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Date:   Sun, 10 Mar 2019 17:35:05 -0300
From:   Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
To:     Ville =?utf-8?B?U3lyasOkbMOk?= <ville.syrjala@linux.intel.com>
Cc:     Shayenne Moura <shayenneluzmoura@gmail.com>,
        Haneen Mohammed <hamohammed.sa@gmail.com>,
        David Airlie <airlied@linux.ie>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] drm/vkms: Solve bug on kms_crc_cursor tests
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On 03/01, Ville Syrjälä wrote:
> On Fri, Mar 01, 2019 at 03:35:35PM -0300, Shayenne Moura wrote:
> > Em sex, 1 de mar de 2019 às 12:26, Ville Syrjälä
> > <ville.syrjala@linux.intel.com> escreveu:
> > >
> > > On Fri, Mar 01, 2019 at 11:55:11AM -0300, Shayenne Moura wrote:
> > > > Em qui, 28 de fev de 2019 às 11:03, Ville Syrjälä
> > > > <ville.syrjala@linux.intel.com> escreveu:
> > > > >
> > > > > On Thu, Feb 28, 2019 at 11:11:07AM +0100, Daniel Vetter wrote:
> > > > > > On Mon, Feb 25, 2019 at 11:26:06AM -0300, Shayenne Moura wrote:
> > > > > > > vkms_crc_work_handle needs the value of the actual frame to
> > > > > > > schedule the workqueue that calls periodically the vblank
> > > > > > > handler and the destroy state functions. However, the frame
> > > > > > > value returned from vkms_vblank_simulate is updated and
> > > > > > > diminished in vblank_get_timestamp because it is not in a
> > > > > > > vblank interrupt, and return an inaccurate value.
> > > > > > >
> > > > > > > Solve this getting the actual vblank frame directly from the
> > > > > > > vblank->count inside the `struct drm_crtc`, instead of using
> > > > > > > the `drm_accurate_vblank_count` function.
> > > > > > >
> > > > > > > Signed-off-by: Shayenne Moura <shayenneluzmoura@gmail.com>
> > > > > >
> > > > > > Sorry for the delay, I'm a bit swamped right now :-/
> > > > > >
> > > > > > Debug work you're doing here is really impressive! But I have no idea
> > > > > > what's going on. It doesn't look like it's just papering over a bug (like
> > > > > > the in_vblank_irq check we've discussed on irc), but I also have no idea
> > > > > > why it works.
> > > > > >
> > > > > > I'll pull in Ville, he understands this better than me.
> > > > >
> > > > > It's not entirely clear what we're trying to fix. From what I can see
> > > > > the crc work seems to be in no way synchronized with page flips, so
> > > > > I'm not sure how all this is really supposed to work.
> > > > >
> > > >
> > > > Hi, Ville!
> > > >
> > > > Thank you for the review! :)
> > > >
> > > > I do not understand well what crc code is doing, but the issue that I found
> > > > is related to the vblank timestamp and frame count.
> > > >
> > > > When vkms handles the crc_cursor it uses the start frame and end frame
> > > > values to verify if it needs to call the function 'drm_crtc_add_crc_entry()'
> > > > for each frame.
> > > >
> > > > However, when getting the frame count, the code is calling the function
> > > > drm_update_vblank_count(dev, pipe, false) and, because of the 'false',
> > > > subtracting the actual vblank timestamp (consequently, the frame count
> > > > value), causing conflicts.
> > >
> > > The in_vblank_irq behavour looks sane to me. What are these conflicts?
> > >
> > 
> > The entire history was:
> >  - I sent the patch with bugfix for vblank extra frame. The patch changed
> >    our function vkms_get_vblank_timestamp() to look like this:
> > 
> > bool vkms_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe,
> >                    int *max_error, ktime_t *vblank_time,
> >                    bool in_vblank_irq)
> > {
> >     struct vkms_device *vkmsdev = drm_device_to_vkms_device(dev);
> >     struct vkms_output *output = &vkmsdev->output;
> > 
> >     *vblank_time = output->vblank_hrtimer.node.expires;
> > 
> > +   if (!in_vblank_irq)
> > +        *vblank_time -= output->period_ns;
> > 
> >     return true;
> > }
> > 
> >  - This patch solve the issue that I was looking for (extra vblank
> > frames on kms_flip).
> > 
> >  - However, kms_cursor_crc tests, which passed before my patch, started to fail.
> > 
> >  - Debugging them, I found that the problem was inside of function
> > `vkms_vblank_simulate()`
> > when it was handling the crc_enabled (inside  if (state && output->crc_enabled))
> > and inside the function vkms_crc_work_handle() too.
> > 
> >  - Following the steps:
> > 1. Inside vkms_vblank_simulate() we call drm_crtc_accurate_vblank_count()
> > 2. In its turn, drm_crtc_accurate_vblank_count() calls the function
> >    drm_update_vblank_count(dev, pipe, false).  /* This false is default */
> > 3. Finally, the “false” used in drm_update_vblank_count(), will be
> >   passed to vkms_get_vblank_timestamp() and the condition “if
> >   (!in_vblank_irq)” will be executed multiple times (we don’t want it).
> > 
> >  - Inside vkms_crc, the issue is that the returned frame value change for
> > every call of drm_crtc_accurate_vblank_count() because
> > in_vblank_irq == false.

