Message-ID: <20191107111019.GA24028@onstation.org>
Date:   Thu, 7 Nov 2019 06:10:19 -0500
From:   Brian Masney <masneyb@...tation.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     Jeffrey Hugo <jeffrey.l.hugo@...il.com>,
        Rob Clark <robdclark@...omium.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Sean Paul <sean@...rly.run>
Subject: Re: [Freedreno] drm/msm: 'pp done time out' errors after async
 commit changes

On Wed, Nov 06, 2019 at 08:58:59AM -0800, Rob Clark wrote:
> On Wed, Nov 6, 2019 at 8:47 AM Jeffrey Hugo <jeffrey.l.hugo@...il.com> wrote:
> >
> > On Wed, Nov 6, 2019 at 9:30 AM Rob Clark <robdclark@...il.com> wrote:
> > >
> > > On Wed, Nov 6, 2019 at 1:13 AM Brian Masney <masneyb@...tation.org> wrote:
> > > >
> > > > On Tue, Nov 05, 2019 at 08:23:27AM -0800, Rob Clark wrote:
> > > > > On Tue, Nov 5, 2019 at 2:08 AM Brian Masney <masneyb@...tation.org> wrote:
> > > > > > The 'pp done time out' errors go away if I revert the following three
> > > > > > commits:
> > > > > >
> > > > > > cd6d923167b1 ("drm/msm/dpu: async commit support")
> > > > > > d934a712c5e6 ("drm/msm: add atomic traces")
> > > > > > 2d99ced787e3 ("drm/msm: async commit support")
> > > > > >
> > > > > > I reverted the first one to fix a compiler error, and the second one so
> > > > > > that the last patch can be reverted without any merge conflicts.
> > > > > >
> > > > > > I see that crtc_flush() calls mdp5_ctl_commit(). I tried to use
> > > > > > crtc_flush_all() in mdp5_flush_commit() and the contents of the frame
> > > > > > buffer dance around the screen like its out of sync. I renamed
> > > > > > crtc_flush_all() to mdp5_crtc_flush_all() and removed the static
> > > > > > declaration. Here's the relevant part of what I tried:
> > > > > >
> > > > > > --- a/drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c
> > > > > > +++ b/drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c
> > > > > > @@ -171,7 +171,15 @@ static void mdp5_prepare_commit(struct msm_kms *kms, struct drm_atomic_state *st
> > > > > >
> > > > > >  static void mdp5_flush_commit(struct msm_kms *kms, unsigned crtc_mask)
> > > > > >  {
> > > > > > -       /* TODO */
> > > > > > +       struct mdp5_kms *mdp5_kms = to_mdp5_kms(to_mdp_kms(kms));
> > > > > > +       struct drm_crtc *crtc;
> > > > > > +
> > > > > > +       for_each_crtc_mask(mdp5_kms->dev, crtc, crtc_mask) {
> > > > > > +               if (!crtc->state->active)
> > > > > > +                       continue;
> > > > > > +
> > > > > > +               mdp5_crtc_flush_all(crtc);
> > > > > > +       }
> > > > > >  }
> > > > > >
> > > > > > Any tips would be appreciated.
> > > > >
> > > > >
> > > > > I think this is along the lines of what we need to enable async commit
> > > > > for mdp5 (but also removing the flush from the atomic-commit path)..
> > > > > the principle behind the async commit is to do all the atomic state
> > > > > commit normally, but defer writing the flush bits.  This way, if you
> > > > > get another async update before the next vblank, you just apply it
> > > > > immediately instead of waiting for vblank.
> > > > >
> > > > > But I guess you are on a command mode panel, if I remember?  Which is
> > > > > a case I didn't have a way to test.  And I'm not entirely about how
> > > > > kms_funcs->vsync_time() should be implemented for cmd mode panels.
> > > >
> > > > Yes, this is a command-mode panel and there's no hardware frame counter
> > > > available. The key to getting the display working on this phone was this
> > > > patch:
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2bab52af6fe68c43b327a57e5ce5fc10eefdfadf
> > > >
> > > > > That all said, I think we should first fix what is broken, before
> > > > > worrying about extending async commit support to mdp5.. which
> > > > > shouldn't hit the async==true path, due to not implementing
> > > > > kms_funcs->vsync_time().
> > > > >
> > > > > What I think is going on is that, in the cmd mode case,
> > > > > mdp5_wait_flush() (indirectly) calls mdp5_crtc_wait_for_pp_done(),
> > > > > which waits for a pp-done irq regardless of whether there is a flush
> > > > > in progress.  Since there is no flush pending, the irq never comes.
> > > > > But the expectation is that kms_funcs->wait_flush() returns
> > > > > immediately if there is nothing to wait for.
> > > >
> > > > I don't think that's happening in this case. I added some pr_info()
> > > > statements to request_pp_done_pending() and mdp5_crtc_pp_done_irq().
> > > > Here's the first two sets of messages that appear in dmesg:
> > > >
> > > > [   14.018907] msm fd900000.mdss: pp done time out, lm=0
> > > > [   14.018993] request_pp_done_pending: HERE
> > > > [   14.074208] mdp5_crtc_pp_done_irq: HERE
> > > > [   14.074368] Console: switching to colour frame buffer device 135x120
> > > > [   14.138938] msm fd900000.mdss: pp done time out, lm=0
> > > > [   14.139021] request_pp_done_pending: HERE
> > > > [   14.158097] mdp5_crtc_pp_done_irq: HERE
> > > >
> > > > The messages go on like this with the same pattern.
> > > >
> > > > I tried two different changes:
> > > >
> > > > 1) I moved the request_pp_done_pending() and corresponding if statement
> > > >    from mdp5_crtc_atomic_flush() and into mdp5_crtc_atomic_begin().
> > > >
> > > > 2) I increased the timeout in wait_for_completion_timeout() by several
> > > >    increments; all the way to 5 seconds.
> > >
> > > increasing the timeout won't help, because the pp-done irq has already
> > > come at the point where we wait for it..
> > >
> > > maybe the easy thing is just add mdp5_crtc->needs_pp, set to true
> > > before requesting, and false when we get the irq.. and then
> > > mdp5_crtc_wait_for_pp_done() just returns if needs_pp==false..
> >
> > On the otherhand, what about trying to make command mode panels
> > resemble video mode panels slightly?  Video mode panels have a vsync
> > counter in hardware, which is missing from command mode - however it
> > seems like the driver/drm framework would prefer such a counter.
> > Would it be a reasonable idea to make a software counter, and just
> > increment it every time the pp_done irq is triggered?
> >
> > I'm just thinking that we'll avoid issues long term by trying to make
> > the code common, rather than diverging it for the two modes.
> >
> 
> *possibly*, but I think we want to account somehow periods where
> display is not updated.
> 
> fwiw, it isn't that uncommon for userspace to use vblanks to "keep
> time" (drive animations for desktop switch, window
> maximize/unmaximize, etc).. it could be a surprise when "vblank" is
> not periodic.

What do you think about using some variation of the current value of
jiffies in the kernel + the number of pp_done IRQs as the software
counter for command-mode panels?

Brian

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