[Intel-gfx] vlv_disable_backlight causing warnings with i915 @ Xorg start

[Andrew Siplas]

At Xorg startup after a fresh compile of the mainline kernel, WARN_ON is
truthy and throws a warning into the kernel's dmesg buffer.

I'm still trying to understand the driver, but it originates here:

--

static void vlv_disable_backlight(struct intel_connector *connector)
{
        struct drm_i915_private *dev_priv =
to_i915(connector->base.dev);
        enum pipe pipe = intel_get_pipe_from_connector(connector);
        u32 tmp;

        if (WARN_ON(pipe != PIPE_A && pipe != PIPE_B))
                return;

        intel_panel_actually_set_backlight(connector, 0);

        tmp = I915_READ(VLV_BLC_PWM_CTL2(pipe));
        I915_WRITE(VLV_BLC_PWM_CTL2(pipe), tmp & ~BLM_PWM_ENABLE);
}

--

Don't know yet what this means and so taking a chance this warning is
something someone here might have some insight into / want to know about
especially given the "...----[ cut here ]----...".

The last two lines are from 5+ minutes after the warning but seem related:


[   67.896233] ------------[ cut here ]------------
[   67.896259] WARNING: CPU: 1 PID: 4276 at drivers/gpu/drm/i915/intel_panel.c:771 vlv_disable_backlight+0x86/0x90
[   67.896261] Modules linked in: iptable_nat nf_nat_ipv4 nf_nat uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 videobuf2_core videodev
[   67.896299] CPU: 1 PID: 4276 Comm: Xorg Not tainted 4.11.0+ #2
[   67.896303] Hardware name: HP HP Notebook/80C5, BIOS F.1E 12/25/2015
[   67.896308] task: ffff8d6e75d79800 task.stack: ffff9a06c1218000
[   67.896318] RIP: 0010:vlv_disable_backlight+0x86/0x90
[   67.896323] RSP: 0018:ffff9a06c121b998 EFLAGS: 00010286
[   67.896330] RAX: 0000000000000029 RBX: ffff8d6e76018000 RCX: ffffffff8ea56a78
[   67.896335] RDX: 0000000000000001 RSI: 0000000000000092 RDI: ffffffff8ece6bcc
[   67.896339] RBP: ffff9a06c121b9b0 R08: 0000000000000029 R09: 00000000000002d8
[   67.896344] R10: 0000000000000000 R11: 0000000000000320 R12: ffff8d6e723c5000
[   67.896348] R13: 00000000ffffffff R14: ffff8d6e7423e800 R15: ffff8d6e74ab9830
[   67.896355] FS:  00007fb8d7e00a40(0000) GS:ffff8d6e7fc80000(0000) knlGS:0000000000000000
[   67.896359] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   67.896364] CR2: 0000557bfe77dfd0 CR3: 0000000274d31000 CR4: 00000000001006e0
[   67.896368] Call Trace:
[   67.896383]  intel_panel_disable_backlight+0x51/0x80
[   67.896391]  intel_edp_backlight_off+0x42/0x50
[   67.896397]  intel_disable_dp+0x70/0xf0
[   67.896407]  intel_encoders_disable.isra.103+0x82/0x90
[   67.896416]  i9xx_crtc_disable+0x51/0x3b0
[   67.896424]  ? intel_crtc_disable_planes+0xd4/0xf0
[   67.896434]  intel_atomic_commit_tail+0x892/0xfb0
[   67.896444]  ? insert_work+0x52/0x70
[   67.896453]  ? __queue_work+0x12e/0x390
[   67.896461]  ? intel_atomic_commit_ready+0x70/0x80
[   67.896470]  intel_atomic_commit+0x3e6/0x4b0
[   67.896479]  ? handle_conflicting_encoders+0x279/0x290
[   67.896489]  drm_atomic_commit+0x46/0x50
[   67.896497]  drm_atomic_helper_set_config+0x6b/0xa0
[   67.896507]  __drm_mode_set_config_internal+0x62/0x110
[   67.896514]  drm_mode_setcrtc+0x4ba/0x5a0
[   67.896524]  drm_ioctl+0x326/0x430
[   67.896532]  ? drm_mode_getcrtc+0x170/0x170
[   67.896543]  do_vfs_ioctl+0x8f/0x5a0
[   67.896552]  ? getnstimeofday64+0x9/0x20
[   67.896561]  SyS_ioctl+0x74/0x80
[   67.896569]  do_syscall_64+0x48/0xb0
[   67.896580]  entry_SYSCALL64_slow_path+0x25/0x25
[   67.896586] RIP: 0033:0x7fb8d5932507
[   67.896590] RSP: 002b:00007ffe8bf43018 EFLAGS: 00003246 ORIG_RAX: 0000000000000010
[   67.896598] RAX: ffffffffffffffda RBX: 0000561b92c11210 RCX: 00007fb8d5932507
[   67.896602] RDX: 00007ffe8bf43140 RSI: 00000000c06864a2 RDI: 0000000000000009
[   67.896607] RBP: 00007ffe8bf43140 R08: 0000561b92c11050 R09: 0000000000000001
[   67.896611] R10: 0000000000000001 R11: 0000000000003246 R12: 00000000c06864a2
[   67.896615] R13: 0000000000000009 R14: 0000561b92c13f70 R15: 00007fb8d7d67000
[   67.896620] Code: df 41 8d b4 05 50 12 06 00 ff 93 f8 06 00 00 5b 41 5c 41 5d 5d c3 48 c7 c6 18 bc 88 8e 48 c7 c7 22 e8 82 8e 31 c0 e8 47 70 b6 ff <0f> ff eb e0 66 0f 1f 44 00 00 55 8b b7 f4 03 00 00 48 89 e5 e8 
[   67.896771] ---[ end trace 528f42628325fc1b ]---
[  490.370803] [drm] Atomic update on pipe (A) took 102 us, max time under evasion is 100 us
[ 2623.216311] [drm] Atomic update on pipe (A) took 415 us, max time under evasion is 100 us

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