The idea presented in these patches is to address link training failure in a way that: a) changes the current happy day scenario as little as possible, to avoid regressions, b) can be implemented the same way by all drm drivers, c) is still opt-in for the drivers and userspace, and opting out doesn't regress the user experience, d) doesn't prevent drivers from implementing better or alternate approaches, possibly without userspace involvement. And, of course, handles all the issues presented.
The solution is to add a "link status" connector property. In the usual happy day scenario, this is always "good". If something fails during or after a mode set, the kernel driver can set the link status to "bad", prune the mode list based on new information as necessary, and send a hotplug uevent for userspace to have it re-check the valid modes through getconnector, and try again. If the theoretical capabilities of the link can't be reached, the mode list is trimmed based on that.
If the userspace is not aware of the property, the user experience is the same as it currently is. If the userspace is aware of the property, it has a chance to improve user experience. If a drm driver does not modify the property (it stays "good"), the user experience is the same as it currently is. A drm driver can also choose to try to handle more of the failures in kernel, hardware not limiting, or it can choose to involve userspace more. Up to the drivers.
The reason for adding the property is to handle link training failures, but it is not limited to DP or link training. For example, if we implement asynchronous setcrtc, we can use this to report any failures in that.
Finally, while DP CTS compliance is advertized (which is great, and could be made to work similarly for all drm drivers), this can be used for the more important goal of improving user experience on link training failures, by avoiding black screens.
Manasi Navare (5): drm: Add a new connector property for link status drm: Set DRM connector link status property drm/i915: Update CRTC state if connector link status property changed drm/i915: Find fallback link rate/lane count drm/i915: Implement Link Rate fallback on Link training failure
drivers/gpu/drm/drm_atomic_helper.c | 7 ++ drivers/gpu/drm/drm_connector.c | 65 +++++++++++++++- drivers/gpu/drm/i915/intel_dp.c | 103 +++++++++++++++++++++++++- drivers/gpu/drm/i915/intel_dp_link_training.c | 28 ++++++- drivers/gpu/drm/i915/intel_drv.h | 7 ++ include/drm/drm_connector.h | 9 ++- include/drm/drm_mode_config.h | 5 ++ include/uapi/drm/drm_mode.h | 4 + 8 files changed, 221 insertions(+), 7 deletions(-)