On 07/25, Daniel Vetter wrote: > On Sat, Jul 25, 2020 at 5:12 AM Sidong Yang <realwakka@xxxxxxxxx> wrote: > > > > On Wed, Jul 22, 2020 at 05:17:05PM +0200, Daniel Vetter wrote: > > > On Wed, Jul 22, 2020 at 4:06 PM Melissa Wen <melissa.srw@xxxxxxxxx> wrote: > > > > > > > > On 07/22, daniel@xxxxxxxx wrote: > > > > > On Wed, Jul 22, 2020 at 08:04:11AM -0300, Melissa Wen wrote: > > > > > > This patch adds a missing drm_crtc_vblank_put op to the pair > > > > > > drm_crtc_vblank_get/put (inc/decrement counter to guarantee vblanks). > > > > > > > > > > > > It clears the execution of the following kms_cursor_crc subtests: > > > > > > 1. pipe-A-cursor-[size,alpha-opaque, NxN-(on-screen, off-screen, sliding, > > > > > > random, fast-moving])] - successful when running individually. > > > > > > 2. pipe-A-cursor-dpms passes again > > > > > > 3. pipe-A-cursor-suspend also passes > > > > > > > > > > > > The issue was initially tracked in the sequential execution of IGT > > > > > > kms_cursor_crc subtest: when running the test sequence or one of its > > > > > > subtests twice, the odd execs complete and the pairs get stuck in an > > > > > > endless wait. In the IGT code, calling a wait_for_vblank before the start > > > > > > of CRC capture prevented the busy-wait. But the problem persisted in the > > > > > > pipe-A-cursor-dpms and -suspend subtests. > > > > > > > > > > > > Checking the history, the pipe-A-cursor-dpms subtest was successful when, > > > > > > in vkms_atomic_commit_tail, instead of using the flip_done op, it used > > > > > > wait_for_vblanks. Another way to prevent blocking was wait_one_vblank when > > > > > > enabling crtc. However, in both cases, pipe-A-cursor-suspend persisted > > > > > > blocking in the 2nd start of CRC capture, which may indicate that > > > > > > something got stuck in the step of CRC setup. Indeed, wait_one_vblank in > > > > > > the crc setup was able to sync things and free all kms_cursor_crc > > > > > > subtests. > > > > > > > > > > > > Tracing and comparing a clean run with a blocked one: > > > > > > - in a clean one, vkms_crtc_atomic_flush enables vblanks; > > > > > > - when blocked, only in next op, vkms_crtc_atomic_enable, the vblanks > > > > > > started. Moreover, a series of vkms_vblank_simulate flow out until > > > > > > disabling vblanks. > > > > > > Also watching the steps of vkms_crtc_atomic_flush, when the very first > > > > > > drm_crtc_vblank_get returned an error, the subtest crashed. On the other > > > > > > hand, when vblank_get succeeded, the subtest completed. Finally, checking > > > > > > the flush steps: it increases counter to hold a vblank reference (get), > > > > > > but there isn't a op to decreased it and release vblanks (put). > > > > > > > > > > > > Cc: Daniel Vetter <daniel@xxxxxxxx> > > > > > > Cc: Rodrigo Siqueira <rodrigosiqueiramelo@xxxxxxxxx> > > > > > > Cc: Haneen Mohammed <hamohammed.sa@xxxxxxxxx> > > > > > > Signed-off-by: Melissa Wen <melissa.srw@xxxxxxxxx> > > > > > > --- > > > > > > drivers/gpu/drm/vkms/vkms_crtc.c | 1 + > > > > > > 1 file changed, 1 insertion(+) > > > > > > > > > > > > diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c > > > > > > index ac85e17428f8..a99d6b4a92dd 100644 > > > > > > --- a/drivers/gpu/drm/vkms/vkms_crtc.c > > > > > > +++ b/drivers/gpu/drm/vkms/vkms_crtc.c > > > > > > @@ -246,6 +246,7 @@ static void vkms_crtc_atomic_flush(struct drm_crtc *crtc, > > > > > > > > > > > > spin_unlock(&crtc->dev->event_lock); > > > > > > > > > > > > + drm_crtc_vblank_put(crtc); > > > > > > > > > > Uh so I reviewed this a bit more carefully now, and I dont think this is > > > > > the correct bugfix. From the kerneldoc of drm_crtc_arm_vblank_event(): > > > > > > > > > > * Caller must hold a vblank reference for the event @e acquired by a > > > > > * drm_crtc_vblank_get(), which will be dropped when the next vblank arrives. > > > > > > > > > > So when we call drm_crtc_arm_vblank_event then the vblank_put gets called > > > > > for us. And that's the only case where we successfully acquired a vblank > > > > > interrupt reference since on failure of drm_crtc_vblank_get (0 indicates > > > > > success for that function, failure negative error number) we directly send > > > > > out the event. > > > > > > > > > > So something else fishy is going on, and now I'm totally confused why this > > > > > even happens. > > > > > > > > > > We also have a pile of WARN_ON checks in drm_crtc_vblank_put to make sure > > > > > we don't underflow the refcount, so it's also not that I think (except if > > > > > this patch creates more WARNING backtraces). > > > > > > > > > > But clearly it changes behaviour somehow ... can you try to figure out > > > > > what changes? Maybe print out the vblank->refcount at various points in > > > > > the driver, and maybe also trace when exactly the fake vkms vblank hrtimer > > > > > is enabled/disabled ... > > > > > > > > :( > > > > > > > > I can check these, but I also have other suspicions. When I place the > > > > drm_crct_vblank_put out of the if (at the end of flush), it not only solve > > > > the issue of blocking on kms_cursor_crc, but also the WARN_ON on kms_flip > > > > doesn't appear anymore (a total cleanup). Just after: > > > > > > > > vkms_output->composer_state = to_vkms_crtc_state(crtc->state); > > > > > > > > looks like there is something stuck around here. > > > > > > Hm do you have the full WARNING for this? Maybe this gives me an idea > > > what's going wrong. > > > > > > > Besides, there is a lock at atomic_begin: > > > > > > > > /* This lock is held across the atomic commit to block vblank timer > > > > * from scheduling vkms_composer_worker until the composer is updated > > > > */ > > > > spin_lock_irq(&vkms_output->lock); > > > > > > > > that seems to be released on atomic_flush and make me suspect something > > > > missing on the composer update. > > > > > > atomic_begin/atomic_flush are symmetric functions an always called > > > around all the plane updates. So having the spin_lock in _begin and > > > the spin_unlock in _flush should be symmetric and correct. > > > > > > If you want to make sure, recompile with CONFIG_PROVE_LOCKING, which > > > should immmediately give you a huge splat in dmesg if there's anything > > > unbalanced with locking. > > > > > > > I'll check all these things and come back with news (hope) :) > > > > > > Have fun chasing stuff :-) > > > > > > Cheers, Daniel > > > > > > > > > > > > > > Thanks, > > > > > > > > Melissa > > > > > > > > > > I'm totally confused about what's going on here now. > > > > > -Daniel > > > > Hi Daniel, Melissa. > > I found something about this problem. > > I traced vblank->refcount that it's important in the problem. > > In normal case, first test run calls commit_tail() and enable vblank in > > atomic_flush(). in drm_vblank_get(), it enable vblank when refcount was zero. > > > > in first test run, it disable crtc for cleanup test. drm_crtc_vblank_off() was > > called by atomic_disable. in this function vblank's refcount was increased for > > prevent subsequent drm_vblank_get() from re-enabling the vblank interrupt. > > and refcount goes one not zero for next test run. > > > > and next test run, drm_vblank_get() was called but it didn't enable vblank > > because refcount was already one. drm_crtc_vblank_on() was called in next. but > > it didn't enable vblank but just increase refcount only. > > > > I think this is why this problem happen. don't know how to fix this correctly. > > should we force to enable vblank after enabling crtc? > > Hm, between drm_crtc_vblank_off and drm_crtc_vblank_on > drm_crtc_vblank_get should fail (and leave the refcount unchanged). > It's convoluted logic, but the check for vblank->enabled should catch > that and return -EINVAL for this case. Does that not happen? > > It would indeed explain the bug (I think, I've been wrong way too many > times with this). > -Daniel > Hi Daniel and Sidong, I don't know if it will be confusing, but I will try to explain in a little more detail (and newbie way) what I saw in this behavior of the refcount (similar to what Sidong evaluated). 1. Starting with the loading of vkms is: In vkms_init: After drm_vblank_init (refcount=0), it calls: vkms_modeset_init --> vkms_output_init ----> drm_mode_config_reset -------> vkms_atomic_crtc_reset (even more inside)--> drm_crtc_vblank_reset that bumps the refcount to prevent vblank_get to enable vblank (refcount=1) 2. So, when we start a subtest, vblank is still disabled and in commit_tail, commit_planes triggers a atomic_begin/flush->vblank_get that return -EINVAL because !vblank->enabled (refcount ends 1) and send_vblank; however the test fails before atomic_enable decrements refcount to 0 and reset timestamp. ** This warning also appears in this very first running: WARNING: CPU: 0 PID: 708 at drivers/gpu/drm/vkms/vkms_crtc.c:91 vkms_get_vblank_timestamp+0x41/0x50 [vkms] In the end, this sequence modeset_disable -> atomic_begin -> atomic_flush: refcount going from 0 to 1 and than drm_vblank_enable prepares to everything going well in the next subtest (because atomic_disable is not called). 3. It could be nice, but in the next subtest (with success), as refcount + vblank_enabled ok, after doind its job, it calls atomic_disable->vblank_off and here refcount ends 1 and vblank disabled (the problem returns). So, we have a kind of good turn and bad turn. I tried different things, but the only relatively stable result was putting the sequence modeset_disable + modeset_enables + commit_planes in the commit_tail. That didn't convince me and then I keep trying things. > > > > Thanks > > -Sidong > > > > > > > > > > > > > crtc->state->event = NULL; > > > > > > } > > > > > > > > > > > > -- > > > > > > 2.27.0 > > > > > > > > > > > > > > > > -- > > > > > Daniel Vetter > > > > > Software Engineer, Intel Corporation > > > > > http://blog.ffwll.ch > > > > > > > > > > > > -- > > > Daniel Vetter > > > Software Engineer, Intel Corporation > > > http://blog.ffwll.ch > > > _______________________________________________ > > > dri-devel mailing list > > > dri-devel@xxxxxxxxxxxxxxxxxxxxx > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel > > > > -- > Daniel Vetter > Software Engineer, Intel Corporation > http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
