On Monday, April 20, 2020 2:22 PM, Pekka Paalanen <ppaala...@gmail.com> wrote:
> On Mon, 20 Apr 2020 10:15:39 +0000 > Simon Ser cont...@emersion.fr wrote: > > > On Monday, April 20, 2020 10:27 AM, Pekka Paalanen ppaala...@gmail.com > > wrote: > > > > > The only "random" KMS state is the properties the userspace KMS > > > program does not know that are set on start-up. I have been assuming > > > that as long as you had fbdev active before the KMS program started, > > > the unknown properties have "harmless" default values. And maybe even at > > > driver device init if fbdev does not exist? > > I meant fbcon, not fbdev above. > > > Note, this is not the case when using e.g. a display manager. In the > > past there have been cases of a display manager setting a hw cursor > > and launching a compositor not supporting hw cursors. This results in > > a stuck hw cursor. > > Indeed. So the display manager might get sensible defaults, but the > session compositor might not. Or maybe boot splash uses KMS already, so > even display manager doesn't get all-defaults state. > > It seems we really do need "sane defaults" from the kernel explicitly. > Writing a userspace tool to save it at boot time before any KMS program > runs would be awkward. Agreed. > > > Btw. I searched for all occurrences of link_status in > > > https://dri.freedesktop.org/docs/drm/gpu/drm-kms.html and it seems it > > > only has two possible values, good and bad, and no mention whether it > > > is writable. Looks like it's writable. There does not seem to be a) an > > > explanation how exactly it needs to the handled (writing it does > > > something? what can you write?) or b) any way discern between kernel > > > and userspace set values like HDCP "Content Protection" has. > > > > User-space needs to reset the value to GOOD when recovering from a BAD > > value. > > What if userspace writes BAD? > > BAD cannot be default state, so getting default state from somewhere > would solve this property's restoring as well. Reading back the true > current value could accidentally return BAD. Interestingly, setting it to BAD is a no-op (BAD is silently discarded): /* Never downgrade from GOOD to BAD on userspace's request here, * only hw issues can do that. * * For an atomic property the userspace doesn't need to be able * to understand all the properties, but needs to be able to * restore the state it wants on VT switch. So if the userspace * tries to change the link_status from GOOD to BAD, driver * silently rejects it and returns a 0. This prevents userspace * from accidently breaking the display when it restores the * state. */ if (state->link_status != DRM_LINK_STATUS_GOOD) state->link_status = val; So restoring the "sane default" would be work, even if the link happens to be BAD when saving said "sane defaults". > Just to reiterate for everyone, the important thing here is to figure > out how userspace is supposed to reset unknown properties to sensible > defaults. Once we know how that should work, we can review whether new > properties support or break that. Yes, that's a good description of the problem. I see two main solutions here: either the kernel provides the default values in its property descriptions (e.g. drmModeGetProperty), either user-space can ask the kernel to reset properties to their default values. _______________________________________________ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
