Re: [RFC PATCH 0/5] drm/panthor: Protected mode support for Mali CSF GPUs

[Maxime Ripard <mripard@kernel.org>]

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On Mon, Feb 03, 2025 at 04:43:23PM +0000, Florent Tomasin wrote:
> Hi Maxime, Nicolas
> 
> On 30/01/2025 17:47, Nicolas Dufresne wrote:
> > Le jeudi 30 janvier 2025 à 17:38 +0100, Maxime Ripard a écrit :
> >> Hi Nicolas,
> >>
> >> On Thu, Jan 30, 2025 at 10:59:56AM -0500, Nicolas Dufresne wrote:
> >>> Le jeudi 30 janvier 2025 à 14:46 +0100, Maxime Ripard a écrit :
> >>>> Hi,
> >>>>
> >>>> I started to review it, but it's probably best to discuss it here.
> >>>>
> >>>> On Thu, Jan 30, 2025 at 01:08:56PM +0000, Florent Tomasin wrote:
> >>>>> Hi,
> >>>>>
> >>>>> This is a patch series covering the support for protected mode execution in
> >>>>> Mali Panthor CSF kernel driver.
> >>>>>
> >>>>> The Mali CSF GPUs come with the support for protected mode execution at the
> >>>>> HW level. This feature requires two main changes in the kernel driver:
> >>>>>
> >>>>> 1) Configure the GPU with a protected buffer. The system must provide a DMA
> >>>>>    heap from which the driver can allocate a protected buffer.
> >>>>>    It can be a carved-out memory or dynamically allocated protected memory region.
> >>>>>    Some system includes a trusted FW which is in charge of the protected memory.
> >>>>>    Since this problem is integration specific, the Mali Panthor CSF kernel
> >>>>>    driver must import the protected memory from a device specific exporter.
> >>>>
> >>>> Why do you need a heap for it in the first place? My understanding of
> >>>> your series is that you have a carved out memory region somewhere, and
> >>>> you want to allocate from that carved out memory region your buffers.
> >>>>
> >>>> How is that any different from using a reserved-memory region, adding
> >>>> the reserved-memory property to the GPU device and doing all your
> >>>> allocation through the usual dma_alloc_* API?
> >>>
> >>> How do you then multiplex this region so it can be shared between
> >>> GPU/Camera/Display/Codec drivers and also userspace ?
> >>
> >> You could point all the devices to the same reserved memory region, and
> >> they would all allocate from there, including for their userspace-facing
> >> allocations.
> > 
> > I get that using memory region is somewhat more of an HW description, and
> > aligned with what a DT is supposed to describe. One of the challenge is that
> > Mediatek heap proposal endup calling into their TEE, meaning knowing the region
> > is not that useful. You actually need the TEE APP guid and its IPC protocol. If
> > we can dell drivers to use a head instead, we can abstract that SoC specific
> > complexity. I believe each allocated addressed has to be mapped to a zone, and
> > that can only be done in the secure application. I can imagine similar needs
> > when the protection is done using some sort of a VM / hypervisor.
> > 
> > Nicolas
> > 
> 
> The idea in this design is to abstract the heap management from the
> Panthor kernel driver (which consumes a DMA buffer from it).
> 
> In a system, an integrator would have implemented a secure heap driver,
> and could be based on TEE or a carved-out memory with restricted access,
> or else. This heap driver would be responsible of implementing the
> logic to: allocate, free, refcount, etc.
> 
> The heap would be retrieved by the Panthor kernel driver in order to
> allocate protected memory to load the FW and allow the GPU to enter/exit
> protected mode. This memory would not belong to a user space process.
> The driver allocates it at the time of loading the FW and initialization
> of the GPU HW. This is a device globally owned protected memory.

The thing is, it's really not clear why you absolutely need to have the
Panthor driver involved there. It won't be transparent to userspace,
since you'd need an extra flag at allocation time, and the buffers
behave differently. If userspace has to be aware of it, what's the
advantage to your approach compared to just exposing a heap for those
secure buffers, and letting userspace allocate its buffers from there?

> When I came across this patch series:
> -
> https://lore.kernel.org/lkml/20230911023038.30649-1-yong.wu@mediatek.com/#t
> I found it could help abstract the interface between the secure heap and
> the integration of protected memory in Panthor.
> 
> A kernel driver would have to find the heap: `dma_heap_find()`, then
> request allocation of a DMA buffer from it. The heap driver would deal
> with the specifities of the protected memory on the system.

Sure, but we still have to address *why* it would be a good idea for the
driver to do it in the first place. The mediatek series had the same
feedback.

Maxime

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