linux-imx

// SPDX-License-Identifier: GPL-2.0

/*

 * DisplayPort CEC-Tunneling-over-AUX support

 *

 * Copyright 2018 Cisco Systems, Inc. and/or its affiliates. All rights reserved.

 */

​

#include <linux/kernel.h>

#include <linux/module.h>

#include <linux/slab.h>

#include <drm/drm_dp_helper.h>

#include <media/cec.h>

​

/*

 * Unfortunately it turns out that we have a chicken-and-egg situation

 * here. Quite a few active (mini-)DP-to-HDMI or USB-C-to-HDMI adapters

 * have a converter chip that supports CEC-Tunneling-over-AUX (usually the

 * Parade PS176), but they do not wire up the CEC pin, thus making CEC

 * useless. Note that MegaChips 2900-based adapters appear to have good

 * support for CEC tunneling. Those adapters that I have tested using

 * this chipset all have the CEC line connected.

 *

 * Sadly there is no way for this driver to know this. What happens is

 * that a /dev/cecX device is created that is isolated and unable to see

 * any of the other CEC devices. Quite literally the CEC wire is cut

 * (or in this case, never connected in the first place).

 *

 * The reason so few adapters support this is that this tunneling protocol

 * was never supported by any OS. So there was no easy way of testing it,

 * and no incentive to correctly wire up the CEC pin.

 *

 * Hopefully by creating this driver it will be easier for vendors to

 * finally fix their adapters and test the CEC functionality.

 *

 * I keep a list of known working adapters here:

 *

 * https://hverkuil.home.xs4all.nl/cec-status.txt

 *

 * Please mail me (hverkuil@xs4all.nl) if you find an adapter that works

 * and is not yet listed there.

 *

 * Note that the current implementation does not support CEC over an MST hub.

 * As far as I can see there is no mechanism defined in the DisplayPort

 * standard to transport CEC interrupts over an MST device. It might be

 * possible to do this through polling, but I have not been able to get that

 * to work.

 */

​

/**

 * DOC: dp cec helpers

 *

 * These functions take care of supporting the CEC-Tunneling-over-AUX

 * feature of DisplayPort-to-HDMI adapters.

 */

​

/*

 * When the EDID is unset because the HPD went low, then the CEC DPCD registers

 * typically can no longer be read (true for a DP-to-HDMI adapter since it is

 * powered by the HPD). However, some displays toggle the HPD off and on for a

 * short period for one reason or another, and that would cause the CEC adapter

 * to be removed and added again, even though nothing else changed.

 *

 * This module parameter sets a delay in seconds before the CEC adapter is