On Fri, 11 Oct 2019 at 08:46, Daniel Vetter <dan...@ffwll.ch> wrote:
> On Thu, Oct 10, 2019 at 7:32 PM Ayan Halder <ayan.hal...@arm.com> wrote:
> > On Thu, Oct 10, 2019 at 03:41:15PM +0200, Neil Armstrong wrote:
> > > Sorry I don't understand, you ask me to limit AFBC to ABGR8888 ?
> >
> > Apologies for the confusion, as per the link, the formats which are
> > suggested with AFBC_FORMAT_MOD_YTR are the BGR/ABGR formats (as
> > listed in the 'AFBC formats' table)
> >
> > Thus, any other permutation of the components might make it incompatible
> > with some other AFBC producers/consumers.
> Uh, that's not how this is supposed to be used. Drivers are meant to
> expose _everything_ they support (bonus if you roughly sort it in
> preference order). Userspace then computes the intersection of
> modifiers/formats supported by all devices it needs to share a buffer
> with. Allowing that was the entire point of modifiers, if we
> artificially limit to the common denominator we're back "only linear
> works everywhere".


A lot of userspace will select for format first, then find a modifier
which can be used with that format. If userspace has specific
knowledge of AFBC and decides that it prefers to use AFBC so will seek
out an alpha format - and make sure everyone fills the channel solid -
then that's fine. But that's putting the cart before the horse.

Not exposing XRGB8888 on the primary plane will break a lot of
userspace, so in this case AFBC would just never really be used.

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