Re: [PATCH v5 06/21] gpu: host1x: Cleanup and refcounting for syncpoints

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On Tue, Mar 23, 2021 at 12:44:28PM +0200, Mikko Perttunen wrote:
> On 3/23/21 12:36 PM, Thierry Reding wrote:
> > On Mon, Jan 11, 2021 at 03:00:04PM +0200, Mikko Perttunen wrote:
> > > Add reference counting for allocated syncpoints to allow keeping
> > > them allocated while jobs are referencing them. Additionally,
> > > clean up various places using syncpoint IDs to use host1x_syncpt
> > > pointers instead.
> > > 
> > > Signed-off-by: Mikko Perttunen <mperttunen@xxxxxxxxxx>
> > > ---
> > > v5:
> > > - Remove host1x_syncpt_put in submit code, as job_put already
> > >    puts the syncpoint.
> > > - Changes due to rebase in VI driver.
> > > v4:
> > > - Update from _free to _put in VI driver as well
> > > ---
> > >   drivers/gpu/drm/tegra/dc.c             |  4 +-
> > >   drivers/gpu/drm/tegra/drm.c            | 14 ++---
> > >   drivers/gpu/drm/tegra/gr2d.c           |  4 +-
> > >   drivers/gpu/drm/tegra/gr3d.c           |  4 +-
> > >   drivers/gpu/drm/tegra/vic.c            |  4 +-
> > >   drivers/gpu/host1x/cdma.c              | 11 ++--
> > >   drivers/gpu/host1x/dev.h               |  7 ++-
> > >   drivers/gpu/host1x/hw/cdma_hw.c        |  2 +-
> > >   drivers/gpu/host1x/hw/channel_hw.c     | 10 ++--
> > >   drivers/gpu/host1x/hw/debug_hw.c       |  2 +-
> > >   drivers/gpu/host1x/job.c               |  5 +-
> > >   drivers/gpu/host1x/syncpt.c            | 75 +++++++++++++++++++-------
> > >   drivers/gpu/host1x/syncpt.h            |  3 ++
> > >   drivers/staging/media/tegra-video/vi.c |  4 +-
> > >   include/linux/host1x.h                 |  8 +--
> > >   15 files changed, 98 insertions(+), 59 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
> > > index 85dd7131553a..033032dfc4b9 100644
> > > --- a/drivers/gpu/drm/tegra/dc.c
> > > +++ b/drivers/gpu/drm/tegra/dc.c
> > > @@ -2129,7 +2129,7 @@ static int tegra_dc_init(struct host1x_client *client)
> > >   		drm_plane_cleanup(primary);
> > >   	host1x_client_iommu_detach(client);
> > > -	host1x_syncpt_free(dc->syncpt);
> > > +	host1x_syncpt_put(dc->syncpt);
> > >   	return err;
> > >   }
> > > @@ -2154,7 +2154,7 @@ static int tegra_dc_exit(struct host1x_client *client)
> > >   	}
> > >   	host1x_client_iommu_detach(client);
> > > -	host1x_syncpt_free(dc->syncpt);
> > > +	host1x_syncpt_put(dc->syncpt);
> > >   	return 0;
> > >   }
> > > diff --git a/drivers/gpu/drm/tegra/drm.c b/drivers/gpu/drm/tegra/drm.c
> > > index e45c8414e2a3..5a6037eff37f 100644
> > > --- a/drivers/gpu/drm/tegra/drm.c
> > > +++ b/drivers/gpu/drm/tegra/drm.c
> > > @@ -171,7 +171,7 @@ int tegra_drm_submit(struct tegra_drm_context *context,
> > >   	struct drm_tegra_syncpt syncpt;
> > >   	struct host1x *host1x = dev_get_drvdata(drm->dev->parent);
> > >   	struct drm_gem_object **refs;
> > > -	struct host1x_syncpt *sp;
> > > +	struct host1x_syncpt *sp = NULL;
> > >   	struct host1x_job *job;
> > >   	unsigned int num_refs;
> > >   	int err;
> > > @@ -298,8 +298,8 @@ int tegra_drm_submit(struct tegra_drm_context *context,
> > >   		goto fail;
> > >   	}
> > > -	/* check whether syncpoint ID is valid */
> > > -	sp = host1x_syncpt_get(host1x, syncpt.id);
> > > +	/* Syncpoint ref will be dropped on job release. */
> > > +	sp = host1x_syncpt_get_by_id(host1x, syncpt.id);
> > 
> > It's a bit odd to replace the comment like that. Perhaps instead of
> > replacing it, just extend it with the note about the lifetime?
> 
> I replaced it because in the past the check was there really to just check
> if the ID is valid (the pointer was thrown away) -- now we actually pass the
> pointer into the job structure, so it serves a more general "get the
> syncpoint" purpose which is clear based on the name of the function. The new
> comment is then a new comment to clarify the lifetime of the reference.

Alright, makes sense.

> > 
> > >   	if (!sp) {
> > >   		err = -ENOENT;
> > >   		goto fail;
> > > @@ -308,7 +308,7 @@ int tegra_drm_submit(struct tegra_drm_context *context,
> > >   	job->is_addr_reg = context->client->ops->is_addr_reg;
> > >   	job->is_valid_class = context->client->ops->is_valid_class;
> > >   	job->syncpt_incrs = syncpt.incrs;
> > > -	job->syncpt_id = syncpt.id;
> > > +	job->syncpt = sp;
> > >   	job->timeout = 10000;
> > >   	if (args->timeout && args->timeout < 10000)
> > > @@ -380,7 +380,7 @@ static int tegra_syncpt_read(struct drm_device *drm, void *data,
> > >   	struct drm_tegra_syncpt_read *args = data;
> > >   	struct host1x_syncpt *sp;
> > > -	sp = host1x_syncpt_get(host, args->id);
> > > +	sp = host1x_syncpt_get_by_id_noref(host, args->id);
> > 
> > Why don't we need a reference here? It's perhaps unlikely, because this
> > function is short-lived, but the otherwise last reference to this could
> > go away at any point after this line and cause sp to become invalid.
> > 
> > In general it's very rare to not have to keep a reference to a reference
> > counted object.
> 
> Having a reference to a syncpoint indicates ownership of the syncpoint.
> Since here we are just reading it, we don't want ownership. (The non _noref
> functions will fail if the syncpoint is not currently allocated, which would
> break this interface.) The host1x_syncpt structure itself always exists even
> if the refcount drops to zero.

Ah... you're right. host1x_syncpt_put() on the last reference doesn't
actually cause the backing memory to be freed. That's a bit counter-
intuitive, but I don't see why that can't work.

> > >   	if (!sp)
> > >   		return -EINVAL;
> > > @@ -395,7 +395,7 @@ static int tegra_syncpt_incr(struct drm_device *drm, void *data,
> > >   	struct drm_tegra_syncpt_incr *args = data;
> > >   	struct host1x_syncpt *sp;
> > > -	sp = host1x_syncpt_get(host1x, args->id);
> > > +	sp = host1x_syncpt_get_by_id_noref(host1x, args->id);
> > 
> > Same here. Or am I missing some other way by which it is ensured that
> > the reference stays around?
> 
> As above, though here we actually mutate the syncpoint even though we don't
> have a reference and as such ownership. But that's just a quirk of this old
> interface allowing incrementing of syncpoints you don't own.

Yeah, doesn't actually make anything worse.

Thierry

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