Re: [PATCHv3 06/30] drm/omap: Add support for render nodes

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Tomi,

(CC'ing Daniel and David)

On Wednesday 29 Mar 2017 11:58:23 Tomi Valkeinen wrote:
> On 29/03/17 11:22, Laurent Pinchart wrote:
> > Hi Tomi,
> > 
> > Thank you for the patch.
> > 
> > On Tuesday 28 Mar 2017 16:07:52 Tomi Valkeinen wrote:
> >> From: Hemant Hariyani <hemanthariyani@ti.com>
> >> 
> >> Add support for render nodes in omap driver and allow required
> >> ioctls to be accessible via render nodes.
> > 
> > But the OMAP DSS doesn't perform rendering... This seems an abuse of
> > render nodes, I think the API should instead be implemented by the GPU
> > driver.
> 
> I agree that the GPU use case described in the patch sounds a bit odd.
> Why not allocate from the GPU driver instead. But here a particular
> issue is that to get TILER buffers we need to ask them from the omapdrm.
> Probably TILER should not be part of omapdrm, but then, where should it
> be...
> 
> We also have writeback in DSS, which can function as a memory to memory
> or capture device. That's not supported in the mainline, but we have
> support in the TI kernel.
> 
> And how about a case where you have the privileged process as KMS
> master, and another process wants to draw to a buffer with the CPU, and
> then give the buffer to the privileged process for displaying.
> 
> So, yes, DSS is not a renderer (well, WB is kind of rendering), but
> isn't it a valid use case to allocate a buffer from omapdrm?

It could be a valid use case, but it's still an API abuse. It starts sounding 
like a DRM core issue to me. The DRIVER_RENDER flag is not documented, so its 
exact meaning isn't defined. I thought it was supposed to flag the device as a 
renderer (GPU).

commit 1793126fcebd7c18834f95d43b55e387a8803aa8
Author: David Herrmann <dh.herrmann@gmail.com>
Date:   Sun Aug 25 18:29:00 2013 +0200

    drm: implement experimental render nodes
    
    Render nodes provide an API for userspace to use non-privileged GPU
    commands without any running DRM-Master. It is useful for offscreen
    rendering, GPGPU clients, and normal render clients which do not perform
    modesetting.

    [...]

You instead use the flag as a way to enable unprivileged clients to allocate 
buffers. If that's what the flag should mean, I wonder if there would be a use 
case for *not* setting it.

> Also, where should a buffer be allocated from, generally? I usually
> think that if a buffer is going to DSS, I allocate it from omapdrm. Then
> again, getting it from the source sounds ok also, i.e. from a v4l2
> capture driver... But if we get it from the source, it may not be usable
> by all the components in the processing pipeline (e.g, SGX requires 32
> pixel aligned buffers).

-- 
Regards,

Laurent Pinchart

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