[PATCH] drm: Try to document legacy DPMS uapi a bit better

[Sean Paul]

On Wed, Sep 20, 2017 at 3:59 PM, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> 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 at ideasonboard.com>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Sean Paul <seanpaul at chromium.org>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Eric Anholt <eric at anholt.net>
> Reviewed-by: Eric Anholt <eric at anholt.net>

v2 is much better, IMO, thanks.

I'd recommend waiting for Laurent's R-b, but fwiw,

Reviewed-by: Sean Paul <seanpaul at chromium.org>

> ---
>  drivers/gpu/drm/drm_connector.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index bb2e60f5feb6..d8ca526ca4ee 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -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
> --
> 2.9.5
>