Re: [PATCH v2 1/8] drm: Disable the cursor plane on atomic contexts with virtualized drivers

["Jonas Ådahl" <jadahl@gmail.com>]

On Thu, May 04, 2023 at 01:39:04PM +0300, Pekka Paalanen wrote:
> On Thu, 4 May 2023 01:50:25 +0000
> Zack Rusin <zackr@vmware.com> wrote:
> 
> > On Wed, 2023-05-03 at 09:48 +0200, Javier Martinez Canillas wrote:
> > > Zack Rusin <zackr@vmware.com> writes:
> > >   
> > > > On Tue, 2023-05-02 at 11:32 +0200, Javier Martinez Canillas wrote:  
> 
> > > > > AFAICT this is the only remaining thing to be addressed for this series ?  
> > > > 
> > > > No, there was more. tbh I haven't had the time to think about whether the above
> > > > makes sense to me, e.g. I'm not sure if having virtualized drivers expose
> > > > "support
> > > > universal planes" and adding another plane which is not universal (the only
> > > > "universal" plane on them being the default one) makes more sense than a flag
> > > > that
> > > > says "this driver requires a cursor in the cursor plane". There's certainly a
> > > > huge
> > > > difference in how userspace would be required to handle it and it's way uglier
> > > > with
> > > > two different cursor planes. i.e. there's a lot of ways in which this could be
> > > > cleaner in the kernel but they all require significant changes to userspace,
> > > > that go
> > > > way beyond "attach hotspot info to this plane". I'd like to avoid approaches
> > > > that
> > > > mean running with atomic kms requires completely separate paths for virtualized
> > > > drivers because no one will ever support and maintain it.
> > > > 
> > > > It's not a trivial thing because it's fundamentally hard to untangle the fact
> > > > the
> > > > virtualized drivers have been advertising universal plane support without ever
> > > > supporting universal planes. Especially because most new userspace in general
> > > > checks
> > > > for "universal planes" to expose atomic kms paths.
> > > >   
> > > 
> > > After some discussion on the #dri-devel, your approach makes sense and the
> > > only contention point is the name of the driver feature flag name. The one
> > > you are using (DRIVER_VIRTUAL) seems to be too broad and generic (the fact
> > > that vkms won't set and is a virtual driver as well, is a good example).
> > > 
> > > Maybe something like DRIVER_CURSOR_HOTSPOT or DRIVER_CURSOR_COMMANDEERING
> > > would be more accurate and self explanatory ?  
> > 
> > Sure, or even more verbose DRIVER_NEEDS_CURSOR_PLANE_HOTSPOT, but it sounds like
> > Pekka doesn't agree with this approach. As I mentioned in my response to him, I'd be
> > happy with any approach that gets paravirtualized drivers working with atomic kms,
> > but atm I don't have enough time to be creating a new kernel subsystem or a new set
> > of uapi's for paravirtualized drivers and then porting mutter/kwin to those.
> 
> It seems I have not been clear enough, apologies. Once more, in short:
> 
> Zack, I'm worried about this statement from you (copied from above):
> 
> > > > I'd like to avoid approaches that mean running with atomic kms
> > > > requires completely separate paths for virtualized drivers
> > > > because no one will ever support and maintain it.
> 
> It feels like you are intentionally limiting your own design options
> for the fear of "no one will ever support it". I'm worried that over
> the coming years, that will lead to a hard to use, hard to maintain
> patchwork of vague or undocumented or just too many little UAPI details.
> 
> Please, don't limit your designs. There are good reasons why nested KMS
> drivers behave fundamentally differently to most KMS hardware drivers.
> Userspace that does not or cannot take that into account is unavoidably
> crippled.

From a compositor side, there is a valid reason to minimize the uAPI
difference between "nested virtual machine" code paths and "running on
actual hardware" code paths, which is to let virtual machines with a
viewer connected to KMS act as a testing environment, rather than a
production environment. Running a production environment in a virtual
machine doesn't really need to use KMS at all.

When using virtual machines for testing, I want to minimize the amount
of differentation between running on hardware and running in the VM
because otherwise the parts that are tested are not the same.

I realize that hotpspots and the cursor moving viewer side contradicts
that to some degree, but still, from a graphical testing witha VM point
of view, one has to compromise, as testing isn't just for the KMS layer,
but for the DE and distribution as a whole.


Jonas

> 
> 
> Thanks,
> pq