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drm: Try to document legacy DPMS uapi a bit better

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Due to inconsistency of how various legacy drivers implemented DPMS
the DPMS uabi has a lot of quirks. Atomic standardizes this, but
drivers using the DPMS support can't rely on that since legacy drivers
still exist.

Laurent asked for this.

v2:
Improve commit message and explain that DPMS doesn't really exist for
the atomic ioctl (it's "ACTIVE" on the CRTC instead).

Text polish from Eric's review.

Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170920225957.16278-1-daniel.vetter@ffwll.ch
This commit is contained in:
Daniel Vetter 2017-09-21 00:59:57 +02:00
parent 6bf2e0324b
commit d0d1aee5f7
1 changed files with 23 additions and 0 deletions
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23
drivers/gpu/drm/drm_connector.c
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@ -719,6 +719,29 @@ DRM_ENUM_NAME_FN(drm_get_tv_subconnector_name,
* callback. For atomic drivers the remapping to the "ACTIVE" property is
* implemented in the DRM core. This is the only standard connector
* property that userspace can change.
*
* Note that this property cannot be set through the MODE_ATOMIC ioctl,
* userspace must use "ACTIVE" on the CRTC instead.
*
* WARNING:
*
* For userspace also running on legacy drivers the "DPMS" semantics are a
* lot more complicated. First, userspace cannot rely on the "DPMS" value
* returned by the GETCONNECTOR actually reflecting reality, because many
* drivers fail to update it. For atomic drivers this is taken care of in
* drm_atomic_helper_update_legacy_modeset_state().
*
* The second issue is that the DPMS state is only well-defined when the
* connector is connected to a CRTC. In atomic the DRM core enforces that
* "ACTIVE" is off in such a case, no such checks exists for "DPMS".
*
* Finally, when enabling an output using the legacy SETCONFIG ioctl then
* "DPMS" is forced to ON. But see above, that might not be reflected in
* the software value on legacy drivers.
*
* Summarizing: Only set "DPMS" when the connector is known to be enabled,
* assume that a successful SETCONFIG call also sets "DPMS" to on, and
* never read back the value of "DPMS" because it can be incorrect.
* PATH:
* Connector path property to identify how this sink is physically
* connected. Used by DP MST. This should be set by calling