[PATCH] drm/vkms: Add a DRM render node to vkms

[Tao Wu(吴涛@Eng)]

On Sat, Jan 7, 2023 at 2:45 AM Simon Ser <contact at emersion.fr> wrote:
>
> On Friday, January 6th, 2023 at 23:28, Tao Wu <lepton at google.com> wrote:
>
> > On Fri, Jan 6, 2023 at 1:54 AM Daniel Vetter daniel at ffwll.ch wrote:
> >
> > > On Thu, Jan 05, 2023 at 01:40:28PM -0800, Tao Wu(吴涛@Eng) wrote:
> > >
> > > > Hi Daniel,
> > > >
> > > > May I know what's the requirement for adding render node support to a
> > > > "gpu"? Why we just export render node for every drm devices?
> > > > I read document here
> > > > https://www.kernel.org/doc/html/v4.8/gpu/drm-uapi.html#render-nodes
> > >
> > > Thus far we've only done it when there's actual rendering capability,
> > > which generally means at least some private ioctls.
> >
> > Hi Daniel, it looks like vgem is exporting render node by default.
> > Per my understanding, vgem provides some DRM API so users can play
> > with graphic buffers. I am feeling it's natural have a v*** device
> > which provide
> > the surperset which vgem and vkms provides, so it sounds like it's
> > natural add rendernode to vkms, or do the opposite, add kms related
> > stuff to vgem. I still don't get the point: what kind of issue it
> > could bring if we just
> > add render node to vkms? If your point is, we don't do that for other
> > kms only devices, then my question is, how about we just enable render
> > node for every DRM driver? what could go wrong with this approach?
>
> This is wrong for at least two reasons:
>
> - A render node has a semantic value: it indicates whether a device has
>   rendering capabilities. If we attach a render node to vkms, we lie
>   because vkms has no such capability.
Thanks, then my question is, why vgem provides render node? What do you
mean by "rendering capabilities" here? Are you meaning things like GPU
acceleration?
> - This would regress user-space. wlroots would no longer accept to
>   start with Pixman on vkms, because it detects a render node on the
>   device.
>
> I'd advise moving away from abusing DRM dumb buffers in Mesa.