[RFC] remoting KMS

[Thomas Hellstrom]

On 05/17/2018 09:20 PM, Daniel Vetter wrote:
> On Thu, May 17, 2018 at 8:23 PM, Thomas Hellstrom <thomas at shipmail.org> wrote:
>> Hi!
>>
>> 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"
>> user-space.
>>
>> 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
> pipeline.

Hi!
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
> this?
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
> prime.

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

Thanks,

Thomas