linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Brian Starkey <Brian.Starkey@arm.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: "Lowry Li (Arm Technology China)" <Lowry.Li@arm.com>,
	Liviu Dudau <Liviu.Dudau@arm.com>,
	"james qian wang (Arm Technology China)"
	<james.qian.wang@arm.com>,
	"maarten.lankhorst@linux.intel.com" 
	<maarten.lankhorst@linux.intel.com>,
	"seanpaul@chromium.org" <seanpaul@chromium.org>,
	"airlied@linux.ie" <airlied@linux.ie>,
	"Julien Yin (Arm Technology China)" <Julien.Yin@arm.com>,
	"maxime.ripard@bootlin.com" <maxime.ripard@bootlin.com>,
	"eric@anholt.net" <eric@anholt.net>,
	"kieran.bingham+renesas@ideasonboard.com" 
	<kieran.bingham+renesas@ideasonboard.com>,
	"sean@poorly.run" <sean@poorly.run>,
	"laurent.pinchart@ideasonboard.com" 
	<laurent.pinchart@ideasonboard.com>,
	"Jonathan Chai (Arm Technology China)" <Jonathan.Chai@arm.com>,
	Ayan Halder <Ayan.Halder@arm.com>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-renesas-soc@vger.kernel.org" 
	<linux-renesas-soc@vger.kernel.org>, nd <nd@arm.com>
Subject: Re: [PATCH v1 2/2] drm: Clear the fence pointer when writeback job signaled
Date: Fri, 2 Aug 2019 10:09:05 +0000	[thread overview]
Message-ID: <20190802100904.blnusnieti3pxgxu@DESKTOP-E1NTVVP.localdomain> (raw)
In-Reply-To: <CAKMK7uH38rxyTyuYRGJ6NBejyUxQ6Qvr1CdjH2kpXiq+3-=t8Q@mail.gmail.com>

Hi Daniel,

On Fri, Aug 02, 2019 at 11:45:13AM +0200, Daniel Vetter wrote:
> On Fri, Aug 2, 2019 at 11:43 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Fri, Aug 2, 2019 at 11:29 AM Brian Starkey <Brian.Starkey@arm.com> wrote:
> > >
> > > Hi Lowry,
> > >
> > > On Thu, Aug 01, 2019 at 06:34:08AM +0000, Lowry Li (Arm Technology China) wrote:
> > > > Hi Brian,
> > > >
> > > > On Wed, Jul 31, 2019 at 09:20:04PM +0800, Brian Starkey wrote:
> > > > > Hi Lowry,
> > > > >
> > > > > Thanks for this cleanup.
> > > > >
> > > > > On Wed, Jul 31, 2019 at 11:04:45AM +0000, Lowry Li (Arm Technology China) wrote:
> > > > > > During it signals the completion of a writeback job, after releasing
> > > > > > the out_fence, we'd clear the pointer.
> > > > > >
> > > > > > Check if fence left over in drm_writeback_cleanup_job(), release it.
> > > > > >
> > > > > > Signed-off-by: Lowry Li (Arm Technology China) <lowry.li@arm.com>
> > > > > > ---
> > > > > >  drivers/gpu/drm/drm_writeback.c | 23 +++++++++++++++--------
> > > > > >  1 file changed, 15 insertions(+), 8 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
> > > > > > index ff138b6..43d9e3b 100644
> > > > > > --- a/drivers/gpu/drm/drm_writeback.c
> > > > > > +++ b/drivers/gpu/drm/drm_writeback.c
> > > > > > @@ -324,6 +324,9 @@ void drm_writeback_cleanup_job(struct drm_writeback_job *job)
> > > > > >   if (job->fb)
> > > > > >           drm_framebuffer_put(job->fb);
> > > > > >
> > > > > > + if (job->out_fence)
> > > > >
> > > > > I'm thinking it might be a good idea to signal the fence with an error
> > > > > here, if it's not already signaled. Otherwise, if there's someone
> > > > > waiting (which there shouldn't be), they're going to be waiting a very
> > > > > long time :-)
> > > > >
> > > > > Thanks,
> > > > > -Brian
> > > > >
> > > > Here it happened at atomic_check failed and test only commit. For both
> > > > cases, the commit has been dropped and it's only a clean up. So here better
> > > > not be treated as an error case:)
> > >
> > > If anyone else has a reference on the fence, then IMO it absolutely is
> > > an error to reach this point without the fence being signaled -
> > > because it means that the fence will never be signaled.
> > >
> > > I don't think the API gives you a way to check if this is the last
> > > reference, so it's safest to just make sure the fence is signalled
> > > before dropping the reference.
> > >
> > > It just feels wrong to me to have the possibility of a dangling fence
> > > which is never going to get signalled; and it's an easy defensive step
> > > to make sure it can never happen.
> > >
> > > I know it _shouldn't_ happen, but we often put in handling for cases
> > > which shouldn't happen, because they frequently do happen :-)
> >
> > We're not as paranoid with the vblank fences either, so not sure why
> > we need to be this paranoid with writeback fences. If your driver
> > grabs anything from the atomic state in ->atomic_check it's buggy
> > anyway.
> >
> > If you want to fix this properly I think we need to move the call to
> > prepare_signalling() in between atomic_check and atomic_commit. Then I
> > think it makes sense to also force-complete the fence on error ...

Well, fair enough. I'm struggling with "that's too paranoid" vs "fix
it properly" though? Is it a "problem" worth fixing or not?

It seems natural to me to do the fence cleanup in the cleanup function
for the object which owns the fence.

> >
> > > > Since for userspace, it should have been failed or a test only case, so
> > > > writebace fence should not be signaled.
> > >
> > > It's not only userspace that can wait on fences (and in fact this
> > > fence will never even reach userspace if the commit fails), the driver
> > > may have taken a copy to use for "something".
> 
> I forgot to add: you can check this by looking at the fence reference
> count. A WARN_ON if that's more than 1 on cleanup (but also for the
> out fences) could be a nice addition.

Do we really want to be looking at the fence internals directly like
that?

Cheers,
-Brian

> -Daniel
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

  reply	other threads:[~2019-08-02 10:09 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-31 11:04 [PATCH v1 0/2] Free the writeback_job when it with an empty fb Lowry Li (Arm Technology China)
2019-07-31 11:04 ` [PATCH v1 1/2] drm: " Lowry Li (Arm Technology China)
2019-07-31 13:03   ` Liviu Dudau
2019-09-23  7:24   ` [v1,1/2] " james qian wang (Arm Technology China)
2019-07-31 11:04 ` [PATCH v1 2/2] drm: Clear the fence pointer when writeback job signaled Lowry Li (Arm Technology China)
2019-07-31 13:15   ` Liviu Dudau
     [not found]     ` <20190801063055.GA17887@lowry.li@arm.com>
2019-08-01  9:58       ` Liviu Dudau
2019-07-31 13:20   ` Brian Starkey
     [not found]     ` <20190801063351.GB17887@lowry.li@arm.com>
2019-08-02  9:29       ` Brian Starkey
2019-08-02  9:43         ` Daniel Vetter
2019-08-02  9:45           ` Daniel Vetter
2019-08-02 10:09             ` Brian Starkey [this message]
2019-08-02 14:06               ` Daniel Vetter
2019-08-02 10:09         ` james qian wang (Arm Technology China)
2019-08-05 13:10   ` Brian Starkey
2019-09-23  7:25   ` [v1,2/2] " james qian wang (Arm Technology China)

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190802100904.blnusnieti3pxgxu@DESKTOP-E1NTVVP.localdomain \
    --to=brian.starkey@arm.com \
    --cc=Ayan.Halder@arm.com \
    --cc=Jonathan.Chai@arm.com \
    --cc=Julien.Yin@arm.com \
    --cc=Liviu.Dudau@arm.com \
    --cc=Lowry.Li@arm.com \
    --cc=airlied@linux.ie \
    --cc=daniel@ffwll.ch \
    --cc=dri-devel@lists.freedesktop.org \
    --cc=eric@anholt.net \
    --cc=james.qian.wang@arm.com \
    --cc=kieran.bingham+renesas@ideasonboard.com \
    --cc=laurent.pinchart@ideasonboard.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-renesas-soc@vger.kernel.org \
    --cc=maarten.lankhorst@linux.intel.com \
    --cc=maxime.ripard@bootlin.com \
    --cc=nd@arm.com \
    --cc=sean@poorly.run \
    --cc=seanpaul@chromium.org \
    --subject='Re: [PATCH v1 2/2] drm: Clear the fence pointer when writeback job signaled' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).