On Fri, Mar 01, 2024 at 02:03:12PM +0100, Jocelyn Falempe wrote: > 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. If we get drivers to use this for some of their own locking we have to keep it enabled unconditionally. Also I think locking that's only conditional on Kconfig is just a bit too suprising to be a good idea irrespective of this specific case. -Sima > > 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@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 > > > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> > > 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> > > --- > > 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__ */ > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
