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

Thierry Reding
2021-03-23T11:22:14Z

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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@nvidia.com>
> > > ---
> > > 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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