[PATCH v4 4/9] drm/i915: Disable tracing points on PREEMPT_RT

[Maarten Lankhorst]

Hey,

I believe the platforms that require spinlocks for reading registers all predate Xe,
so it should be possible to enable tracepoints on Xe.

I do see there is still a path there that takes locks, through intel_crtc_get_vblank_counter -> drm_crtc_accurate_vblank_count().

I would recommend changing intel_crtc_get_vblank_counter to use drm_vblank_count(),
which doesn't take locks.

This reduces vblank accuracy slightly on old platforms and gen11+ DSI, but I would say for most usecases it has the same accuracy, with the benefit of keeping tracepoints enabled. The only users for intel_crtc_get_vblank_counter() are debug functions.

With that change, something like: 
"#if defined(CONFIG_PREEMPT_RT) && defined(I915) && !defined(NOTRACE)"

would disable tracepoints only on i915.

Kind regards,
~Maarten

Den 2025-07-14 kl. 17:39, skrev Sebastian Andrzej Siewior:
> 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.
> 
> Acked-by: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
> Reported-by: Luca Abeni <lucabe72 at gmail.com>
> Cc: Steven Rostedt <rostedt at goodmis.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
> ---
>  drivers/gpu/drm/i915/display/intel_display_trace.h | 4 ++++
>  drivers/gpu/drm/i915/i915_trace.h                  | 4 ++++
>  drivers/gpu/drm/i915/intel_uncore_trace.h          | 4 ++++
>  3 files changed, 12 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_display_trace.h b/drivers/gpu/drm/i915/display/intel_display_trace.h
> index 27ebc32cb61a5..a519d94700c36 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_trace.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_trace.h
> @@ -13,6 +13,10 @@
>  #if !defined(__INTEL_DISPLAY_TRACE_H__) || defined(TRACE_HEADER_MULTI_READ)
>  #define __INTEL_DISPLAY_TRACE_H__
>  
> +#if defined(CONFIG_PREEMPT_RT) && !defined(NOTRACE)
> +#define NOTRACE
> +#endif
> +
>  #include <linux/string.h>
>  #include <linux/string_helpers.h>
>  #include <linux/types.h>
> diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
> index 7ed41ce9b7085..6b87ef6005c69 100644
> --- a/drivers/gpu/drm/i915/i915_trace.h
> +++ b/drivers/gpu/drm/i915/i915_trace.h
> @@ -6,6 +6,10 @@
>  #if !defined(_I915_TRACE_H_) || defined(TRACE_HEADER_MULTI_READ)
>  #define _I915_TRACE_H_
>  
> +#if defined(CONFIG_PREEMPT_RT) && !defined(NOTRACE)
> +#define NOTRACE
> +#endif
> +
>  #include <linux/stringify.h>
>  #include <linux/types.h>
>  #include <linux/tracepoint.h>
> diff --git a/drivers/gpu/drm/i915/intel_uncore_trace.h b/drivers/gpu/drm/i915/intel_uncore_trace.h
> index f13ff71edf2db..3c67e267fb602 100644
> --- a/drivers/gpu/drm/i915/intel_uncore_trace.h
> +++ b/drivers/gpu/drm/i915/intel_uncore_trace.h
> @@ -7,6 +7,10 @@
>  #if !defined(__INTEL_UNCORE_TRACE_H__) || defined(TRACE_HEADER_MULTI_READ)
>  #define __INTEL_UNCORE_TRACE_H__
>  
> +#if defined(CONFIG_PREEMPT_RT) && !defined(NOTRACE)
> +#define NOTRACE
> +#endif
> +
>  #include "i915_reg_defs.h"
>  
>  #include <linux/types.h>