Re: [PATCH] drm: drivers may provide multiple primary planes per CRTC

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Fri, 7 Aug 2020 11:07:06 +0200
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Thu, Aug 06, 2020 at 10:33:31AM +0000, Simon Ser wrote:
> > Some drivers may expose primary planes compatible with multiple CRTCs.
> > Make this clear in the docs: the current wording may be misunderstood as
> > "exactly one primary plane per CRTC".
> > 
> > Signed-off-by: Simon Ser <contact@emersion.fr>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > ---
> >  drivers/gpu/drm/drm_plane.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> > index b7b90b3a2e38..108a922e8c23 100644
> > --- a/drivers/gpu/drm/drm_plane.c
> > +++ b/drivers/gpu/drm/drm_plane.c
> > @@ -49,8 +49,8 @@
> >   * &struct drm_plane (possibly as part of a larger structure) and registers it
> >   * with a call to drm_universal_plane_init().
> >   *
> > - * Cursor and overlay planes are optional. All drivers should provide one
> > - * primary plane per CRTC to avoid surprising userspace too much. See enum
> > + * Cursor and overlay planes are optional. All drivers should provide at least
> > + * one primary plane per CRTC to avoid surprising userspace too much. See enum  
> 
> I think that's even more confusing, since this reads like there could be
> multiple primary planes for a specific CRTC. That's not the case, there'
> only one pointer going from drm_crtc->primary to a drm_plane in the
> kernel.

There could be multiple primary planes *usable* for a specific CRTC but
just one used at a time, right?

> The problem is that userspace doesn't have a drm_property to read this
> pointer, and needs to guess.
> 
> I thought the rule is:
> 
> Nth primary plane (or cursor) is the primary plane for the Nth crtc.
> Enumaration with increasing drm kms object ids.

Why is that needed? With universal planes, I thought
drmModePlane::possible_crtcs bitmask is trustworthy?

In the legacy KMS UAPI you can't even pick your primary plane, because
it's implied in drmModeSetCrtc(), right?

> And I guess we should explain that on some hw any plane (including primary
> ones, since that's only a sw construct) can be freely assinged to crtc.
> 
> Yes it's probably the most gloriously bonkers uapi we've come up with.
> Might be so bad that a libdrm helper to look up the primary plane for a
> crtc (or it's cursor plane if it exists) would be in order :-)

I'm not sure I see the bonkers there.


Thanks,
pq

> 
> Cheers, Daniel
> 
> 
> >   * drm_plane_type for a more in-depth discussion of these special uapi-relevant
> >   * plane types. Special planes are associated with their CRTC by calling
> >   * drm_crtc_init_with_planes().
> > -- 
> > 2.28.0
> > 
> >   
> 


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