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

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Zack Rusin <zackr@xxxxxxxxxx> writes:

> On Tue, 2023-05-02 at 11:32 +0200, Javier Martinez Canillas wrote:
>> !! External Email
>> Daniel Vetter <daniel@xxxxxxxx> writes:
>> > On Mon, Jul 11, 2022 at 11:32:39PM -0400, Zack Rusin wrote:
>> > > From: Zack Rusin <zackr@xxxxxxxxxx>
>> > > 
>> > > Cursor planes on virtualized drivers have special meaning and require
>> > > that the clients handle them in specific ways, e.g. the cursor plane
>> > > should react to the mouse movement the way a mouse cursor would be
>> > > expected to and the client is required to set hotspot properties on it
>> > > in order for the mouse events to be routed correctly.
>> > > 
>> > > This breaks the contract as specified by the "universal planes". Fix it
>> > > by disabling the cursor planes on virtualized drivers while adding
>> > > a foundation on top of which it's possible to special case mouse cursor
>> > > planes for clients that want it.
>> > > 
>> > > Disabling the cursor planes makes some kms compositors which were broken,
>> > > e.g. Weston, fallback to software cursor which works fine or at least
>> > > better than currently while having no effect on others, e.g. gnome-shell
>> > > or kwin, which put virtualized drivers on a deny-list when running in
>> > > atomic context to make them fallback to legacy kms and avoid this issue.
>> > > 
>> > > Signed-off-by: Zack Rusin <zackr@xxxxxxxxxx>
>> > > Fixes: 681e7ec73044 ("drm: Allow userspace to ask for universal plane list
>> > > (v2)")
>> [...]
>> > > diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
>> > > index f6159acb8856..c4cd7fc350d9 100644
>> > > --- a/include/drm/drm_drv.h
>> > > +++ b/include/drm/drm_drv.h
>> > > @@ -94,6 +94,16 @@ enum drm_driver_feature {
>> > >       * synchronization of command submission.
>> > >       */
>> > >      DRIVER_SYNCOBJ_TIMELINE         = BIT(6),
>> > > +    /**
>> > > +     * @DRIVER_VIRTUAL:
>> > > +     *
>> > > +     * Driver is running on top of virtual hardware. The most significant
>> > > +     * implication of this is a requirement of special handling of the
>> > > +     * cursor plane (e.g. cursor plane has to actually track the mouse
>> > > +     * cursor and the clients are required to set hotspot in order for
>> > > +     * the cursor planes to work correctly).
>> > > +     */
>> > > +    DRIVER_VIRTUAL                  = BIT(7),
>> > 
>> > I think the naming here is unfortunate, because people will vonder why
>> > e.g. vkms doesn't set this, and then add it, and confuse stuff completely.
>> > 
>> > Also it feels a bit wrong to put this onto the driver, when really it's a
>> > cursor flag. I guess you can make it some kind of flag in the drm_plane
>> > structure, or a new plane type, but putting it there instead of into the
>> > "random pile of midlayer-mistake driver flags" would be a lot better.
>> > 
>> > Otherwise I think the series looks roughly how I'd expect it to look.
>> > -Daniel
>> > 
>> 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).

would be more accurate and self explanatory ?

> The other thing blocking this series was the testing of all the edge cases, I think
> Simon and Pekka had some ideas for things to test (e.g. run mutter with support for
> this and wayland without support for this in at the same time in different consoles
> and see what happens). I never had the time to do that either.

I understand that every new feature needs tests but I fail to see why
the bar is higher for this feature than others? I would prefer if this
series are not blocked due some potential issues on hypothetical corner
cases that might not happen in practice. Or do people really run two or
more compositors on different console and switch between them ?

>> Zack, are you planning to re-spin a v3 of this patch-set? Asking because
>> we want to take virtio-gpu out of the atomic KMS deny list in mutter, but
>> first need this to land.
>> If you think that won't be able to do it in the short term, Bilal (Cc'ed)
>> or me would be glad to help with that.
> This has been on my todo for a while I just never had the time to go through all the
> remaining issues. Fundamentally it's not so much a technical issue anymore, it's
> about picking the least broken solution and trying to make the best out of a pretty
> bad situation. In general it's hard to paint a bikeshed if all you have is a million
> shades of gray ;)

Agreed. And I believe that other than the driver cap name, everyone agrees
with the color of your bikeshed :)

Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat

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