2 files changed, 105 insertions, 0 deletions
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index 9d8acf7a10eb..06d7777a881f 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -506,6 +506,16 @@ struct drm_mode_config {
 	struct list_head plane_list;
 
 	/**
+	 * @panic_lock:
+	 *
+	 * Raw spinlock used to protect critical sections of code that access
+	 * the display hardware or modeset software state, which the panic
+	 * printing code must be protected against. See drm_panic_trylock(),
+	 * drm_panic_lock() and drm_panic_unlock().
+	 */
+	struct raw_spinlock panic_lock;
+
+	/**
 	 * @num_crtc:
 	 *
 	 * Number of CRTCs on this device linked with &drm_crtc.head. This is invariant over the lifetime
diff --git a/include/drm/drm_panic.h b/include/drm/drm_panic.h
new file mode 100644
index 000000000000..967e02ccc6d5
--- /dev/null
+++ b/include/drm/drm_panic.h
@@ -0,0 +1,95 @@
+/* SPDX-License-Identifier: GPL-2.0 or MIT */
+#ifndef __DRM_PANIC_H__
+#define __DRM_PANIC_H__
+
+#include <drm/drm_device.h>
+/*
+ * Copyright (c) 2024 Intel
+ */
+
+/**
+ * drm_panic_trylock - try to enter the panic printing critical section
+ * @dev: struct drm_device
+ * @flags: unsigned long irq flags you need to pass to the unlock() counterpart
+ *
+ * This function must be called by any panic printing code. The panic printing
+ * attempt must be aborted if the trylock fails.
+ *
+ * Panic printing code can make the following assumptions while holding the
+ * panic lock:
+ *
+ * - Anything protected by drm_panic_lock() and drm_panic_unlock() pairs is safe
+ *   to access.
+ *
+ * - Furthermore the panic printing code only registers in drm_dev_unregister()
+ *   and gets removed in drm_dev_unregister(). This allows the panic code to
+ *   safely access any state which is invariant in between these two function
+ *   calls, like the list of planes &drm_mode_config.plane_list or most of the
+ *   struct drm_plane structure.
+ *
+ * Specifically thanks to the protection around plane updates in
+ * drm_atomic_helper_swap_state() the following additional guarantees hold:
+ *
+ * - It is safe to deference the drm_plane.state pointer.
+ *
+ * - Anything in struct drm_plane_state or the driver's subclass thereof which
+ *   stays invariant after the atomic check code has finished is safe to access.
+ *   Specifically this includes the reference counted pointers to framebuffer
+ *   and buffer objects.
+ *
+ * - Anything set up by &drm_plane_helper_funcs.fb_prepare and cleaned up
+ *   &drm_plane_helper_funcs.fb_cleanup is safe to access, as long as it stays
+ *   invariant between these two calls. This also means that for drivers using
+ *   dynamic buffer management the framebuffer is pinned, and therefer all
+ *   relevant datastructures can be accessed without taking any further locks
+ *   (which would be impossible in panic context anyway).
+ *
+ * - Importantly, software and hardware state set up by
+ *   &drm_plane_helper_funcs.begin_fb_access and
+ *   &drm_plane_helper_funcs.end_fb_access is not safe to access.
+ *
+ * Drivers must not make any assumptions about the actual state of the hardware,
+ * unless they explicitly protected these hardware access with drm_panic_lock()
+ * and drm_panic_unlock().
+ *
+ * Return:
+ * %0 when failing to acquire the raw spinlock, nonzero on success.
+ */
+#define drm_panic_trylock(dev, flags) \
+	raw_spin_trylock_irqsave(&(dev)->mode_config.panic_lock, flags)
+
+/**
+ * drm_panic_lock - protect panic printing relevant state
+ * @dev: struct drm_device
+ * @flags: unsigned long irq flags you need to pass to the unlock() counterpart
+ *
+ * This function must be called to protect software and hardware state that the
+ * panic printing code must be able to rely on. The protected sections must be
+ * as small as possible. It uses the irqsave/irqrestore variant, and can be
+ * called from irq handler. Examples include:
+ *
+ * - Access to peek/poke or other similar registers, if that is the way the
+ *   driver prints the pixels into the scanout buffer at panic time.
+ *
+ * - Updates to pointers like &drm_plane.state, allowing the panic handler to
+ *   safely deference these. This is done in drm_atomic_helper_swap_state().
+ *
+ * - An state that isn't invariant and that the driver must be able to access
+ *   during panic printing.
+ */
+
+#define drm_panic_lock(dev, flags) \
+	raw_spin_lock_irqsave(&(dev)->mode_config.panic_lock, flags)
+
+/**
+ * drm_panic_unlock - end of the panic printing critical section
+ * @dev: struct drm_device
+ * @flags: irq flags that were returned when acquiring the lock
+ *
+ * Unlocks the raw spinlock acquired by either drm_panic_lock() or
+ * drm_panic_trylock().
+ */
+#define drm_panic_unlock(dev, flags) \
+	raw_spin_unlock_irqrestore(&(dev)->mode_config.panic_lock, flags)
+
+#endif /* __DRM_PANIC_H__ */