linux-kernel - [PATCH v3 6/9] leds: lm3692x: Make sure we don't exceed the maximum led current

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Message-Id: <5826b77d42521595e93d01d53475a8881cad1875.1578134779.git.agx@sigxcpu.org>
Date:   Sat,  4 Jan 2020 11:54:22 +0100
From:   Guido Günther <agx@...xcpu.org>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>, Dan Murphy <dmurphy@...com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 6/9] leds: lm3692x: Make sure we don't exceed the maximum led current

The current is given by the formular from page 12 of
https://www.ti.com/lit/ds/symlink/lm36922.pdf. We use this to limit the
led's max_brightness using the led-max-microamp DT property.

The formula for the lm36923 is identical according to the data sheet.

Signed-off-by: Guido Günther <agx@...xcpu.org>
Acked-by: Pavel Machek <pavel@....cz>
---
 drivers/leds/leds-lm3692x.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/leds/leds-lm3692x.c b/drivers/leds/leds-lm3692x.c
index ff20560a8263..2e24316566f5 100644
--- a/drivers/leds/leds-lm3692x.c
+++ b/drivers/leds/leds-lm3692x.c
@@ -6,6 +6,7 @@
 #include <linux/i2c.h>
 #include <linux/init.h>
 #include <linux/leds.h>
+#include <linux/log2.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/of.h>
@@ -320,11 +321,29 @@ static int lm3692x_init(struct lm3692x_led *led)
 	return ret;
 }
 
+static enum led_brightness lm3692x_max_brightness(struct lm3692x_led *led,
+						  u32 max_cur)
+{
+	u32 max_code;
+
+	/* see p.12 of LM36922 data sheet for brightness formula */
+	if (led->brightness_ctrl & LM3692X_MAP_MODE_EXP) {
+		/*  228 =~ 1.0 / log2(1.003040572) */
+		max_code = ilog2(max_cur/50) * 228;
+	} else {
+		max_code = ((max_cur * 1000) - 37806) / 12195;
+	}
+	if (max_code > 0x7FF)
+		max_code = 0x7FF;
+
+	return max_code >> 3;
+}
+
 static int lm3692x_probe_dt(struct lm3692x_led *led)
 {
 	struct fwnode_handle *child = NULL;
 	struct led_init_data init_data = {};
-	u32 ovp;
+	u32 ovp, max_cur;
 	bool exp_mode;
 	int ret;
 
@@ -397,6 +416,10 @@ static int lm3692x_probe_dt(struct lm3692x_led *led)
 		return ret;
 	}
 
+	ret = fwnode_property_read_u32(child, "led-max-microamp", &max_cur);
+	led->led_dev.max_brightness = ret ? LED_FULL :
+		lm3692x_max_brightness(led, max_cur);
+
 	init_data.fwnode = child;
 	init_data.devicename = led->client->name;
 	init_data.default_label = ":";
-- 
2.23.0