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

[Simon Ser]

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.