Hi Ville,
 
> OK. So why is it changing? AFAICS it should not change unless the
> timer was moved forward in between the calls.

Yes Ville, you’re right. We have to update it only when the function
vkms_vblank_simulate() is invoked (timer move forward), and FWIU we do
it when we call drm_crtc_handle_vblank(). However, the current
implementation of vkms, has a call to drm_crtc_accurate_vblank_count()
inside the vkms_vblank_simulate() which is a problem because it also
update the vblank value. FWIU, this patch fixes this issue by taking the
count value in the data struct instead of call
drm_crtc_accurate_vblank_count() which will avoid the extra update.

Thanks,
Best Regards
 
> > 
> >  - To solve this, I used the value already calculated on vblank->count,
> > instead of using the helper function that updates the value.
> > 
> > Shayenne
> > 
> > > >
> > > > Does it make sense? I am not sure about this crc code behavior.
> > > >
> > > > Shayenne
> > > >
> > > > > > -Daniel
> > > > > >
> > > > > > > ---
> > > > > > >  drivers/gpu/drm/vkms/vkms_crc.c  | 4 +++-
> > > > > > >  drivers/gpu/drm/vkms/vkms_crtc.c | 4 +++-
> > > > > > >  2 files changed, 6 insertions(+), 2 deletions(-)
> > > > > > >
> > > > > > > diff --git a/drivers/gpu/drm/vkms/vkms_crc.c b/drivers/gpu/drm/vkms/vkms_crc.c
> > > > > > > index d7b409a3c0f8..09a8b00ef1f1 100644
> > > > > > > --- a/drivers/gpu/drm/vkms/vkms_crc.c
> > > > > > > +++ b/drivers/gpu/drm/vkms/vkms_crc.c
> > > > > > > @@ -161,6 +161,8 @@ void vkms_crc_work_handle(struct work_struct *work)
> > > > > > >     struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
> > > > > > >     struct vkms_device *vdev = container_of(out, struct vkms_device,
> > > > > > >                                             output);
> > > > > > > +   unsigned int pipe = drm_crtc_index(crtc);
> > > > > > > +   struct drm_vblank_crtc *vblank = &crtc->dev->vblank[pipe];
> > > > > > >     struct vkms_crc_data *primary_crc = NULL;
> > > > > > >     struct vkms_crc_data *cursor_crc = NULL;
> > > > > > >     struct drm_plane *plane;
> > > > > > > @@ -196,7 +198,7 @@ void vkms_crc_work_handle(struct work_struct *work)
> > > > > > >     if (primary_crc)
> > > > > > >             crc32 = _vkms_get_crc(primary_crc, cursor_crc);
> > > > > > >
> > > > > > > -   frame_end = drm_crtc_accurate_vblank_count(crtc);
> > > > > > > +   frame_end = vblank->count;
> > > > > > >
> > > > > > >     /* queue_work can fail to schedule crc_work; add crc for
> > > > > > >      * missing frames
> > > > > > > diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> > > > > > > index 8a9aeb0a9ea8..9bf3268e2e92 100644
> > > > > > > --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> > > > > > > +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> > > > > > > @@ -10,6 +10,8 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
> > > > > > >                                               vblank_hrtimer);
> > > > > > >     struct drm_crtc *crtc = &output->crtc;
> > > > > > >     struct vkms_crtc_state *state = to_vkms_crtc_state(crtc->state);
> > > > > > > +   unsigned int pipe = drm_crtc_index(crtc);
> > > > > > > +   struct drm_vblank_crtc *vblank = &crtc->dev->vblank[pipe];
> > > > > > >     u64 ret_overrun;
> > > > > > >     bool ret;
> > > > > > >
> > > > > > > @@ -20,7 +22,7 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
> > > > > > >             DRM_ERROR("vkms failure on handling vblank");
> > > > > > >
> > > > > > >     if (state && output->crc_enabled) {
> > > > > > > -           u64 frame = drm_crtc_accurate_vblank_count(crtc);
> > > > > > > +           u64 frame = vblank->count;
> > > > > > >
> > > > > > >             /* update frame_start only if a queued vkms_crc_work_handle()
> > > > > > >              * has read the data
> > > > > > > --
> > > > > > > 2.17.1
> > > > > > >
> > > > > >
> > > > > > --
> > > > > > Daniel Vetter
> > > > > > Software Engineer, Intel Corporation
> > > > > > http://blog.ffwll.ch
> > > > > > _______________________________________________
> > > > > > dri-devel mailing list
> > > > > > dri-devel@lists.freedesktop.org
> > > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > > > >
> > > > > --
> > > > > Ville Syrjälä
> > > > > Intel
> > >
> > > --
> > > Ville Syrjälä
> > > Intel
> 
> -- 
> Ville Syrjälä
> Intel

-- 
Rodrigo Siqueira
https://siqueira.tech
Graduate Student
Department of Computer Science
University of São Paulo