Re: [RFC 4/4] drm: Add NVIDIA Tegra support

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On Thu, Apr 12, 2012 at 9:25 AM, Thierry Reding
<thierry.reding@xxxxxxxxxxxxxxxxx> wrote:
> * Alex Deucher wrote:
>> On Thu, Apr 12, 2012 at 5:33 AM, Thierry Reding
>> <thierry.reding@xxxxxxxxxxxxxxxxx> wrote:
>> > In other words I would like to use the Tegra hardware to render content into
>> > a framebuffer (using potentially the 3D engine or HW accelerated video
>> > decoding blocks) but display that framebuffer with a CRTC registered by a
>> > different driver (perhaps provided by a PCIe or USB device).
>> >
>> > I think such a setup would be possible if the CRTC registration can be
>> > decoupled from the DRM driver. Perhaps sdrm even supports that already?
>> You should be able to do something like that already with dma_buf and
>> the drm prime infrastructure.  There's even a drm driver for the udl
>> USB framebuffer devices.
> Using DRM PRIME requires user-space to be involved. I was thinking more along
> the lines of allowing a dumb DRM driver that only provides a CRTC to register
> with another driver so that it shows up as an output for the latter DRM
> device.

Even then it could get tricky with synchronization depending on the hw.

> Then again, having user-space control this may be more flexible. Performance-
> wise both should be about the same, right? What I don't quite understand yet
> is how the DMABUF would be synchronized on both ends. Is there some infra-
> structure to account for it or would I have to export two buffers and flip
> them during the vblank of the "consumer"?

Performance should be about the same.  The tricky part is
synchronization.  dma_buf is for sharing.  There are ideas to make
generic sync objects, but for now, it's up to the userspace components
to handle it.

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