[PATCH] drm/doc: Document that modifiers are always required for fb

[Daniel Vetter]

On Wed, Sep 02, 2020 at 02:59:49PM +0000, Simon Ser wrote:
> On Wednesday, September 2, 2020 4:29 PM, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> 
> > On Wed, Sep 2, 2020 at 2:49 PM Simon Ser contact at emersion.fr wrote:
> >
> > > On Wednesday, September 2, 2020 2:44 PM, Daniel Vetter daniel.vetter at ffwll.ch wrote:
> > >
> > > > > I suppose something similar happens in user-space: gbm_bo_create
> > > > > without modifiers needs to properly set the implicit modifier, ie.
> > > > > gbm_bo_get_modifier needs to return the effective modifier. Is this
> > > > > something already documented?
> > > >
> > > > I don't think that happens, but it has come up in discussions. It's
> > > > kinda different scenario though: getfb2 is for cross-compositor stuff,
> > > > enabling smooth transitions at boot-up and when switching. So you have
> > > > a legit reason for mixing modifier-aware userspace with
> > > > non-modifier-aware userspace. And the modifier-aware userspace would
> > > > like that everything works with modifiers consistently, including
> > > > getfb2. But gbm is just within a single process, and that should
> > > > either run all with modifiers, or not at all, since these worlds just
> > > > dont mix well. Hence I'm not seeing much use for that, -modesetting
> > > > being a confused mess nonwithstanding :-)
> > >
> > > Well… There's also the case where some legacy Wayland client talks to a
> > > modifier-aware compositor. gbm_bo_import would be called without a
> > > modifier, but the compositor expects gbm_bo_get_modifier to work.
> > > Also, wlroots will call gbm_bo_create without a modifier to only let
> > > the driver pick "safe" modifiers in case passing the full list of
> > > modifiers results in a black screen. Later on wlroots will call
> > > gbm_bo_get_modifier to figure out what modifier the driver picked.
> >
> > gbm_bo_import is a different thing from gbm_bo_create. Former I agree
> > should figure out the right modifiers (and I think it does that, at
> > least on intel mesa). For gbm_bo_create I'm not sure we should/need to
> > require that.
> 
> I guess the compositor will just forward the value returned by
> gbm_bo_get_modifier in any case, so returning INVALID would be fine
> too (to mean "implicit modifier").
> 
> In both the create and import cases, other metadata like pitches and
> offsets should be correctly set I think?

Well if you have a modifier format underneath, the non-modifiered offsets
and pitches might be pure fiction. Also, they might not be sufficient, if
the modifier adds more planes.

So I'm not sure how we can let the "implicit modifier" go through once a
stack is converted to support modifiers. In a way modifiers are one-way
compatible only: implicit modifiers -> explicit modifiers should be
well-defined, the other way just looses information and doesn't work.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch