linux-kernel - [PATCH v3 08/12] drm/bridge: ti-sn65dsi86: Power things properly for reading the EDID

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Date:   Fri,  2 Apr 2021 15:28:42 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     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>
Cc:     Linus W <linus.walleij@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        robdclark@...omium.org, Stephen Boyd <swboyd@...omium.org>,
        Steev Klimaszewski <steev@...i.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        linux-arm-msm@...r.kernel.org,
        Stanislav Lisovskiy <stanislav.lisovskiy@...el.com>,
        Douglas Anderson <dianders@...omium.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: [PATCH v3 08/12] drm/bridge: ti-sn65dsi86: Power things properly for reading the EDID

eDP panels won't provide their EDID unless they're powered on. Let's
chain a power-on before we read the EDID. This roughly matches what
was done in 'parade-ps8640.c'.

NOTE: The old code attempted to call pm_runtime_get_sync() before
reading the EDID. While that was enough to power the bridge chip on,
it wasn't enough to talk to the panel for two reasons:
1. 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.
2. Even if we fixed things to ignore HPD, the EDID still wouldn't read
   if the panel wasn't on.

ALSO NOTE: Without the future patch ("drm/panel: panel-simple: Use
runtime pm to avoid excessive unprepare / prepare") there will be boot
speed implications here. Specifically we'll power the panel on to read
the EDID, then fully off. Then we'll likely have to wait the minimum
time between power off and power on.

Fixes: 58074b08c04a ("drm/bridge: ti-sn65dsi86: Read EDID blob over DDC")
Signed-off-by: Douglas Anderson <dianders@...omium.org>
---

Changes in v3:
- Rebased now that we're not moving EDID caching to the core.
- Separating out patch to block AUX channel when not powered.
- Added note about boot speed implications.

 drivers/gpu/drm/bridge/ti-sn65dsi86.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
index 6390bc58f29a..543590801a8e 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
@@ -129,6 +129,7 @@
  * @dp_lanes:     Count of dp_lanes we're using.
  * @ln_assign:    Value to program to the LN_ASSIGN register.
  * @ln_polrs:     Value for the 4-bit LN_POLRS field of SN_ENH_FRAME_REG.
+ * @pre_enabled:  If true then pre_enable() has run.
  *
  * @gchip:        If we expose our GPIOs, this is used.
  * @gchip_output: A cache of whether we've set GPIOs to output.  This
@@ -157,6 +158,7 @@ struct ti_sn_bridge {
 	int				dp_lanes;
 	u8				ln_assign;
 	u8				ln_polrs;
+	bool				pre_enabled;
 
 #if defined(CONFIG_OF_GPIO)
 	struct gpio_chip		gchip;
@@ -270,12 +272,17 @@ static int ti_sn_bridge_connector_get_modes(struct drm_connector *connector)
 {
 	struct ti_sn_bridge *pdata = connector_to_ti_sn_bridge(connector);
 	struct edid *edid = pdata->edid;
+	bool was_enabled;
 	int num;
 
 	if (!edid) {
-		pm_runtime_get_sync(pdata->dev);
+		was_enabled = pdata->pre_enabled;
+
+		if (!was_enabled)
+			drm_bridge_chain_pre_enable(&pdata->bridge);
 		edid = pdata->edid = drm_get_edid(connector, &pdata->aux.ddc);
-		pm_runtime_put(pdata->dev);
+		if (!was_enabled)
+			drm_bridge_chain_post_disable(&pdata->bridge);
 	}
 
 	if (edid && drm_edid_is_valid(edid)) {
@@ -846,12 +853,16 @@ static void ti_sn_bridge_pre_enable(struct drm_bridge *bridge)
 			   HPD_DISABLE);
 
 	drm_panel_prepare(pdata->panel);
+
+	pdata->pre_enabled = true;
 }
 
 static void ti_sn_bridge_post_disable(struct drm_bridge *bridge)
 {
 	struct ti_sn_bridge *pdata = bridge_to_ti_sn_bridge(bridge);
 
+	pdata->pre_enabled = false;
+
 	drm_panel_unprepare(pdata->panel);
 
 	clk_disable_unprepare(pdata->refclk);
-- 
2.31.0.208.g409f899ff0-goog