[PATCH] drm: enable render nodes wherever buffer sharing is supported

[Liviu Dudau]

On Wed, Apr 29, 2020 at 09:51:19AM -0700, Peter Collingbourne wrote:
> On Wed, Apr 29, 2020 at 9:17 AM Brian Starkey <brian.starkey at arm.com> wrote:
> >
> > Hi Peter,
> >
> > On Mon, Apr 27, 2020 at 01:05:13PM -0700, Peter Collingbourne wrote:
> > > Render nodes are not just useful for devices supporting GPU hardware
> > > acceleration. Even on devices that only support dumb frame buffers,
> > > they are useful in situations where composition (using software
> > > rasterization) and KMS are done in different processes with buffer
> > > sharing being used to send frame buffers between them. This is the
> > > situation on Android, where surfaceflinger is the compositor and the
> > > composer HAL uses KMS to display the buffers. Thus it is beneficial
> > > to expose render nodes on all devices that support buffer sharing.
> >
> > If I understand your problem right, the issue is that you're getting
> > your buffers in minigbm via pl111, which means you want a render node
> > just for buffer allocation? Then HWComposer will import those on the
> > primary node for displaying.
> 
> Correct.
> 
> > I'm not at all familiar with how minigbm sits in Android, but on our
> > platforms where the Display and GPU devices are different, we allocate
> > via ion in Gralloc, and import those into both the GPU and Display.
> > I think that should work on pl111 too - if you can allocate contiguous
> > memory from ion (or dma-buf heaps) in minigbm, then you don't need the
> > render node.
> 
> Those contiguous memory regions would not necessarily be compatible
> with the pl111 device as far as I know -- according to [1], on some
> devices, a designated memory region must be used for the framebuffer
> and therefore the memory region allocated in CMA would not be
> compatible. That being said, FVP doesn't appear to be one of those
> devices, so in principle that might work for FVP (provided that the
> user provides a sufficiently large cma=X kernel command line
> argument), but not for those other devices.

We have been doing that with mali-dp drivers for years. Because most of
our dev environments are on FPGAs, we needed to use the local RAM on
those boards so we've had to assign a memory region to the driver that
in turn was using CMA. We've made heavy use of 'reserved-memory' and
'memory-region' nodes in the DT to link the two together, but things
worked out quite well. Because the 'reserved-memory' sub-node was marked
as 'compatible="shared-dma-pool"', gralloc and ION could be used to
allocate memory from it.

Best regards,
Liviu

> 
> Peter
> 
> [1] https://www.kernel.org/doc/Documentation/devicetree/bindings/display/arm%2Cpl11x.txt

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