• Manasi Navare's avatar
    drm: Add a new connector atomic property for link status · 40ee6fbe
    Manasi Navare authored
    At the time userspace does setcrtc, we've already promised the mode
    would work. The promise is based on the theoretical capabilities of
    the link, but it's possible we can't reach this in practice. The DP
    spec describes how the link should be reduced, but we can't reduce
    the link below the requirements of the mode. Black screen follows.
    
    One idea would be to have setcrtc return a failure. However, it
    already should not fail as the atomic checks have passed. It would
    also conflict with the idea of making setcrtc asynchronous in the
    future, returning before the actual mode setting and link training.
    
    Another idea is to train the link "upfront" at hotplug time, before
    pruning the mode list, so that we can do the pruning based on
    practical not theoretical capabilities. However, the changes for link
    training are pretty drastic, all for the sake of error handling and
    DP compliance, when the most common happy day scenario is the current
    approach of link training at mode setting time, using the optimal
    parameters for the mode. It is also not certain all hardware could do
    this without the pipe on; not even all our hardware can do this. Some
    of this can be solved, but not trivially.
    
    Both of the above ideas also fail to address link degradation *during*
    operation.
    
    The solution is to add a new "link-status" connector property in order
    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.
    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" and issue a hotplug uevent for userspace to have it
    re-check the valid modes through GET_CONNECTOR IOCTL, and try modeset
    again. If the theoretical capabilities of the link can't be reached,
    the mode list is trimmed based on that.
    
    v7 by Jani:
    * Rebase, simplify set property while at it, checkpatch fix
    v6:
    * Fix a typo in kernel doc (Sean Paul)
    v5:
    * Clarify doc for silent rejection of atomic properties by driver (Daniel Vetter)
    v4:
    * Add comments in kernel-doc format (Daniel Vetter)
    * Update the kernel-doc for link-status (Sean Paul)
    v3:
    * Fixed a build error (Jani Saarinen)
    v2:
    * Removed connector->link_status (Daniel Vetter)
    * Set connector->state->link_status in drm_mode_connector_set_link_status_property
    (Daniel Vetter)
    * Set the connector_changed flag to true if connector->state->link_status changed.
    * Reset link_status to GOOD in update_output_state (Daniel Vetter)
    * Never allow userspace to set link status from Good To Bad (Daniel Vetter)
    Reviewed-by: default avatarSean Paul <seanpaul@chromium.org>
    Reviewed-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
    Reviewed-by: default avatarJani Nikula <jani.nikula@intel.com>
    Acked-by: default avatarTony Cheng <tony.cheng@amd.com>
    Acked-by: default avatarHarry Wentland <harry.wentland@amd.com>
    Cc: Jani Nikula <jani.nikula@linux.intel.com>
    Cc: Daniel Vetter <daniel.vetter@intel.com>
    Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
    Cc: Chris Wilson <chris@chris-wilson.co.uk>
    Cc: Sean Paul <seanpaul@chromium.org>
    Signed-off-by: default avatarManasi Navare <manasi.d.navare@intel.com>
    Signed-off-by: default avatarJani Nikula <jani.nikula@intel.com>
    Acked-by: Eric Anholt <eric@anholt.net> (for the -modesetting patch)
    Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
    Link: http://patchwork.freedesktop.org/patch/msgid/0182487051aa9f1594820e35a4853de2f8747b4e.1481883920.git.jani.nikula@intel.com
    40ee6fbe
drm_mode.h 18.7 KB