drm/i915: Avoid waking the engines just to check if they are idle

drm/i915: Avoid waking the engines just to check if they are idle

Exploit that reads of the ring registers return 0 from the engine when
it is idle and we do not apply forcewake to know that if the engine is
idle then both reads will be identical (and so we interpret the ring as
idle).

The ulterior motive is to try and reduce the number of spurious wakeups
to avoid untimely death, such as:

<3> [85.046836] [drm:fw_domains_get [i915]] *ERROR* render: timed out waiting for forcewake ack request.
<4> [85.051916] ------------[ cut here ]------------
<4> [85.051917] GT thread status wait timed out
<4> [85.051963] WARNING: CPU: 2 PID: 2195 at drivers/gpu/drm/i915/intel_uncore.c:303 __gen6_gt_wait_for_thread_c0+0x6e/0xa0 [i915]
<4> [85.051964] Modules linked in: snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic i915 x86_pkg_temp_thermal coretemp mei_hdcp crct10dif_pclmul crc32_pclmul snd_hda_intel ghash_clmulni_intel snd_hda_codec broadcom bcm_phy_lib i2c_i801 snd_hwdep snd_hda_core tg3 snd_pcm ptp pps_core mei_me mei prime_numbers lpc_ich
<4> [85.051980] CPU: 2 PID: 2195 Comm: drm_read Tainted: G     U            5.0.0-rc8-CI-CI_DRM_5662+ #1
<4> [85.051981] Hardware name: Dell Inc. XPS 8300  /0Y2MRG, BIOS A06 10/17/2011
<4> [85.052012] RIP: 0010:__gen6_gt_wait_for_thread_c0+0x6e/0xa0 [i915]
<4> [85.052015] Code: 8b 92 5c 80 13 00 83 e2 07 75 d5 5b 5d c3 80 3d 5b 6a 1a 00 00 75 f4 48 c7 c7 38 21 31 a0 c6 05 4b 6a 1a 00 01 e8 e2 84 ea e0 <0f> 0b eb dd 80 3d 3a 6a 1a 00 00 75 98 48 c7 c6 08 21 31 a0 48 c7
<4> [85.052016] RSP: 0018:ffffc9000043bd00 EFLAGS: 00010086
<4> [85.052019] RAX: 0000000000000000 RBX: ffff888217c50000 RCX: 0000000000000000
<4> [85.052020] RDX: 0000000000000007 RSI: ffffffff820cb141 RDI: 00000000ffffffff
<4> [85.052022] RBP: 00000013cd30f2fb R08: 0000000000000000 R09: 0000000000000001
<4> [85.052024] R10: ffffc9000043bce0 R11: 0000000000000000 R12: ffff888217c50ee0
<4> [85.052025] R13: 0000000000000001 R14: 00000000ffffffff R15: ffff888218076530
<4> [85.052028] FS:  00007fc79d049980(0000) GS:ffff888227a80000(0000) knlGS:0000000000000000
<4> [85.052029] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4> [85.052031] CR2: 00007f782e2940f8 CR3: 000000022458e006 CR4: 00000000000606e0
<4> [85.052033] Call Trace:
<4> [85.052064]  gen6_read32+0x14e/0x250 [i915]
<4> [85.052096]  intel_engine_is_idle+0x7d/0x180 [i915]
<4> [85.052126]  intel_engines_are_idle+0x29/0x50 [i915]
<4> [85.052153]  i915_drop_caches_set+0x21c/0x290 [i915]
<4> [85.052160]  simple_attr_write+0xb0/0xd0
<4> [85.052165]  full_proxy_write+0x51/0x80
<4> [85.052170]  __vfs_write+0x31/0x190
<4> [85.052176]  ? rcu_read_lock_sched_held+0x6f/0x80
<4> [85.052178]  ? rcu_sync_lockdep_assert+0x29/0x50
<4> [85.052181]  ? __sb_start_write+0x152/0x1f0
<4> [85.052183]  ? __sb_start_write+0x163/0x1f0
<4> [85.052187]  vfs_write+0xbd/0x1b0
<4> [85.052191]  ksys_write+0x50/0xc0
<4> [85.052196]  do_syscall_64+0x55/0x190
<4> [85.052200]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
<4> [85.052202] RIP: 0033:0x7fc79c9d3281
<4> [85.052204] Code: c3 0f 1f 84 00 00 00 00 00 48 8b 05 59 8d 20 00 c3 0f 1f 84 00 00 00 00 00 8b 05 8a d1 20 00 85 c0 75 16 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 57 f3 c3 0f 1f 44 00 00 41 54 55 49 89 d4 53
<4> [85.052206] RSP: 002b:00007fffa4a0a7f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
<4> [85.052208] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007fc79c9d3281
<4> [85.052210] RDX: 0000000000000005 RSI: 00007fffa4a0a880 RDI: 0000000000000008
<4> [85.052212] RBP: 00007fffa4a0a820 R08: 0000000000000000 R09: 0000000000000000
<4> [85.052213] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fc79c9bc718
<4> [85.052215] R13: 0000000000000003 R14: 00007fc79c9c1628 R15: 00007fc79c9bdd80
<4> [85.052223] irq event stamp: 71630
<4> [85.052226] hardirqs last  enabled at (71629): [<ffffffff8197b64c>] _raw_spin_unlock_irqrestore+0x4c/0x60
<4> [85.052228] hardirqs last disabled at (71630): [<ffffffff8197b4bd>] _raw_spin_lock_irqsave+0xd/0x50
<4> [85.052231] softirqs last  enabled at (70444): [<ffffffff81c0033a>] __do_softirq+0x33a/0x4b9
<4> [85.052234] softirqs last disabled at (70433): [<ffffffff810b51b1>] irq_exit+0xd1/0xe0
<4> [85.052264] WARNING: CPU: 2 PID: 2195 at drivers/gpu/drm/i915/intel_uncore.c:303 __gen6_gt_wait_for_thread_c0+0x6e/0xa0 [i915]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190227114958.32438-1-chris@chris-wilson.co.uk