On Tuesday, January 31st, 2023 at 12:13, Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote: > On Tue, 31 Jan 2023 10:06:39 +0000 > Simon Ser <contact@xxxxxxxxxxx> wrote: > > > On Tuesday, January 31st, 2023 at 10:25, Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote: > > > > > indeed, what about simply using a 1x1 framebuffer for real? Why was that > > > approach rejected? > > > > Ideally we don't want to allocate any GPU memory for the solid-fill > > stuff. And if we special-case 1x1 FB creation to not be backed by real > > GPU memory then we hit several situations where user-space expects a > > real FB but there isn't: for instance, GETFB2 converts from FB object > > ID to GEM handles. Even if we make GETFB2 fail and accept that this > > breaks user-space, then there is no way for user-space to recover the > > FB color for flicker-free transitions and such. > > > > This is all purely from a uAPI PoV, completely ignoring the potential > > issues with the internal kernel abstractions which might not be suitable > > for this either. > > I mean a real 1x1 buffer: a dumb buffer. > > It would be absolutely compatible with anything existing, because it is > a real FB. As a dumb buffer it would be trivial to write into and read > out. As 1x1 it would be tiny (one page?). Even if something needs to > raw-access uncached memory over 33 MHz PCI bus or whatever the worst > case is, it's just one pixel, so it's fast enough, right? And it only > needs to be read once when set, like USB display drivers do. The driver > does not need to manually apply any color operations, because none are > supported in this special case. > > One can put all these limitations and even pixel format in the plane > property that tells userspace that a 1x1 FB works here. > > To recap, the other alternatives under discussion I see right now are: > > - this proposal of dedicated fill color property > - stuffing something new into FB_ID property > > There is also the question of other kinds of plane content sources like > live camera feeds where userspace won't be shovelling each frame > individually like we do now. > > 1x1 dumb buffer is not as small and lean as a dedicated fill color > property, but the UAPI design questions seem to be much less. What's > the best trade-off and for whom? By "real memory" yes I mean the 1 page. Using a real buffer also brings back other discussions, e.g. the one about which pixel formats to accept.
