[Intel-gfx] [PATCH 5/8] drm/i915: Don't disable interrupts on PREEMPT_RT during atomic updates

Sebastian Andrzej Siewior
2021-12-14T14:03:43Z

From: Mike Galbraith <umgwanakikbuti@gmail.com>

Commit
8d7849db3eab7 ("drm/i915: Make sprite updates atomic")

started disabling interrupts across atomic updates. This breaks on PREEMPT_RT
because within this section the code attempt to acquire spinlock_t locks which
are sleeping locks on PREEMPT_RT.

According to the comment the interrupts are disabled to avoid random delays and
not required for protection or synchronisation.
If this needs to happen with disabled interrupts on PREEMPT_RT, and the
whole section is restricted to register access then all sleeping locks
need to be acquired before interrupts are disabled and some function
maybe moved after enabling interrupts again.
This includes:
- prepare_to_wait() + finish_wait() due its wake queue.
- drm_crtc_vblank_put() -> vblank_disable_fn() drm_device::vbl_lock.
- skl_pfit_enable(), intel_update_plane(), vlv_atomic_update_fifo() and
maybe others due to intel_uncore::lock
- drm_crtc_arm_vblank_event() due to drm_device::event_lock and
drm_device::vblank_time_lock.

Don't disable interrupts on PREEMPT_RT during atomic updates.

[bigeasy: drop local locks, commit message]

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
drivers/gpu/drm/i915/display/intel_crtc.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_crtc.c b/drivers/gpu/drm/i915/display/intel_crtc.c
index 254e67141a776..7a39029b083f4 100644
--- a/drivers/gpu/drm/i915/display/intel_crtc.c
+++ b/drivers/gpu/drm/i915/display/intel_crtc.c
@@ -425,7 +425,8 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
*/
intel_psr_wait_for_idle(new_crtc_state);

- local_irq_disable();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_disable();

crtc->debug.min_vbl = min;
crtc->debug.max_vbl = max;
@@ -450,11 +451,13 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
break;
}

- local_irq_enable();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_enable();

timeout = schedule_timeout(timeout);

- local_irq_disable();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_disable();
}

finish_wait(wq, &wait);
@@ -487,7 +490,8 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
return;

irq_disable:
- local_irq_disable();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_disable();
}

#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_VBLANK_EVADE)
@@ -566,7 +570,8 @@ void intel_pipe_update_end(struct intel_crtc_state *new_crtc_state)
new_crtc_state->uapi.event = NULL;
}

- local_irq_enable();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ local_irq_enable();

/* Send VRR Push to terminate Vblank */
intel_vrr_send_push(new_crtc_state);
--
2.34.1