Could be the same as Bug 96914

Related bugs:
*ERROR* CPU pipe B FIFO underrun:
Bug 94605, Bug 94884, Bug 96226, Bug 96851

*ERROR* too many voltage retries, give up:
Bug 96436

*ERROR* failed to start channel equalization:
Bug 96614

Created attachment 125154 [details]
APL_suspend-to-ram_200-cycles-with-success_all-errors

With new kernel and without GuC, DUT did 200 cycles with success but kernel log contains a lot of errors.

Platform: APL system
CPU Name : Intel(R) Genuine Processor @ 1.1 GHz (family: 6, model: 12, stepping: 9) 4 cores
QDF : Q6HE
SoC : B1
CRB : Apollo Lake DDR3L RVP1A FAB2
Reworks : R19, R20

Software 
Bios: 144_B10 APLK_B0_IFWI_X64_R_2016_06_27_0956_SPI_RVP1.bin from \\gar\ec\proj\ba\CCG\APL BIOS\External\BIOS_Release\Daily\v144_10_2016_WW27.1\IFWI\IFWI_RVP1_Release\IFWI
KSC: 1.15
Linux distribution: Ubuntu 16.04 64 bits
Kernel: 4.7.0-rc7 895a714 from http://cgit.freedesktop.org/drm-intel/ with https://patchwork.freedesktop.org/patch/99445/ applied
  commit 895a714b0b596cfcbe82065f99376ad02d369125
  Author: Daniel Vetter <daniel.vetter@ffwll.ch>
  Date:   Mon Jul 18 14:35:39 2016 +0200
  drm-intel-nightly: 2016y-07m-18d-12h-35m-15s UTC integration manifest
drm: libdrm-2.4.68-15 2212a64 from git://anongit.freedesktop.org/mesa/drm
mesa: mesa-11.2.2 3a9f628from git://anongit.freedesktop.org/mesa/mesa
cairo: 1.15.2 db8a7f1 from git://anongit.freedesktop.org/cairo
xserver: xorg-server-1.18.0-460 e8e3675 from git://git.freedesktop.org/git/xorg/xserver
xf86-video-intel: 2.99.917-676 26f8ab5 from git://git.freedesktop.org/git/xorg/driver/xf86-video-intel
libva: libva-1.7.0-26 c36971c from git://git.freedesktop.org/git/vaapi/libva
vaapi-intel-driver: 1.7.0-53 bcde10d from git://git.freedesktop.org/git/vaapi/intel-driver
DMC 1.07 from https://01.org/linuxgraphics/downloads/broxton-dmc-1.07

Could you try the following:

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 78beb7e..3c0c2d6 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -13682,12 +13682,14 @@ static void intel_atomic_commit_tail(struct drm_atomic_state *state)
 			 */
 			intel_check_cpu_fifo_underruns(dev_priv);
 			intel_check_pch_fifo_underruns(dev_priv);
-
-			if (!crtc->state->active)
-				intel_update_watermarks(crtc);
 		}
 	}
 
+	for_each_crtc_in_state(state, crtc, old_crtc_state, i) {
+		if (needs_modeset(crtc->state) && !crtc->state->active)
+			intel_update_watermarks(crtc);
+	}
+
 	/* Only after disabling all output pipelines that will be changed can we
 	 * update the the output configuration. */
 	intel_modeset_update_crtc_state(state);

Ping: could you try the patch at
https://lists.freedesktop.org/archives/intel-gfx/2016-July/101462.html

Christophe, please try with Imre's patch (also available at https://patchwork.freedesktop.org/series/10206/)

Created attachment 127291 [details]
APL__dmesg__with-patch-applied

ERROR CPU pipe B FIFO underrun is not reproduced when suspending to RAM with the patch applied but I see it earlier in my log:

user@BXTP6:~/Desktop$ dmesg |grep ERROR
[   14.343070] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun
[  207.578429] [drm:intel_dp_start_link_train [i915]] *ERROR* failed to enable link training
[  207.611828] [drm:intel_dp_start_link_train [i915]] *ERROR* failed to start channel equalization

I also see 2 GPU Hangs, I reported it here: bug 98247

Platform BXT-P: APL system
CPU Name : Intel(R) Genuine Processor @ 1.1 GHz (family: 6, model: 12, stepping: 9) 4 cores
QDF : Q6HE
SoC : B1
CRB : Apollo Lake DDR3L RVP1A FAB2
Reworks: R19, R20

Software 
Bios: 144_B10 APLK_B0_IFWI_X64_R_2016_06_27_0956_SPI_RVP1.bin from \\gar\ec\proj\ba\CCG\APL BIOS\External\BIOS_Release\Daily\v144_10_2016_WW27.1\IFWI\IFWI_RVP1_Release\IFWI
KSC: 1.15
Linux distribution: Ubuntu 16.04 64 bits
DMC 1.07
GuC 8.7

Kernel: 4.8.0 f79322d from http://cgit.freedesktop.org/drm-intel/
  commit f79322df75bf6193458ec601a345ba6bd86befca
  Author: Chris Wilson <chris@chris-wilson.co.uk>
  Date:   Tue Oct 11 10:16:23 2016 +0100
  drm-intel-nightly: 2016y-10m-11d-09h-16m-03s UTC integration manifest
with applied https://patchwork.freedesktop.org/patch/100578/

libdrm-2.4.71 a44c9c3 from git://anongit.freedesktop.org/mesa/drm
mesa: mesa-12.0.0 8b06176 from git://anongit.freedesktop.org/mesa/mesa
cairo 1.15.2 db8a7f1 from git://anongit.freedesktop.org/cairo
xorg-server-1.18.99.901-76 97a8353 from git://git.freedesktop.org/git/xorg/xserver
xf86-video-intel 2.99.917-712 696f58f from git://git.freedesktop.org/git/xorg/driver/xf86-video-intel
libva-1.7.2-38 3b7e499 from git://git.freedesktop.org/git/vaapi/libva 
vaapi-intel-driver: 1.7.2-133 dd73514 from git://git.freedesktop.org/git/vaapi/intel-driver

External screens: ASUS PB238Q (HDMI), LG 25UM55D (DP)

Christophe, is this still valid?

Luis could you test this with latest configuration, this can happen next week after current activities.

Issue is not seen on my side with the following config:

============================================
 Software information
============================================
Kernel version                  : 4.9.0-drm-tip-ww52-commit-6096aee+
Linux distribution              : Ubuntu 16.04.1 LTS
Architecture                    : 64-bit
Gfx stack code                  : 2121337496
Mesa version                    : 13.0.2 (git-c9e993b
Xorg-Xserver version            : 1.19.0
DRM version                     : 2.4.74
VAAPI version                   : Intel i965 driver for Intel(R) Broxton - 1.7.3
Cairo version                   : 1.15.2
Intel GPU Tools version         : Tag [intel-gpu-tools-1.17] / Commit [e631bb5]
Kernel driver in use            : i915
Hardware acceleration           : Enabled
Bios revision                   : 159.20
Bios release date               : 10/11/2016
KSC revision                    : 1.6

============================================
 Firmwares information
============================================
DMC fw loaded                   : yes
DMC version                     : 1.7