On 05/17/2018 09:20 PM, Daniel Vetter wrote:
On Thu, May 17, 2018 at 8:23 PM, Thomas Hellstrom <tho...@shipmail.org> wrote:

I'm currently working on a remoting KMS backend, and now I thought it would
be a good time to get some feedback on a very rough design:

The idea is that we want to be get enough information on the backend side of
KMS to be able to remote the display system over VNC or something similar.
Initially I'd plug in an open-source VNC server as the "reference"

The solution should be general enough to plug into any driver, including
VKMS, either mirroring an existing crtc/connector or adding a virtual one.
The driver would typically only have to implement an efficient readback
command, optionally with a diff-map or damage rect that tracks changed
content for the VNC server to be able to do its work efficiently.

This would all be forwarded to a user-space interface and protocol with a
slightly more elaborate user-space event-mechanism than the one we have in
drm now. In particular we want to be able to read partial events, and resume
event reading after a partial event. We would also want to be able to send
commands either through an ioctl() or using write(). Current test code is
using write().

Since the VNC server in our application typically will be system-wide, we
need to resurrect a control-node functionality. I was trying to use sysfs
attributes for this, but while sysfs supports rudimentary poll
functionality. It's a bit too rudimentary and tracking client lifetime
becomes racy. Perhaps control nodes renamed to "remoting" nodes or something
similar. It might be that masters want to attach a VNC server to their
current display and in that case we probably would want to expand primary
nodes with this functionality as well.

Now to the open-source  problem. As mentioned, the reference user-space will
be a solution based on libvncserver. In fact there will be a small
user-space interface library that libvncserver can interact with, similar to
libdrm. However, I envision in the long run VMWare would be interested to
use other remoting protocols with closed source protocol encoders, using the
open source reference library. In theory, this solution would also allow for
simple (open- or closed source) user-space display drivers, very similar to
DisplayLink's evdi solution, even if that's not a use-case VMWare is
currently interested in.

Thoughts and comments appreciated.
Just two very quick comments:

- For the uapi - can't we just use v4l for the output side? That gives
you buffer handoff, syncing (v4l is gaining dma_fence support) and
pretty much everything you might want. And we've talked about building
kms->v4l pipelines in a bunch of other use-cases already (like feeding
into video encoders for widi and stuff like that). Would give you the
userspace for free, only thing we really need is to (maybe, not sure
you'd even need that) expose what kms output corresponds to what v4l

Thanks for the comments.

I'll take a look whether this is possible to accomplish with v4l. Hadn't thought of that. However, I doubt it will be possible to support all things we need to support. Initially we started looking at this in the context of readback connectors but buffer management and readbacks are only a subset of what we want to do. We also need to pipe a subset of the (atomic) state and its updates, the handling of which actually becomes the vast major part of the code. We also need to feed data back, similar to the vmwgfx update_layout ioctl, that sets suggested x and y position, preferred mode, and, on readback, obtain damage information. Perhaps it's possible to construct some kind of side-channel.

- You say "The solution should be general enough to plug into any
driver" - is the idea that any hw driver would automatically support
No, the initial idea is that any hw driver that wants to could use helpers and a readback implementation to support this

Given that prime-like setups are getting ever more common
(mobile is essentially all prime with display and offloaded rendering)
I think it'd be cleanest if we don't tie that virtual output to other
drivers, but bind things together using buffer sharing/dma-fences and

We do need a front end, though, I assume you're suggestion is that we add support to vkms only. That would work for the main VMware use-case, but not for a general use-case where a user would want to share his existing screen using VNC.

  Adding v4l output to vkms is actually something we've already
discussed a bit on irc, but with the use-case that you could just
watch what your desktop/window manager is doing under a specific
simulated setup (e.g. pll sharing constraints, or special plane
features, or whatever other specific thing you want to test your
compositor against but isn't supported on all physical hw/drivers).

Cheers, Daniel



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