linux-kernel - Re: [PATCH v8 2/3] drm: bridge: Add support for Cadence MHDP DPI/DP bridge

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Message-ID: <a2f2ff9d-0c52-12d9-23c5-bab35ef8f8f6@ti.com>
Date:   Fri, 14 Aug 2020 11:22:09 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Swapnil Jakhade <sjakhade@...ence.com>
CC:     <airlied@...ux.ie>, <daniel@...ll.ch>, <robh+dt@...nel.org>,
        <a.hajda@...sung.com>, <narmstrong@...libre.com>,
        <jonas@...boo.se>, <jernej.skrabec@...l.net>,
        <dri-devel@...ts.freedesktop.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <mparab@...ence.com>,
        <yamonkar@...ence.com>, <jsarha@...com>, <nsekhar@...com>,
        <praneeth@...com>
Subject: Re: [PATCH v8 2/3] drm: bridge: Add support for Cadence MHDP DPI/DP
 bridge

On 11/08/2020 05:36, Laurent Pinchart wrote:

>> +static int cdns_mhdp_connector_init(struct cdns_mhdp_device *mhdp)
>> +{
>> +	u32 bus_format = MEDIA_BUS_FMT_RGB121212_1X36;
>> +	struct drm_connector *conn = &mhdp->connector;
>> +	struct drm_bridge *bridge = &mhdp->bridge;
>> +	int ret;
>> +
>> +	if (!bridge->encoder) {
>> +		DRM_ERROR("Parent encoder object not found");
>> +		return -ENODEV;
>> +	}
>> +
>> +	conn->polled = DRM_CONNECTOR_POLL_HPD;
>> +
>> +	ret = drm_connector_init(bridge->dev, conn, &cdns_mhdp_conn_funcs,
>> +				 DRM_MODE_CONNECTOR_DisplayPort);
>> +	if (ret) {
>> +		DRM_ERROR("Failed to initialize connector with drm\n");
>> +		return ret;
>> +	}
>> +
>> +	drm_connector_helper_add(conn, &cdns_mhdp_conn_helper_funcs);
>> +
>> +	ret = drm_display_info_set_bus_formats(&conn->display_info,
>> +					       &bus_format, 1);
>> +	if (ret)
>> +		return ret;
>> +
>> +	conn->display_info.bus_flags = DRM_BUS_FLAG_DE_HIGH;
> 
> Aren't these supposed to be retrieved from the display ? Why do we need
> to override them here ?

DE_HIGH is meant for the display controller. I think this should be in bridge->timings->input_bus_flags

>> +static int cdns_mhdp_atomic_check(struct drm_bridge *bridge,
>> +				  struct drm_bridge_state *bridge_state,
>> +				  struct drm_crtc_state *crtc_state,
>> +				  struct drm_connector_state *conn_state)
>> +{
>> +	struct cdns_mhdp_device *mhdp = bridge_to_mhdp(bridge);
>> +	const struct drm_display_mode *mode = &crtc_state->adjusted_mode;
>> +	int ret;
>> +
>> +	if (!mhdp->plugged)
>> +		return 0;
>> +
>> +	mutex_lock(&mhdp->link_mutex);
>> +
>> +	if (!mhdp->link_up) {
>> +		ret = cdns_mhdp_link_up(mhdp);
>> +		if (ret < 0)
>> +			goto err_check;
>> +	}
> 
> atomic_check isn't supposed to access the hardware. Is there a reason
> this is needed ?

We have been going back and forth with this. The basic problem is that to understand which
videomodes can be used, you need to do link training to see the bandwidth available.

I'm not sure if we strictly need to do LT in atomic check, though... It would then pass modes that
can't be used, but perhaps that's not a big issue.

I think the main point with doing LT in certain places is to filter the list of video modes passed
to userspace, as we can't pass the modes from EDID directly without filtering them based on the link
bandwidth.

 Tomi

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