On 2019/06/28, Matt Roper wrote: > On Fri, Jun 28, 2019 at 05:14:51PM +0100, Emil Velikov wrote: > > Hi Matt, > > > > Thanks for the enlightening input :-) > > > > On 2019/06/25, Matt Roper wrote: > > > > > PLANE_CURSOR is basically just an indication that that specific plane is > > > the one that's also hooked up to the legacy cursor ioctls; like Ville > > > says, it shouldn't directly indicate that the plane is less > > > feature-capable than other planes. You can either detect the true > > > capabilities of the cursor plane by checking for the presence/absence of > > > other plane properties and/or experimenting with atomic TEST_ONLY > > > commits to see what's really possible. > > > > > Interesting, my understanding was the plane type was a hint about the > > capabilities. Although yes, userspace must check via TEST_ONLY to ensure > > the properties chosen will work. > > > > > > > The ideal solution for Intel gen9 hardware would have been to just never > > > have the driver advertise or program the dedicated hardware cursor at > > > all, but to instead expose the top-most universal plane to userspace, > > > describe it as PLANE_CURSOR, and route the legacy cursor ioctl's to that > > > plane instead. That would allow legacy cursor behavior to work as > > > usual, but would also allow atomic userspace to use the plane in a more > > > full-featured manner. I wrote patches to do exactly this a couple years > > > ago, but sadly we discovered that the universal planes on gen9 have a > > > slight alpha blending defect that the dedicated hardware cursor does not > > > exhibit. Thus replacing the hardware cursor with the topmost universal > > > plane led to a slight regression for existing users and we had to scrap > > > the whole idea. :-( > > > > > > For reference, the relevant patch from a few years ago is here: > > > https://patchwork.kernel.org/patch/9398571/ > > > > > In that thread you mention: > > > > "... I believe the color correction settings are different for the > > universal plane vs the cursor plane (which causes IGT CRC mismatches at > > the moment and may be visually noticeable if you have good eyes); that > > shouldn't be hard to track down and fix." > > > > Yet above you mention that universal planes have alpha blending defect. > > Did you confirm that with HW/simulation teams or is that based on the > > documentation? I would love to read a bit more on the topic. > > > > In particular, but not limited to, if this defect is applicable only for > > plane3 or literally all universal planes. > > We only figured out exactly what was going on a while after I wrote that > message in the thread, but we did ultimately confirm the problem with > the hardware architects. Sadly, all of the gen9 universal planes suffer > from the slight blending issue and only the dedicated hardware cursor is > immune because it has some special bypass logic. > > Specifically, you'd usually expect blending between two planes' pixels > to give you a final color value of > > bottom * (1.0-alpha) + top * alpha > > However due to the way the alpha values are interpreted in the hardware, > there's a problematic case when the top plane's pixel alpha is 0. You > wind up getting > > bottom * (255/256) + top * 0 = .996 * bottom > > meaning pixels with a fully transparent plane above them are very > slightly fainter than pixels that didn't go through blending. You'll > pretty much only perceive the difference when you have transparent > pixels at the edge of a plane (and even then only if you have a good > monitor and good eyes). Of course "transparent pixels at the edge of > plane" is pretty common when you're using the plane as a mouse pointer. > You wind up with a faint ghost rectangle around your mouse pointer. :-( > So we have a couple of problems here: - general faided bottom image - not much we can do here, also an issue even w/o using plane3 as cursor.
- ghosting - w/a: make sure the edge of a plane, is never within the viewport (scanout region) Since the fading is a generic problem, we might as well make use of plane3 regardless. What I'm proposing is that we pick up dimensions X and Y as cut-off. Everything larger will use plane3, smaller - cursor. To determine X/Y I see two approaches: - based off the max cursor size exposed by DRM drivers - say Zx larger - consider the current resolution +/- delta We could also extrapolate that with the fact that most drivers expose and hence user-space handle cursor identical width and height. Personally, I'm inclined to pick 512x512 as a threshold and re-spin your patches. Let me know what you think. Thanks, Emil _______________________________________________ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx
