[PATCH 1/3] drm/atomic-helpers: remove legacy_cursor_update hacks

[Daniel Vetter]

On Thu, Oct 22, 2020 at 09:36:23AM -0400, Kazlauskas, Nicholas wrote:
> On 2020-10-21 12:32 p.m., Daniel Vetter wrote:
> > The stuff never really worked, and leads to lots of fun because it
> > out-of-order frees atomic states. Which upsets KASAN, among other
> > things.
> > 
> > For async updates we now have a more solid solution with the
> > ->atomic_async_check and ->atomic_async_commit hooks. Support for that
> > for msm and vc4 landed. nouveau and i915 have their own commit
> > routines, doing something similar.
> > 
> > For everyone else it's probably better to remove the use-after-free
> > bug, and encourage folks to use the async support instead. The
> > affected drivers which register a legacy cursor plane and don't either
> > use the new async stuff or their own commit routine are: amdgpu,
> > atmel, mediatek, qxl, rockchip, sti, sun4i, tegra, virtio, and vmwgfx.
> > 
> > Inspired by an amdgpu bug report.
> > 
> > v2: Drop RFC, I think with amdgpu converted over to use
> > atomic_async_check/commit done in
> > 
> > commit 674e78acae0dfb4beb56132e41cbae5b60f7d662
> > Author: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
> > Date:   Wed Dec 5 14:59:07 2018 -0500
> > 
> >      drm/amd/display: Add fast path for cursor plane updates
> > 
> > we don't have any driver anymore where we have userspace expecting
> > solid legacy cursor support _and_ they are using the atomic helpers in
> > their fully glory. So we can retire this.
> > 
> > References: https://bugzilla.kernel.org/show_bug.cgi?id=199425
> > Cc: mikita.lipski at amd.com
> > Cc: Michel Dänzer <michel at daenzer.net>
> > Cc: harry.wentland at amd.com
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> 
> I'm fine with the idea but it looks like we need modification to amdgpu to
> not break anything:
> 
> if (state->legacy_cursor_update) {
> /* ... */
> 		state->async_update =
> 			!drm_atomic_helper_async_check(dev, state);
> 
> 
> We only check async update for legacy_cursor_updates here which won't cover
> the atomic path. I think it's safe to drop the check here but that should
> probably be done before or as part of this series.

This part is fine, you're essentially duplicating what the helpers are
doing too. I'm not sure whether we should lift this to core atomic
semantics or something else, but should be all ok as-is. Might still be
good to test this in case something isn't 100% complete and amdgpu atomic
commit still relies on legacy_cursor_update semantics somewhere.

But after this patch your atomic code and atomic helpers check/commit
functions match (I think), so we /should/ be good.

Cheers, Daniel

> 
> Regards,
> Nicholas Kazlauskas
> 
> > ---
> >   drivers/gpu/drm/drm_atomic_helper.c | 13 -------------
> >   1 file changed, 13 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index a7bcb4b4586c..549a31e6042c 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -1481,13 +1481,6 @@ drm_atomic_helper_wait_for_vblanks(struct drm_device *dev,
> >   	int i, ret;
> >   	unsigned crtc_mask = 0;
> > -	 /*
> > -	  * Legacy cursor ioctls are completely unsynced, and userspace
> > -	  * relies on that (by doing tons of cursor updates).
> > -	  */
> > -	if (old_state->legacy_cursor_update)
> > -		return;
> > -
> >   	for_each_oldnew_crtc_in_state(old_state, crtc, old_crtc_state, new_crtc_state, i) {
> >   		if (!new_crtc_state->active)
> >   			continue;
> > @@ -2106,12 +2099,6 @@ int drm_atomic_helper_setup_commit(struct drm_atomic_state *state,
> >   			continue;
> >   		}
> > -		/* Legacy cursor updates are fully unsynced. */
> > -		if (state->legacy_cursor_update) {
> > -			complete_all(&commit->flip_done);
> > -			continue;
> > -		}
> > -
> >   		if (!new_crtc_state->event) {
> >   			commit->event = kzalloc(sizeof(*commit->event),
> >   						GFP_KERNEL);
> > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch