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From: "james qian wang (Arm Technology China)" <james.qian.wang@arm.com>
To: Brian Starkey <Brian.Starkey@arm.com>
Cc: "Lowry Li (Arm Technology China)" <Lowry.Li@arm.com>,
	Liviu Dudau <Liviu.Dudau@arm.com>,
	"maarten.lankhorst@linux.intel.com" 
	<maarten.lankhorst@linux.intel.com>,
	"seanpaul@chromium.org" <seanpaul@chromium.org>,
	"airlied@linux.ie" <airlied@linux.ie>,
	"daniel@ffwll.ch" <daniel@ffwll.ch>,
	"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 10:09:26 +0000	[thread overview]
Message-ID: <20190802100919.GA26225@jamwan02-TSP300> (raw)
In-Reply-To: <20190802092920.4la5cwrltv2m6dke@DESKTOP-E1NTVVP.localdomain>

On Fri, Aug 02, 2019 at 05:29:20PM +0800, Brian Starkey 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 :-)
> 
> > 
> > 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".
>

Maybe we can add a wb_fence canceling into complete_signaling() for the
atomic_check failed cleanup like the crtc->out_fence.

Then if in this place we still can got a fence here, that must be a
error we signal and WARN it

Thanks
James

> Cheers,
> -Brian
> 
> > 
> > Best regards,
> > Lowry
> > > > +		dma_fence_put(job->out_fence);
> > > > +
> > > >  	kfree(job);
> > > >  }
> > 
> > -- 
> > Regards
> > Lowry

  parent 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
2019-08-02 14:06               ` Daniel Vetter
2019-08-02 10:09         ` james qian wang (Arm Technology China) [this message]
2019-08-05 13:10   ` Brian Starkey
2019-09-23  7:25   ` [v1,2/2] " james qian wang (Arm Technology China)

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