[PATCH 1/2] drm/i915: Disable tracepoints for PREEMPT_RT

[Maarten Lankhorst <dev@lankhorst.se>]

Luca Abeni reported this:
| BUG: scheduling while atomic: kworker/u8:2/15203/0x00000003
| CPU: 1 PID: 15203 Comm: kworker/u8:2 Not tainted 4.19.1-rt3 #10
| Call Trace:
|  rt_spin_lock+0x3f/0x50
|  gen6_read32+0x45/0x1d0 [i915]
|  g4x_get_vblank_counter+0x36/0x40 [i915]
|  trace_event_raw_event_i915_pipe_update_start+0x7d/0xf0 [i915]

The tracing events use trace_intel_pipe_update_start() among other events
use functions acquire spinlock_t locks which are transformed into
sleeping locks on PREEMPT_RT. A few trace points use
intel_get_crtc_scanline(), others use ->get_vblank_counter() wich also
might acquire a sleeping locks on PREEMPT_RT.
At the time the arguments are evaluated within trace point, preemption
is disabled and so the locks must not be acquired on PREEMPT_RT.

Based on this I don't see any other way than disable trace points on
PREMPT_RT.

[mlankhorst]
The original patch was insufficient, and since the tracing
infrastructure does not allow for partial disabling of tracepoints.

Completely disable tracing for the entire i915 driver in PREEMPT_RT,
a separate fix for display tracepoints on xe is added to make those
work.

Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reported-by: Luca Abeni <lucabe72@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Co-developed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
---
 drivers/gpu/drm/i915/Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index 8535434430728..ba825d7f6ddf5 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -13,6 +13,11 @@ subdir-ccflags-$(CONFIG_DRM_I915_WERROR) += -Werror
 # drivers. Define I915 when building i915.
 subdir-ccflags-y += -DI915
 
+# Disable tracepoints on i915 for PREEMPT_RT, unfortunately
+# it's an all or nothing flag. You cannot selectively disable
+# only some tracepoints.
+subdir-ccflags-$(CONFIG_PREEMPT_RT) += -DNOTRACE
+
 subdir-ccflags-y += -I$(src)
 
 # Please keep these build lists sorted!
-- 
2.50.1