On Tue, Apr 09, 2024 at 06:30:40PM +0200, 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 Please annotate that v10 and later is your work, credit where credit is due and all that :-) > 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 &drm_mode_config.plane_list so that the hyperlinks work. Maybe double-check the generated html for everything at the end of the series to make sure all the hyperlinks do work and there's no typos. > + * 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(). I think the Returns section should be deleted here, that was for v10? Otherwise Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx> on your changes here. -Sima > + */ > + > +#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__ */ > -- > 2.44.0 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
