linux-kernel - Re: [PATCH v4 25/27] drm/bridge: ti-sn65dsi86: Don't read EDID blob over DDC

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Message-ID: <YILyL+V4tgNX8mck@builder.lan>
Date:   Fri, 23 Apr 2021 11:13:35 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Sam Ravnborg <sam@...nborg.org>, Wolfram Sang <wsa@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>, robdclark@...omium.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Stanislav Lisovskiy <stanislav.lisovskiy@...el.com>,
        Steev Klimaszewski <steev@...i.org>,
        linux-arm-msm@...r.kernel.org, Linus W <linus.walleij@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Robert Foss <robert.foss@...aro.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 25/27] drm/bridge: ti-sn65dsi86: Don't read EDID blob
 over DDC

On Fri 16 Apr 17:39 CDT 2021, Douglas Anderson wrote:

> This is really just a revert of commit 58074b08c04a ("drm/bridge:
> ti-sn65dsi86: Read EDID blob over DDC"), resolving conflicts.
> 
> The old code failed to read the EDID properly in a very important
> case: before the bridge's pre_enable() was called. The way things need
> to work:
> 1. Read the EDID.
> 2. Based on the EDID, decide on video settings and pixel clock.
> 3. Enable the bridge w/ the desired settings.
> 
> The way things were working:
> 1. Try to read the EDID but fail; fall back to hardcoded values.
> 2. Based on hardcoded values, decide on video settings and pixel clock.
> 3. Enable the bridge w/ the desired settings.
> 4. Try again to read the EDID, it works now!
> 5. Realize that the hardcoded settings weren't quite right.
> 6. Disable / reenable the bridge w/ the right settings.
> 
> The reasons for the failures were twofold:
> a) Since we never ran the bridge chip's pre-enable then we never set
>    the bit to ignore HPD. This meant the bridge chip didn't even _try_
>    to go out on the bus and communicate with the panel.
> b) Even if we fixed things to ignore HPD, the EDID still wouldn't read
>    if the panel wasn't on.
> 
> Instead of reverting the code, we could fix it to set the HPD bit and
> also power on the panel. However, it also works nicely to just let the
> panel code read the EDID. Now that we've split the driver up we can
> expose the DDC AUX channel bus to the panel node. The panel can take
> charge of reading the EDID.
> 
> NOTE: in order for things to work, anyone that needs to read the EDID
> will need to add something that looks like this to their panel in the
> dts:
>   ddc-i2c-bus = <&sn65dsi86_bridge>;
> 
> Presumably it's OK to land this without waiting for users to add the
> dts property since the EDID reading was a bit broken anyway, was
> "recently" added, and we know we must have the fallback mode to use
> (since the EDID reading was a bit broken).
> 
> Suggested-by: Andrzej Hajda <a.hajda@...sung.com>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>

Regards,
Bjorn

> ---
> 
> (no changes since v1)
> 
>  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 22 ----------------------
>  1 file changed, 22 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> index 8253098bcdbf..62904dfdee0a 100644
> --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> @@ -125,7 +125,6 @@
>   * @connector:    Our connector.
>   * @host_node:    Remote DSI node.
>   * @dsi:          Our MIPI DSI source.
> - * @edid:         Detected EDID of eDP panel.
>   * @refclk:       Our reference clock.
>   * @panel:        Our panel.
>   * @enable_gpio:  The GPIO we toggle to enable the bridge.
> @@ -156,7 +155,6 @@ struct ti_sn65dsi86 {
>  	struct drm_dp_aux		aux;
>  	struct drm_bridge		bridge;
>  	struct drm_connector		connector;
> -	struct edid			*edid;
>  	struct device_node		*host_node;
>  	struct mipi_dsi_device		*dsi;
>  	struct clk			*refclk;
> @@ -404,24 +402,6 @@ connector_to_ti_sn65dsi86(struct drm_connector *connector)
>  static int ti_sn_bridge_connector_get_modes(struct drm_connector *connector)
>  {
>  	struct ti_sn65dsi86 *pdata = connector_to_ti_sn65dsi86(connector);
> -	struct edid *edid = pdata->edid;
> -	int num, ret;
> -
> -	if (!edid) {
> -		pm_runtime_get_sync(pdata->dev);
> -		edid = pdata->edid = drm_get_edid(connector, &pdata->aux.ddc);
> -		pm_runtime_put_autosuspend(pdata->dev);
> -	}
> -
> -	if (edid && drm_edid_is_valid(edid)) {
> -		ret = drm_connector_update_edid_property(connector, edid);
> -		if (!ret) {
> -			num = drm_add_edid_modes(connector, edid);
> -			if (num)
> -				return num;
> -		}
> -	}
> -
>  	return drm_panel_get_modes(pdata->panel, connector);
>  }
>  
> @@ -1330,8 +1310,6 @@ static void ti_sn_bridge_remove(struct auxiliary_device *adev)
>  		mipi_dsi_device_unregister(pdata->dsi);
>  	}
>  
> -	kfree(pdata->edid);
> -
>  	drm_bridge_remove(&pdata->bridge);
>  
>  	of_node_put(pdata->host_node);
> -- 
> 2.31.1.368.gbe11c130af-goog
>