On Thursday, March 28, 2024 8:03:43 AM EDT Jocelyn Falempe wrote:
> From: Daniel Vetter <daniel.vetter@xxxxxxxx>
>
> Rough sketch for the locking of drm panic printing code. The upshot of
> this approach is that we can pretty much entirely rely on the atomic
> commit flow, with the pair of raw_spin_lock/unlock providing any
> barriers we need, without having to create really big critical
> sections in code.
>
> This also avoids the need that drivers must explicitly update the
> panic handler state, which they might forget to do, or not do
> consistently, and then we blow up in the worst possible times.
>
> It is somewhat racy against a concurrent atomic update, and we might
> write into a buffer which the hardware will never display. But there's
> fundamentally no way to avoid that - if we do the panic state update
> explicitly after writing to the hardware, we might instead write to an
> old buffer that the user will barely ever see.
>
> Note that an rcu protected deference of plane->state would give us the
> the same guarantees, but it has the downside that we then need to
> protect the plane state freeing functions with call_rcu too. Which
> would very widely impact a lot of code and therefore doesn't seem
> worth the complexity compared to a raw spinlock with very tiny
> critical sections. Plus rcu cannot be used to protect access to
> peek/poke registers anyway, so we'd still need it for those cases.
>
> Peek/poke registers for vram access (or a gart pte reserved just for
> panic code) are also the reason I've gone with a per-device and not
> per-plane spinlock, since usually these things are global for the
> entire display. Going with per-plane locks would mean drivers for such
> hardware would need additional locks, which we don't want, since it
> deviates from the per-console takeoverlocks design.
>
> Longer term it might be useful if the panic notifiers grow a bit more
> structure than just the absolute bare
> EXPORT_SYMBOL(panic_notifier_list) - somewhat aside, why is that not
> EXPORT_SYMBOL_GPL ... If panic notifiers would be more like console
> drivers with proper register/unregister interfaces we could perhaps
> reuse the very fancy console lock with all it's check and takeover
> semantics that John Ogness is developing to fix the console_lock mess.
> But for the initial cut of a drm panic printing support I don't think
> we need that, because the critical sections are extremely small and
> only happen once per display refresh. So generally just 60 tiny locked
> sections per second, which is nothing compared to a serial console
> running a 115kbaud doing really slow mmio writes for each byte. So for
> now the raw spintrylock in drm panic notifier callback should be good
> enough.
>
> Another benefit of making panic notifiers more like full blown
> consoles (that are used in panics only) would be that we get the two
> stage design, where first all the safe outputs are used. And then the
> dangerous takeover tricks are deployed (where for display drivers we
> also might try to intercept any in-flight display buffer flips, which
> if we race and misprogram fifos and watermarks can hang the memory
> controller on some hw).
>
> For context the actual implementation on the drm side is by Jocelyn
> and this patch is meant to be combined with the overall approach in
> v7 (v8 is a bit less flexible, which I think is the wrong direction):
>
> https://lore.kernel.org/dri-devel/20240104160301.185915-1-jfalempe@xxxxxxxxxx/
>
> Note that the locking is very much not correct there, hence this
> separate rfc.
>
> v2:
> - fix authorship, this was all my typing
> - some typo oopsies
> - link to the drm panic work by Jocelyn for context
>
> v10:
> - Use spinlock_irqsave/restore (John Ogness)
>
> v11:
> - Use macro instead of inline functions for drm_panic_lock/unlock (John Ogness)
>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Jocelyn Falempe <jfalempe@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> Cc: Lukas Wunner <lukas@xxxxxxxxx>
> Cc: Petr Mladek <pmladek@xxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: John Ogness <john.ogness@xxxxxxxxxxxxx>
> Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Maxime Ripard <mripard@xxxxxxxxxx>
> Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxxx>
> Cc: Daniel Vetter <daniel@xxxxxxxx>
> Signed-off-by: Jocelyn Falempe <jfalempe@xxxxxxxxxx>
> ---
>� drivers/gpu/drm/drm_atomic_helper.c |�� 4 ++
>� drivers/gpu/drm/drm_drv.c���������� |�� 1 +
>� include/drm/drm_mode_config.h������ |� 10 +++
>� include/drm/drm_panic.h������������ | 100 ++++++++++++++++++++++++++++
>� 4 files changed, 115 insertions(+)
>� create mode 100644 include/drm/drm_panic.h
>
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 39ef0a6addeb..fb97b51b38f1 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -38,6 +38,7 @@
>� #include <drm/drm_drv.h>
>� #include <drm/drm_framebuffer.h>
>� #include <drm/drm_gem_atomic_helper.h>
> +#include <drm/drm_panic.h>
>� #include <drm/drm_print.h>
>� #include <drm/drm_self_refresh_helper.h>
>� #include <drm/drm_vblank.h>
> @@ -3016,6 +3017,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
>� ��� ��� ��� ��� � bool stall)
>� {
>� ��� int i, ret;
> +��� unsigned long flags;
>� ��� struct drm_connector *connector;
>� ��� struct drm_connector_state *old_conn_state, *new_conn_state;
>� ��� struct drm_crtc *crtc;
> @@ -3099,6 +3101,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
>� ��� ��� }
>� ��� }
>�
> +��� drm_panic_lock(state->dev, flags);
>� ��� for_each_oldnew_plane_in_state(state, plane, old_plane_state, new_plane_state, i) {
>� ��� ��� WARN_ON(plane->state != old_plane_state);
>�
> @@ -3108,6 +3111,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
>� ��� ��� state->planes[i].state = old_plane_state;
>� ��� ��� plane->state = new_plane_state;
>� ��� }
> +��� drm_panic_unlock(state->dev, flags);
>�
>� ��� for_each_oldnew_private_obj_in_state(state, obj, old_obj_state, new_obj_state, i) {
>� ��� ��� WARN_ON(obj->state != old_obj_state);
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 243cacb3575c..c157500b3135 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -638,6 +638,7 @@ static int drm_dev_init(struct drm_device *dev,
>� ��� mutex_init(&dev->filelist_mutex);
>� ��� mutex_init(&dev->clientlist_mutex);
>� ��� mutex_init(&dev->master_mutex);
> +��� raw_spin_lock_init(&dev->mode_config.panic_lock);
>�
>� ��� ret = drmm_add_action_or_reset(dev, drm_dev_init_release, NULL);
>� ��� if (ret)
> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index 973119a9176b..e79f1a557a22 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -505,6 +505,16 @@ 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:
>� ��� *
> diff --git a/include/drm/drm_panic.h b/include/drm/drm_panic.h
> new file mode 100644
> index 000000000000..68f57710d2d1
> --- /dev/null
> +++ b/include/drm/drm_panic.h
> @@ -0,0 +1,100 @@
> +/* 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().
> + *
> + * Returns:
> + *
> + * 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.
> + *
> + * Returns:
> + *
> + * The irqflags needed to call drm_panic_unlock().
> + */
> +
> +#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__ */
>
I can't offer much feedback, except that I have tested this in QEMU with SimpleDRM and it works great
�
Thanks!
