink snd_hda_ext_core iwlmvm intel_rapl_msr intel_rapl_common intel_tcc_cooling x86_pkg_temp_thermal intel_powerclamp mac80211 coretemp kvm_intel snd_hda_codec_hdmi kvm snd_hda_codec_realtek snd_hda_codec_generic uvcvideo libarc4 snd_hda_intel snd_intel_dspcfg snd_hda_codec iwlwifi videobuf2_vmalloc videobuf2_memops uvc irqbypass btusb videobuf2_v4l2 snd_seq_midi crct10dif_pclmul hid_multitouch crc32_pclmul snd_seq_midi_event btrtl snd_hwdep videodev polyval_clmulni polyval_generic snd_rawmidi
  ghash_clmulni_intel aesni_intel btintel crypto_simd snd_hda_core cryptd snd_seq btbcm ee1004 8250_dw videobuf2_common btmtk rapl nls_iso8859_1 mei_hdcp thunderbolt bluetooth intel_cstate wmi_bmof intel_wmi_thunderbolt cfg80211 snd_pcm mc snd_seq_device i2c_i801 r8169 ecdh_generic snd_timer i2c_smbus ecc snd mei_me intel_lpss_pci mei ahci intel_lpss soundcore realtek libahci idma64 intel_pch_thermal i2c_hid_acpi i2c_hid acpi_pad sch_fq_codel msr parport_pc ppdev lp parport efi_pstore ip_tables x_tables autofs4 dm_crypt raid10 raid456 libcrc32c async_raid6_recov async_memcpy async_pq async_xor xor async_tx raid6_pq raid1 raid0 joydev input_leds hid_generic usbhid hid nouveau i915 drm_ttm_helper gpu_sched drm_gpuvm drm_exec i2c_algo_bit drm_buddy ttm drm_display_helper drm_kms_helper cec rc_core drm nvme nvme_core mxm_wmi xhci_pci xhci_pci_renesas video wmi pinctrl_cannonlake mac_hid
 ---[ end trace 0000000000000000 ]---

Fix this by avoiding the divide if bpp is 0.

Fixes: c1d6a22b7219 ("drm/dp: Add helpers to calculate the link BW overhead")
Cc: stable@vger.kernel.org
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
---
 drivers/gpu/drm/display/drm_dp_helper.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index b1ca3a1100da..d51c1bcee258 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -3982,6 +3982,12 @@ int drm_dp_bw_overhead(int lane_count, int hactive,
 	u32 overhead = 1000000;
 	int symbol_cycles;
 
+	if (lane_count == 0 || hactive == 0 || bpp_x16 == 0) {
+		DRM_DEBUG_KMS("Invalid BW overhead params: lane_count %d, hactive %d, bpp_x16 %.04d\n",
+			      lane_count, hactive, bpp_x16);
+		return 0;
+	}
+
 	/*
 	 * DP Standard v2.1 2.6.4.1
 	 * SSC downspread and ref clock variation margin:
-- 
2.39.2