[PATCH 1/3] drm: panel: simple: Allow timing constraints, not fixed delays

[Douglas Anderson]

The simple panel code currently allows panels to define fixed delays
at certain stages of initialization.  These work OK, but they don't
really map all that clearly to the requirements presented in many
panel datasheets.  Instead of defining a fixed delay, those datasheets
provide a timing diagram and specify a minimum amount of time that
needs to pass from event A to event B.

Because of the way things are currently defined, most panels end up
over-delaying.  One prime example here is that a number of panels I've
looked at define the amount of time that must pass between turning a
panel off and turning it back on again.  Since there is no way to
specify this, many developers have listed this as the "unprepare"
delay.  However, if nobody ever tried to turn the panel on again in
the next 500 ms (or whatever the delay was) then this delay was
pointless.  It's better to do the delay only in the case that someone
tried to turn the panel on too quickly.

Let's support specifying delays as constraints.  We'll start with the
one above and also a second one: the minimum time between prepare
being done and doing the enable.  On the panel I'm looking at, there's
an 80 ms minimum time between HPD being asserted by the panel and
setting the backlight enable GPIO.  By specifying as a constraint we
can enforce this without over-delaying.  Specifically the link
training is allowed to happen in parallel with this delay so adding a
fixed 80 ms delay isn't ideal.

Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

 drivers/gpu/drm/panel/panel-simple.c | 51 ++++++++++++++++++++++++----
 1 file changed, 44 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
index 2be358fb46f7..cbbe71a2a940 100644
--- a/drivers/gpu/drm/panel/panel-simple.c
+++ b/drivers/gpu/drm/panel/panel-simple.c
@@ -92,6 +92,19 @@ struct panel_desc {
 		unsigned int unprepare;
 	} delay;
 
+	/**
+	 * @prepare_to_enable_ms: If this many milliseconds hasn't passed after
+	 *                        prepare finished, add a delay to the start
+	 *                        of enable.
+	 * @unprepare_to_prepare_ms: If this many milliseconds hasn't passed
+	 *                           unprepare finished, add a delay to the
+	 *                           start of prepare.
+	 */
+	struct {
+		unsigned int prepare_to_enable_ms;
+		unsigned int unprepare_to_prepare_ms;
+	} timing_constraints;
+
 	u32 bus_format;
 	u32 bus_flags;
 	int connector_type;
@@ -99,10 +112,12 @@ struct panel_desc {
 
 struct panel_simple {
 	struct drm_panel base;
-	bool prepared;
 	bool enabled;
 	bool no_hpd;
 
+	ktime_t prepared_time;
+	ktime_t unprepared_time;
+
 	const struct panel_desc *desc;
 
 	struct regulator *supply;
@@ -230,6 +245,21 @@ static int panel_simple_get_non_edid_modes(struct panel_simple *panel,
 	return num;
 }
 
+static void panel_simple_enforce_constraint(ktime_t start_ktime,
+					    unsigned int min_ms)
+{
+	ktime_t now_ktime, min_ktime;
+
+	if (!min_ms)
+		return;
+
+	min_ktime = ktime_add(start_ktime, ms_to_ktime(min_ms));
+	now_ktime = ktime_get();
+
+	if (ktime_before(now_ktime, min_ktime))
+		msleep(ktime_to_ms(ktime_sub(min_ktime, now_ktime)) + 1);
+}
+
 static int panel_simple_disable(struct drm_panel *panel)
 {
 	struct panel_simple *p = to_panel_simple(panel);
@@ -249,18 +279,19 @@ static int panel_simple_unprepare(struct drm_panel *panel)
 {
 	struct panel_simple *p = to_panel_simple(panel);
 
-	if (!p->prepared)
+	if (!p->prepared_time)
 		return 0;
 
 	gpiod_set_value_cansleep(p->enable_gpio, 0);
 
 	regulator_disable(p->supply);
 
+	p->prepared_time = 0;
+	p->unprepared_time = ktime_get();
+
 	if (p->desc->delay.unprepare)
 		msleep(p->desc->delay.unprepare);
 
-	p->prepared = false;
-
 	return 0;
 }
 
@@ -296,9 +327,12 @@ static int panel_simple_prepare(struct drm_panel *panel)
 	int err;
 	int hpd_asserted;
 
-	if (p->prepared)
+	if (p->prepared_time)
 		return 0;
 
+	panel_simple_enforce_constraint(p->unprepared_time,
+					p->desc->timing_constraints.unprepare_to_prepare_ms);
+
 	err = regulator_enable(p->supply);
 	if (err < 0) {
 		dev_err(panel->dev, "failed to enable supply: %d\n", err);
@@ -333,7 +367,7 @@ static int panel_simple_prepare(struct drm_panel *panel)
 		}
 	}
 
-	p->prepared = true;
+	p->prepared_time = ktime_get();
 
 	return 0;
 }
@@ -348,6 +382,9 @@ static int panel_simple_enable(struct drm_panel *panel)
 	if (p->desc->delay.enable)
 		msleep(p->desc->delay.enable);
 
+	panel_simple_enforce_constraint(p->prepared_time,
+					p->desc->timing_constraints.prepare_to_enable_ms);
+
 	p->enabled = true;
 
 	return 0;
@@ -514,7 +551,7 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
 		return -ENOMEM;
 
 	panel->enabled = false;
-	panel->prepared = false;
+	panel->prepared_time = 0;
 	panel->desc = desc;
 
 	panel->no_hpd = of_property_read_bool(dev->of_node, "no-hpd");
-- 
2.29.0.rc2.309.g374f81d7ae-goog