Re: [PATCH v5 2/2] accel: Add Arm Ethos-U NPU driver

Matthew Brost
2025-10-18T07:42:42Z

On Fri, Oct 17, 2025 at 11:43:51PM -0700, Matthew Brost wrote:
> On Fri, Oct 17, 2025 at 10:37:46AM -0500, Rob Herring wrote:
> > On Thu, Oct 16, 2025 at 11:25:34PM -0700, Matthew Brost wrote:
> > > On Thu, Oct 16, 2025 at 04:06:05PM -0500, Rob Herring (Arm) wrote:
> > > > Add a driver for Arm Ethos-U65/U85 NPUs. The Ethos-U NPU has a
> > > > relatively simple interface with single command stream to describe
> > > > buffers, operation settings, and network operations. It supports up to 8
> > > > memory regions (though no h/w bounds on a region). The Ethos NPUs
> > > > are designed to use an SRAM for scratch memory. Region 2 is reserved
> > > > for SRAM (like the downstream driver stack and compiler). Userspace
> > > > doesn't need access to the SRAM.
> >
> > Thanks for the review.
> >
> > [...]
> >
> > > > +static struct dma_fence *ethosu_job_run(struct drm_sched_job *sched_job)
> > > > +{
> > > > + struct ethosu_job *job = to_ethosu_job(sched_job);
> > > > + struct ethosu_device *dev = job->dev;
> > > > + struct dma_fence *fence = NULL;
> > > > + int ret;
> > > > +
> > > > + if (unlikely(job->base.s_fence->finished.error))
> > > > + return NULL;
> > > > +
> > > > + fence = ethosu_fence_create(dev);
> > >
> > > Another reclaim issue: ethosu_fence_create allocates memory using
> > > GFP_KERNEL. Since we're already in the DMA fence signaling path
> > > (reclaim), this can lead to a deadlock.
> > >
> > > Without too much thought, you likely want to move this allocation to
> > > ethosu_job_do_push, but before taking dev->sched_lock or calling
> > > drm_sched_job_arm.
> > >
> > > We really should fix the DRM scheduler work queue to be tainted with
> > > reclaim. If I recall correctly, we'd need to update the work queue
> > > layer. Let me look into that—I've seen this type of bug several times,
> > > and lockdep should be able to catch it.
> >
> > Likely the rocket driver suffers from the same issues...
> >
>
> I am not surprised by this statement.
>
> > >
> > > > + if (IS_ERR(fence))
> > > > + return fence;
> > > > +
> > > > + if (job->done_fence)
> > > > + dma_fence_put(job->done_fence);
> > > > + job->done_fence = dma_fence_get(fence);
> > > > +
> > > > + ret = pm_runtime_get_sync(dev->base.dev);
> > >
> > > I haven't looked at your PM design, but this generally looks quite
> > > dangerous with respect to reclaim. For example, if your PM resume paths
> > > allocate memory or take locks that allocate memory underneath, you're
> > > likely to run into issues.
> > >
> > > A better approach would be to attach a PM reference to your job upon
> > > creation and release it upon job destruction. That would be safer and
> > > save you headaches in the long run.
> >
> > Our PM is nothing more than clock enable/disable and register init.
> >
> > If the runtime PM API doesn't work and needs special driver wrappers,
> > then I'm inclined to just not use it and manage clocks directly (as
> > that's all it is doing).
> >
>
> Yes, then you’re probably fine. More complex drivers can do all sorts of
> things during a PM wake, which is why PM wakes should generally be the
> outermost layer. I still suggest, to future-proof your code, that you
> move the PM reference to an outer layer.
>

Also, taking a PM reference in a function call — as opposed to tying it
to a object's lifetime — is risky. It can quickly lead to imbalances in
PM references if things go sideways or function calls become unbalanced.
Depending on how your driver uses the DRM scheduler, this seems like a
real possibility.

Matt

> > >
> > > This is what we do in Xe [1] [2].
> > >
> > > Also, in general, this driver has been reviewed (RB’d), but it's not
> > > great that I spotted numerous issues within just five minutes. I suggest
> > > taking a step back and thoroughly evaluating everything this driver is
> > > doing.
> >
> > Well, if it is hard to get simple drivers right, then it's a problem
> > with the subsystem APIs IMO.
> >
>
> Yes, agreed. We should have assertions and lockdep annotations in place
> to catch driver-side misuses. This is the second driver I’ve randomly
> looked at over the past year that has broken DMA fencing and reclaim
> rules. I’ll take an action item to fix this in the DRM scheduler, but
> I’m afraid I’ll likely break multiple drivers in the process as misuess
> / lockdep will complain.
>
> Matt
>
> > Rob