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

[Daniel Vetter]

On Wed, Sep 2, 2020 at 1:02 PM Pekka Paalanen <ppaalanen at gmail.com> wrote:
>
> On Wed,  2 Sep 2020 12:24:40 +0200
> Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> > Even for legacy userspace, since otherwise GETFB2 is broken and if you
> > switch between modifier-less and modifier-aware compositors, smooth
> > transitions break.
> >
> > Also it's just best practice to make sure modifiers are invariant for
> > a given drm_fb, and that a modifier-aware kms drivers only has one
> > place to store them, ignoring any old implicit bo flags or whatever
> > else might float around.
> >
> > Motivated by some irc discussion with Bas about amdgpu modifier
> > support.
> >
> > Fixes: 455e00f1412f ("drm: Add getfb2 ioctl")
> > Cc: Daniel Stone <daniels at collabora.com>
> > Cc: Juston Li <juston.li at intel.com>
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > Cc: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
> > Cc: Marek Olšák <maraeo at gmail.com>
> > Cc: "Wentland, Harry" <harry.wentland at amd.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> >  include/drm/drm_mode_config.h | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> > index a18f73eb3cf6..5ffbb4ed5b35 100644
> > --- a/include/drm/drm_mode_config.h
> > +++ b/include/drm/drm_mode_config.h
> > @@ -58,6 +58,12 @@ struct drm_mode_config_funcs {
> >        * actual modifier used if the request doesn't have it specified,
> >        * ie. when (@mode_cmd->flags & DRM_MODE_FB_MODIFIERS) == 0.
> >        *
> > +      * IMPORTANT: These implied modifiers for legacy userspace must be
> > +      * stored in struct &drm_framebuffer, including all relevant metadata
> > +      * like &drm_framebuffer.pitches and &drm_framebuffer.offsets if the
> > +      * modifier enables additional planes beyond the fourcc pixel format
> > +      * code. This is required by the GETFB2 ioctl.
> > +      *
> >        * If the parameters are deemed valid and the backing storage objects in
> >        * the underlying memory manager all exist, then the driver allocates
> >        * a new &drm_framebuffer structure, subclassed to contain
> > @@ -915,6 +921,13 @@ struct drm_mode_config {
> >        * @allow_fb_modifiers:
> >        *
> >        * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> > +      *
> > +      * IMPORTANT:
> > +      *
> > +      * If this is set the driver must fill out the full implicit modifier
> > +      * information in their &drm_mode_config_funcs.fb_create hook for legacy
> > +      * userspace which does not set modifiers. Otherwise the GETFB2 ioctl is
> > +      * broken for modifier aware userspace.
> >        */
> >       bool allow_fb_modifiers;
> >
>
> Hi,
>
> are there any drivers that would infer this information at
> modeset/pageflip/atomic ioctl time instead of AddFB/AddFB2 time?

Currently no, the only driver that infers anything for legacy is i915.
Proposed amdgpu modifier patches don't work like that, but I think Bas
is working on adding the modifier inference at addfb time for legacy
userspace.

> Userspace may be creating the FB once per buffer and keep re-using
> that over several render/display cycles. If a driver was changing the
> "effective modifiers" dynamically, userspace could break.

Yeah this is why I want to lock this all down, since effective
modifiers that change for a drm_fb object which is supposed to have
all invariant metadata just isn't great uapi.
-Daniel

>
>
> Thanks,
> pq



-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch