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Date: Fri, 1 Mar 2024 14:03:12 +0100
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Subject: Re: [RFC] drm/panic: Add drm panic locking
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To: Daniel Vetter <daniel.vetter@ffwll.ch>,
 DRI Development <dri-devel@lists.freedesktop.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
 Daniel Vetter <daniel.vetter@intel.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 "Peter Zijlstra (Intel)" <peterz@infradead.org>,
 Lukas Wunner <lukas@wunner.de>, Petr Mladek <pmladek@suse.com>,
 Steven Rostedt <rostedt@goodmis.org>, John Ogness
 <john.ogness@linutronix.de>, Sergey Senozhatsky <senozhatsky@chromium.org>,
 Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
 Maxime Ripard <mripard@kernel.org>, Thomas Zimmermann <tzimmermann@suse.de>,
 David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>
References: <20240301100516.2516297-1-daniel.vetter@ffwll.ch>
 <20240301103903.2538083-1-daniel.vetter@ffwll.ch>
From: Jocelyn Falempe <jfalempe@redhat.com>
In-Reply-To: <20240301103903.2538083-1-daniel.vetter@ffwll.ch>
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Thanks for the patch.

I think it misses to initialize the lock, so we need to add a 
raw_spin_lock_init() in the drm device initialization.

Also I'm wondering if it make sense to put that under the 
CONFIG_DRM_PANIC flag, so that if you don't enable it, panic_lock() and 
panic_unlock() would be no-op.
But that may not work if the driver uses this lock to protect some 
register access.

Best regards,

-- 

Jocelyn

On 01/03/2024 11:39, Daniel Vetter wrote:
> 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@redhat.com/
> 
> 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
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Jocelyn Falempe <jfalempe@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
> Cc: Lukas Wunner <lukas@wunner.de>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: John Ogness <john.ogness@linutronix.de>
> Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> ---
>   drivers/gpu/drm/drm_atomic_helper.c |  3 +
>   include/drm/drm_mode_config.h       | 10 +++
>   include/drm/drm_panic.h             | 99 +++++++++++++++++++++++++++++
>   3 files changed, 112 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 40c2bd3e62e8..5a908c186037 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>
> @@ -3086,6 +3087,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
>   		}
>   	}
>   
> +	drm_panic_lock(state->dev);
>   	for_each_oldnew_plane_in_state(state, plane, old_plane_state, new_plane_state, i) {
>   		WARN_ON(plane->state != old_plane_state);
>   
> @@ -3095,6 +3097,7 @@ 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);
>   
>   	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/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..f2135d03f1eb
> --- /dev/null
> +++ b/include/drm/drm_panic.h
> @@ -0,0 +1,99 @@
> +/* 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
> + *
> + * 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.
> + */
> +static inline int drm_panic_trylock(struct drm_device *dev)
> +{
> +	return raw_spin_trylock(&dev->mode_config.panic_lock);
> +}
> +
> +/**
> + * drm_panic_lock - protect panic printing relevant state
> + * @dev: struct drm_device
> + *
> + * 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. 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.
> + *
> + * Call drm_panic_unlock() to unlock the locked spinlock.
> + */
> +static inline void drm_panic_lock(struct drm_device *dev)
> +{
> +	return raw_spin_lock(&dev->mode_config.panic_lock);
> +}
> +
> +/**
> + * drm_panic_unlock - end of the panic printing critical section
> + * @dev: struct drm_device
> + *
> + * Unlocks the raw spinlock acquired by either drm_panic_lock() or
> + * drm_panic_trylock().
> + */
> +static inline void drm_panic_unlock(struct drm_device *dev)
> +{
> +	raw_spin_unlock(&dev->mode_config.panic_lock);
> +}
> +
> +#endif /* __DRM_PANIC_H__ */