[RFC PATCH 2/2] vc4: introduce DMA-BUF heap

[Maxime Ripard]

Hi Maira,

On Thu, Nov 09, 2023 at 09:14:29AM -0300, Maira Canal wrote:
> On 11/9/23 04:45, Simon Ser wrote:
> > User-space sometimes needs to allocate scanout-capable memory for
> > GPU rendering purposes. On a vc4/v3d split render/display SoC, this
> > is achieved via DRM dumb buffers: the v3d user-space driver opens
> > the primary vc4 node, allocates a DRM dumb buffer there, exports it
> > as a DMA-BUF, imports it into the v3d render node, and renders to it.
> > 
> > However, DRM dumb buffers are only meant for CPU rendering, they are
> > not intended to be used for GPU rendering. Primary nodes should only
> > be used for mode-setting purposes, other programs should not attempt
> > to open it. Moreover, opening the primary node is already broken on
> > some setups: systemd grants permission to open primary nodes to
> > physically logged in users, but this breaks when the user is not
> > physically logged in (e.g. headless setup) and when the distribution
> > is using a different init (e.g. Alpine Linux uses openrc).
> > 
> > We need an alternate way for v3d to allocate scanout-capable memory.
> 
> For me, it is a bit unclear how we import the vc4 DMA-BUF heap into v3d.
> Should we create an IOCTL for it on v3d?

dma-heaps are separate device files (under /dev/dma_heap) that have
their specific uapi to allocate buffers:
https://elixir.bootlin.com/linux/latest/source/drivers/dma-buf/dma-heap.c#L126

You then get a dma-buf handle you can import into whatever device you
want.

Maxime
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