drm/bridge: ti-sn65dsi86: Remove the mystery delay

Let's solve the mystery of commit bf1178c98930 ("drm/bridge:
ti-sn65dsi86: Add mystery delay to enable()").  Specifically the
reason we needed that mystery delay is that we weren't paying
attention to HPD.

Looking at the datasheet for the same panel that was tested for the
original commit, I see there's a timing "t3" that times from power on
to the aux channel being operational.  This time is specced as 0 - 200
ms.  The datasheet says that the aux channel is operational at exactly
the same time that HPD is asserted.

Scoping the signals on this board showed that HPD was asserted 84 ms
after power was asserted.  That very closely matches the magic 70 ms
delay that we had.  ...and actually, in my testing the 70 ms wasn't
quite enough of a delay and some percentage of the time the display
didn't come up until I bumped it to 100 ms (presumably 84 ms would
have worked too).

To solve this, we tried to hook up the HPD signal in the bridge.
...but in doing so we found that that the bridge didn't report that
HPD was asserted until ~280 ms after we powered it (!).  This is
explained by looking at the sn65dsi86 datasheet section "8.4.5.1 HPD
(Hot Plug/Unplug Detection)".  Reading there we see that the bridge
isn't even intended to report HPD until 100 ms after it's asserted.
...but that would have left us at 184 ms.  The extra 100 ms
(presumably) comes from this part in the datasheet:

> The HPD state machine operates off an internal ring oscillator. The
> ring oscillator frequency will vary [ ... ]. The min/max range in
> the HPD State Diagram refers to the possible times based off
> variation in the ring oscillator frequency.

Given that the 280 ms we'll end up delaying if we hook up HPD is
_slower_ than the 200 ms we could just hardcode, for now we'll solve
the problem by just hardcoding a 200 ms delay in the panel driver
using the patch in this series ("drm/panel: simple: Support panels
with HPD where HPD isn't connected").

If we later find a panel that needs to use this bridge where we need
HPD then we'll have to come up with some new code to handle it.  Given
the silly debouncing in the bridge chip, though, it seems unlikely.

One last note is that I tried to solve this through another way: In
ti_sn_bridge_enable() I tried to use various combinations of
dp_dpcd_writeb() and dp_dpcd_readb() to detect when the aux channel
was up.  In theory that would let me detect _exactly_ when I could
continue and do link training.  Unfortunately even if I did an aux
transfer w/out waiting I couldn't see any errors.  Possibly I could
keep looping over link training until it came back with success, but
that seemed a little overly hacky to me.

Reviewed-by: Sean Paul <sean@poorly.run>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181025222134.174583-4-dianders@chromium.org

drivers/gpu/drm/bridge/ti-sn65dsi86.c

@@ -458,18 +458,6 @@ static void ti_sn_bridge_enable(struct drm_bridge *bridge)
 	unsigned int val;
 	int ret;
 
-	/*
-	 * FIXME:
-	 * This 70ms was found necessary by experimentation. If it's not
-	 * present, link training fails. It seems like it can go anywhere from
-	 * pre_enable() up to semi-auto link training initiation below.
-	 *
-	 * Neither the datasheet for the bridge nor the panel tested mention a
-	 * delay of this magnitude in the timing requirements. So for now, add
-	 * the mystery delay until someone figures out a better fix.
-	 */
-	msleep(70);
-
 	/* DSI_A lane config */
 	val = CHA_DSI_LANES(4 - pdata->dsi->lanes);
 	regmap_update_bits(pdata->regmap, SN_DSI_LANES_REG,
@@ -536,7 +524,22 @@ static void ti_sn_bridge_pre_enable(struct drm_bridge *bridge)
 	/* configure bridge ref_clk */
 	ti_sn_bridge_set_refclk_freq(pdata);
 
-	/* in case drm_panel is connected then HPD is not supported */
+	/*
+	 * HPD on this bridge chip is a bit useless.  This is an eDP bridge
+	 * so the HPD is an internal signal that's only there to signal that
+	 * the panel is done powering up.  ...but the bridge chip debounces
+	 * this signal by between 100 ms and 400 ms (depending on process,
+	 * voltage, and temperate--I measured it at about 200 ms).  One
+	 * particular panel asserted HPD 84 ms after it was powered on meaning
+	 * that we saw HPD 284 ms after power on.  ...but the same panel said
+	 * that instead of looking at HPD you could just hardcode a delay of
+	 * 200 ms.  We'll assume that the panel driver will have the hardcoded
+	 * delay in its prepare and always disable HPD.
+	 *
+	 * If HPD somehow makes sense on some future panel we'll have to
+	 * change this to be conditional on someone specifying that HPD should
+	 * be used.
+	 */
 	regmap_update_bits(pdata->regmap, SN_HPD_DISABLE_REG, HPD_DISABLE,
 			   HPD_DISABLE);