Re: [PATCH v2 2/2] drm/vc4: Allocated/liberate the binner BO at firstopen/lastclose

[Paul Kocialkowski <paul.kocialkowski@bootlin.com>]

Hi,

Le mercredi 20 mars 2019 à 09:58 -0700, Eric Anholt a écrit :
> Paul Kocialkowski <paul.kocialkowski@bootlin.com> writes:
> 
> > The binner BO is a pre-requisite to GPU operations, so we must ensure
> > that it is always allocated when the GPU is in use. Currently, we are
> > allocating it at probe time and liberating/allocating it during runtime
> > pm cycles.
> > 
> > First, since the binner buffer is only required for GPU rendering, it's
> > a waste to allocate it when the driver probes since internal users of
> > the driver (such as fbcon) won't try to use the GPU.
> > 
> > Move the allocation/liberation to the firstopen/lastclose instead to
> > only allocate it when userspace has opened the device and adapt the IRQ
> > handler to return early when no binner BO was allocated yet.
> > 
> > Second, because the buffer is allocated from the same pool as other GPU
> > buffers, we might run into a situation where we are out of memory at
> > runtime resume. This causes the binner BO allocation to fail and results
> > in all subsequent operations to fail, resulting in a major hang in
> > userspace.
> > 
> > As a result, keep the buffer alive during runtime pm.
> > 
> > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
> > ---
> > diff --git a/drivers/gpu/drm/vc4/vc4_irq.c b/drivers/gpu/drm/vc4/vc4_irq.c
> > index 4cd2ccfe15f4..efaba2b02f6c 100644
> > --- a/drivers/gpu/drm/vc4/vc4_irq.c
> > +++ b/drivers/gpu/drm/vc4/vc4_irq.c
> > @@ -64,6 +64,9 @@ vc4_overflow_mem_work(struct work_struct *work)
> >  	struct vc4_exec_info *exec;
> >  	unsigned long irqflags;
> >  
> > +	if (!bo)
> > +		return;
> > +
> >  	bin_bo_slot = vc4_v3d_get_bin_slot(vc4);
> >  	if (bin_bo_slot < 0) {
> >  		DRM_ERROR("Couldn't allocate binner overflow mem\n");
> 
> Hmm.  We take the OOM IRQ on poweron, have no bin BO since nobody's
> opened yet, and leave it.  Do we ever get the OOM IRQ again after that?
> Seems like vc4_allocate_bin_bo() might need to kick something so that we
> can fill an OOM request.

I just had a look and it seems that we do get the OOM interrupt again
after the bin BO is allocated. Actually, I can see it kicking from time
to time when using X with glamor.

From what I understood, this looks fairly legitimate. Should we be
worried about this?

Cheers,

Paul

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