Capturing the restrictions of the BSpec pages bellow:

SKL and CNL do not support MST in DDI E, DDI E only support 2 lanes
and it is mostly used to support a 4 lanes eDP panel together with
DDI A.
ICL's DDI E support MST just like other ports but DDI A is still eDP
and MIPI only.
TGL supports MST in any DDI, including DDI A but TGL has it's own
ddi_pre_enable_dp function already without any warning.

[  215.579791] ------------[ cut here ]------------
[  215.579794] WARN_ON(is_mst && (port == PORT_A || port == PORT_E))
[  215.579875] WARNING: CPU: 0 PID: 268 at drivers/gpu/drm/i915/display/intel_ddi.c:3576 intel_ddi_pre_enable+0x124/0xea0 [i915]
[  215.579878] Modules linked in: snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic i915 btusb btrtl btbcm btintel bluetooth prime_numbers snd_hda_intel snd_intel_dspcfg snd_hda_codec e1000e snd_hwdep snd_hda_core asix mei_hdcp cdc_ether x86_pkg_temp_thermal mei_me snd_pcm r8152 coretemp usbnet mei crct10dif_pclmul mii ptp ecdh_generic crc32_pclmul i2c_i801 ecc pps_core ghash_clmulni_intel thunderbolt
[  215.579905] CPU: 0 PID: 268 Comm: kworker/0:2 Tainted: G        W         5.4.0-rc8-zeh+ #1307
[  215.579907] Hardware name: Intel Corporation Ice Lake Client Platform/IceLake U DDR4 SODIMM PD RVP TLC, BIOS ICLSFWR1.R00.3201.A00.1905140358 05/14/2019
[  215.579912] Workqueue: events_long drm_dp_mst_link_probe_work
[  215.579975] RIP: 0010:intel_ddi_pre_enable+0x124/0xea0 [i915]
[  215.579978] Code: ff 8b 7c 24 10 89 44 24 30 85 ff 74 1f f7 44 24 18 fb ff ff ff 75 15 48 c7 c6 98 fa 48 a0 48 c7 c7 d3 df 4a a0 e8 cf d5 d0 e0 <0f> 0b 0f b6 4c 24 2c 41 8b b5 04 06 00 00 4c 89 e7 41 0f b6 95 0c
[  215.579980] RSP: 0018:ffffc90001a5f990 EFLAGS: 00010286
[  215.579984] RAX: 0000000000000000 RBX: ffff88848356a000 RCX: 0000000000000000
[  215.579986] RDX: 0000000000001df1 RSI: ffff88849340c998 RDI: ffffffff821489c5
[  215.579989] RBP: ffff88848356a000 R08: 00000000c021a419 R09: 0000000000000000
[  215.579991] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88848356a118
[  215.579994] R13: ffff88847f39c000 R14: ffff88847fe70000 R15: ffff88848356a000
[  215.579996] FS:  0000000000000000(0000) GS:ffff88849f800000(0000) knlGS:0000000000000000
[  215.579999] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  215.580001] CR2: 000055d3d5a26bc0 CR3: 0000000480ba6005 CR4: 0000000000760ef0
[  215.580004] PKRU: 55555554
[  215.580006] Call Trace:
[  215.580014]  ? drm_dp_mst_topology_put_port+0x6f/0x130
[  215.580072]  intel_mst_pre_enable_dp+0x14b/0x170 [i915]
[  215.580129]  intel_encoders_pre_enable+0x76/0x90 [i915]
[  215.580191]  haswell_crtc_enable+0x84/0x880 [i915]
[  215.580266]  intel_update_crtc+0x1e4/0x200 [i915]
[  215.580333]  skl_commit_modeset_enables+0x287/0x420 [i915]
[  215.580405]  intel_atomic_commit_tail+0x332/0x14e0 [i915]
[  215.580410]  ? queue_work_on+0x41/0x70
[  215.580489]  intel_atomic_commit+0x31e/0x350 [i915]
[  215.580500]  drm_client_modeset_commit_atomic+0x18b/0x220
[  215.580523]  drm_client_modeset_commit_force+0x4d/0x180
[  215.580531]  drm_fb_helper_restore_fbdev_mode_unlocked+0x46/0xa0
[  215.580538]  drm_fb_helper_set_par+0x27/0x50
[  215.580543]  drm_fb_helper_hotplug_event.part.0+0xa7/0xc0
[  215.580549]  drm_kms_helper_hotplug_event+0x21/0x30
[  215.580553]  process_one_work+0x25b/0x5b0
[  215.580566]  worker_thread+0x4b/0x3b0
[  215.580578]  kthread+0x100/0x140
[  215.580581]  ? process_one_work+0x5b0/0x5b0
[  215.580585]  ? kthread_park+0x80/0x80
[  215.580591]  ret_from_fork+0x24/0x50
[  215.580603] irq event stamp: 1393930
[  215.580606] hardirqs last  enabled at (1393929): [<ffffffff8112a013>] vprintk_emit+0x143/0x330
[  215.580609] hardirqs last disabled at (1393930): [<ffffffff81001cfa>] trace_hardirqs_off_thunk+0x1a/0x20
[  215.580613] softirqs last  enabled at (1393434): [<ffffffff81c00389>] __do_softirq+0x389/0x47f
[  215.580618] softirqs last disabled at (1393423): [<ffffffff810b7199>] irq_exit+0xa9/0xc0
[  215.580621] ---[ end trace afd44ea9caa6373e ]---

BSpec: 4217
BSpec: 14004
BSpec: 20584
BSpec: 50583
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107170922.153612-2-jose.souza@intel.com