Message-ID: <3ykaewmjjwkp3y2f3gf5jvqketicd4p2xqyajqtfnsxci36qlm@twidtyj2kgbw>
Date: Thu, 30 Jan 2025 14:46:57 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Florent Tomasin <florent.tomasin@....com>
Cc: Vinod Koul <vkoul@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Boris Brezillon <boris.brezillon@...labora.com>, Steven Price <steven.price@....com>, 
	Liviu Dudau <liviu.dudau@....com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Benjamin Gaignard <benjamin.gaignard@...labora.com>, Brian Starkey <Brian.Starkey@....com>, 
	John Stultz <jstultz@...gle.com>, "T . J . Mercier" <tjmercier@...gle.com>, 
	Christian König <christian.koenig@....com>, Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, Yong Wu <yong.wu@...iatek.com>, dmaengine@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org, 
	linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org, nd@....com, 
	Akash Goel <akash.goel@....com>
Subject: Re: [RFC PATCH 0/5] drm/panthor: Protected mode support for Mali CSF
 GPUs

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?

Or do you expect to have another dma-buf heap that would call into the
firmware to perform the allocations?

The semantics of the CMA heap allocations is a concern too.

Another question is how would you expect something like a secure
video-playback pipeline to operate with that kind of interface. Assuming
you have a secure codec, you would likely get that protected buffer from
the codec, right? So the most likely scenario would be to then import
that dma-buf into the GPU driver, but not allocate the buffer from it.

Overall, I think a better abstraction would be to have a heap indeed to
allocate your protected buffers from, and then import them in the
devices that need them. The responsibility would be on the userspace to
do so, but it already kind of does with your design anyway.

Maxime

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