[PATCH 4/8] drm/panthor: Add support for protected memory allocation in panthor

[Chia-I Wu]

On Mon, May 18, 2026 at 12:16 AM Boris Brezillon
<boris.brezillon at collabora.com> wrote:
>
> On Wed, 13 May 2026 12:31:32 -0700
> Chia-I Wu <olvaffe at gmail.com> wrote:
>
> > On Tue, May 12, 2026 at 8:39 AM Liviu Dudau <liviu.dudau at arm.com> wrote:
> > >
> > > On Tue, May 12, 2026 at 04:11:11PM +0200, Boris Brezillon wrote:
> > > > On Tue, 12 May 2026 14:47:27 +0100
> > > > Liviu Dudau <liviu.dudau at arm.com> wrote:
> > > >
> > > > > On Thu, May 07, 2026 at 01:53:56PM +0200, Boris Brezillon wrote:
> > > > > > On Thu, 7 May 2026 11:02:26 +0200
> > > > > > Marcin Ślusarz <marcin.slusarz at arm.com> wrote:
> > > > > >
> > > > > > > On Tue, May 05, 2026 at 06:15:23PM +0200, Boris Brezillon wrote:
> > > > > > > > > @@ -277,9 +286,21 @@ int panthor_device_init(struct panthor_device *ptdev)
> > > > > > > > >                     return ret;
> > > > > > > > >     }
> > > > > > > > >
> > > > > > > > > +   /* If a protected heap name is specified but not found, defer the probe until created */
> > > > > > > > > +   if (protected_heap_name && strlen(protected_heap_name)) {
> > > > > > > >
> > > > > > > > Do we really need this strlen() > 0? Won't dma_heap_find() fail is the
> > > > > > > > name is "" already?
> > > > > > >
> > > > > > > If dma_heap_find() will fail, then the whole probe with fail too.
> > > > > > > This check prevents that.
> > > > > >
> > > > > > Yeah, that's also a questionable design choice. I mean, we can
> > > > > > currently probe and boot the FW even though we never setup the
> > > > > > protected FW sections, so why should we defer the probe here? Can't we
> > > > > > just retry the next time a group with the protected bit is created and
> > > > > > fail if we can find a protected heap?
> > > > >
> > > > > The problem we have with the current firmware is that it does a number of setup steps at "boot"
> > > > > time only. One of the steps is preparing its internal structures for when it enters protected
> > > > > mode and it stores them in the buffer passed in at firmware loading. We cannot later run the
> > > > > process when we have a group with protected mode set.
> > > >
> > > > No, but we can force a full/slow reset and have that thing
> > > > re-initialized, can't we? I mean, that's basically what we do when a
> > > > fast reset fails: we re-initialize all the sections and reset again, at
> > > > which point the FW should start from a fresh state, and be able to
> > > > properly initialize the protected-related stuff if protected sections
> > > > are populated. Am I missing something?
> > >
> > > Right, we can do that. For some reason I keep associating the reset with the
> > > error handling and not with "normal" operations.
> > I kind of hope we end up with either
> >
> >  - panthor knows the exact heap to use and fails with EPROBE_DEFER if
> > the heap is missing, or
> >  - panthor gets a dma-buf from userspace and does the full reset
> >    - userspace also needs to provide a dma-buf for each protected
> > group for the suspend buffer
> >
> > than something in-between. The latter is more ad-hoc and basically
> > kicks the issue to the userspace.
>
> Indeed, the second option is more ad-hoc, but when you think about it,
> userspace has to have this knowledge, because it needs to know the
> dma-heap to use for buffer allocation that cross a device boundary
> anyway. Think about frames produced by a video decoder, and composited
> by the GPU into a protected scanout buffer that's passed to the KMS
> device. Why would the GPU driver be source of truth when it comes to
> choosing the heap to use to allocate protected buffers for the video
> decoder or those used for the display?
I don't think the GPU driver is ever the source of truth. If the
system integrator wants to specify the source of truth (SoT) from
kernel space, they should use the device tree (or module params /
config options). If they want to specify the SoT in userspace, then we
don't really care how it is done other than providing an ioctl.
Panthor is always on the receiving end.

If we don't want to delay this functionality, but it takes time to
converge on SoT, maybe a solution that is not a long-term promise can
work? Of the options on the table (dt, module params, kconfig options,
ioctls), a kconfig option, potentially marked as experimental, seems
like a good candidate.

>
> >
> > For the former, expressing the relation in DT seems to be the best,
> > but only if possible :-). Otherwise, a kconfig option (instead of
> > module param) should be easier to work with.
> >
> > Looking at the userspace implementation, can we also have an panthor
> > ioctl to return the heap to userspace?
>
> Yes, it's something we can add, but again, I'm questioning the
> usefulness of this: how can we ensure the heap used by panthor to
> allocate its protected FW buffers is suitable for scanout buffers
> (buffers that can be used by display drivers). There needs to be a glue
> leaving in usersland and taking the decision, and I'm not too sure
> trusting any of the component in the chain (vdec, gpu, display) is the
> right thing to do.
The heap returned by panthor is only for panfrost/panvk. It says
nothing about compatibility with other components on the system.