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SubjectRe: [PATCH v4 4/4] drm/vc4: Allocate binner bo when starting to use the V3D
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Hi,

Le mercredi 03 avril 2019 à 11:53 -0700, Eric Anholt a écrit :
> Paul Kocialkowski <paul.kocialkowski@bootlin.com> writes:
>
> > The binner bo is not required until the V3D is in use, so avoid
> > allocating it at probe and do it on the first non-dumb BO allocation.
> > Keep track of which clients are using the V3D and liberate the buffer
> > when there is none left.
> >
> > We also want to keep it alive during runtime suspend/resume to avoid
> > failing to allocate it at resume. This happens when the CMA pool is
> > full at that point and results in a hard crash.
> >
> > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
> > ---
> > drivers/gpu/drm/vc4/vc4_bo.c | 32 ++++++++++++++++++++++++++++++++
> > drivers/gpu/drm/vc4/vc4_drv.c | 9 +++++++++
> > drivers/gpu/drm/vc4/vc4_drv.h | 4 ++++
> > drivers/gpu/drm/vc4/vc4_v3d.c | 13 -------------
> > 4 files changed, 45 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/vc4/vc4_bo.c b/drivers/gpu/drm/vc4/vc4_bo.c
> > index 88ebd681d7eb..b941f09b9378 100644
> > --- a/drivers/gpu/drm/vc4/vc4_bo.c
> > +++ b/drivers/gpu/drm/vc4/vc4_bo.c
> > @@ -799,6 +799,30 @@ vc4_prime_import_sg_table(struct drm_device *dev,
> > return obj;
> > }
> >
> > +static int vc4_prepare_bin_bo(struct drm_device *dev,
> > + struct drm_file *file_priv)
> > +{
> > + struct vc4_file *vc4file = file_priv->driver_priv;
> > + struct vc4_dev *vc4 = to_vc4_dev(dev);
> > + int ret;
> > +
> > + if (!vc4->v3d)
> > + return -ENODEV;
> > +
> > + if (!vc4file->needs_bin_bo) {
> > + atomic_inc(&vc4->bin_bo_usecnt);
> > + vc4file->needs_bin_bo = true;
> > + }
> > +
> > + if (!vc4->bin_bo) {
> > + ret = vc4_v3d_allocate_bin_bo(vc4);
> > + if (ret)
> > + return ret;
> > + }
> > +
>
> This atomic usage looks really racy. For example, multiple clients
> could call allocate at the same time and leak one. Or this timeline:
>
> us them
> dec count to 0
> inc count
> check bin_bo
> free bin_bo

Oh, you're definitely right. Sorry I missed that.

> vc4_v3d_allocate_bin_bo should probably be a vc4_v3d_bin_bo_get()
> returning a kref on the BO, called under a lock protecting both one
> file_priv being dereferenced by multiple threads in the kernel at the
> same time (so file_priv doesn't try to double-get its ref) and multiple
> file_privs trying to get the bin_bo at once.

Sounds good, I'll look into it and spin up a new revision soon.

Cheers,

Paul

--
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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