On Tue, 29 Jun 2021 13:39:18 +0200 Werner Sembach wse@tuxedocomputers.com wrote:
Am 29.06.21 um 13:17 schrieb Pekka Paalanen:
On Tue, 29 Jun 2021 08:12:54 +0000 Simon Ser contact@emersion.fr wrote:
On Tuesday, June 22nd, 2021 at 09:15, Pekka Paalanen ppaalanen@gmail.com wrote:
yes, I think this makes sense, even if it is a property that one can't tell for sure what it does before hand.
Using a pair of properties, preference and active, to ask for something and then check what actually worked is good for reducing the combinatorial explosion caused by needing to "atomic TEST_ONLY commit" test different KMS configurations. Userspace has a better chance of finding a configuration that is possible.
OTOH, this has the problem than in UI one cannot tell the user in advance which options are truly possible. Given that KMS properties are rarely completely independent, and in this case known to depend on several other KMS properties, I think it is good enough to know after the fact.
If a driver does not use what userspace prefers, there is no way to understand why, or what else to change to make it happen. That problem exists anyway, because TEST_ONLY commits do not give useful feedback but only a yes/no.
By submitting incremental atomic reqs with TEST_ONLY (i.e. only changing one property at a time), user-space can discover which property makes the atomic commit fail.
That works if the properties are independent of each other. Color range, color format, bpc and more may all be interconnected, allowing only certain combinations to work.
If all these properties have "auto" setting too, then it would be possible to probe each property individually, but that still does not tell which combinations are valid.
If you probe towards a certain configuration by setting the properties one by one, then depending on the order you pick the properties, you may come to a different conclusion on which property breaks the configuration.
My mind crossed another point that must be considered: When plugin in a Monitor a list of possible Resolutions+Framerate combinations is created for xrandr and other userspace (I guess by atomic checks? but I don't know).
Hi,
I would not think so, but I hope to be corrected if I'm wrong.
My belief is that the driver collects a list of modes from EDID, some standard modes, and maybe some other hardcoded modes, and then validates each entry against all the known limitations like vertical and horizontal frequency limits, discarding modes that do not fit.
Not all limitations are known during that phase, which is why KMS property "link-status" exists. When userspace actually programs a mode (not a TEST_ONLY commit), the link training may fail. The kernel prunes the mode from the list and sets the link status property to signal failure, and sends a hotplug uevent. Userspace needs to re-check the mode list and try again.
That is a generic escape hatch for when TEST_ONLY commit succeeds, but in reality the hardware cannot do it, you just cannot know until you actually try for real. It causes end user visible flicker if it happens on an already running connector, but since it usually happens when turning a connector on to begin with, there is no flicker to be seen, just a small delay in finding a mode that works.
During this drm properties are already considered, which is no problem atm because as far as i can tell there is currently no drm property that would make a certain Resolutions+Framerate combination unreachable that would be possible with everything on default.
I would not expect KMS properties to be considered at all. It would reject modes that are actually possible if the some KMS properties were changed. So at least going forward, current KMS property values cannot factor in.
However for example forcing YCbCr420 encoding would limit the available resolutions (my screen for example only supports YCbCr420 on 4k@60 and @50Hz and on no other resolution or frequency (native is 2560x1440@144Hz).
So would a "force color format" that does not get resetted on repluging/reenabling a monitor break the output, for example, of an not updated xrandr, unaware of this new property?
Yes, not because the mode list would be missing the mode, but because actually setting the mode would fail.
RandR in particular is problematic, because it does not actually understand any KMS properties, it is merely a relay. So anything that *uses* RandR protocol or xrandr command would also need to be patched to understand the new properties.
The kernel automatically resetting *some* properties in *some* occasions seems really fragile and complicated to me, which is why I'm a lot more keen to see a "reset everything to sensible defaults" generic mechanism added to KMS.
Thanks, pq