linux-kernel - [PATCH] backlight: pwm_bl: Fix brightness levels for non-DT case.

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Message-Id: <20180824155402.2822-1-enric.balletbo@collabora.com>
Date:   Fri, 24 Aug 2018 17:54:02 +0200
From:   Enric Balletbo i Serra <enric.balletbo@...labora.com>
To:     linux-kernel@...r.kernel.org, robert.jarzmik@...e.fr,
        daniel.thompson@...aro.org, lee.jones@...aro.org
Cc:     kernel@...labora.com, heiko@...ech.de, linux-pwm@...r.kernel.org,
        linux-fbdev@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        dri-devel@...ts.freedesktop.org, Jingoo Han <jingoohan1@...il.com>
Subject: [PATCH] backlight: pwm_bl: Fix brightness levels for non-DT case.

Commit '88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED
linearly to human eye")' allows the possibility to compute a default
brightness table when there isn't the brightness-levels property in the
DT. Unfortunately the changes made broke the pwm backlight for the
non-DT boards.

Usually, the non-DT boards don't pass the brightness levels via platform
data, instead, they set the max_brightness in their platform data and the
driver calculates the level without a table. The ofending patch assumed
that when there is no brightness levels table we should create one, but this
is clearly wrong for the non-DT case.

After this patch the code handles the DT and the non-DT case taking in
consideration also if max_brightness is set or not. The default table is
only created when neither, brightness-levels and max_brightness, are
set.

The patch also fixes another issue found by Robert. Before this patch
looks like a division by 0 was possible when state.period returned 0 by
pwm_get_state(). This is because pwm_get_state() was called before
pwm_apply_args() so state.period was not initialized. The patch moves
the pwm_apply_args() call before any call to pwm_get_state().

Fixes: '88ba95bedb79 ("backlight: pwm_bl: Compute brightness of LED linearly to human eye")'
Reported-by: Robert Jarzmik <robert.jarzmik@...e.fr>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
---

 drivers/video/backlight/pwm_bl.c | 50 ++++++++++++++++++++++++--------
 1 file changed, 38 insertions(+), 12 deletions(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index bdfcc0a71db1..19770f7bffe4 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -539,7 +539,33 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 
 	dev_dbg(&pdev->dev, "got pwm for backlight\n");
 
-	if (!data->levels) {
+	/*
+	 * FIXME: pwm_apply_args() should be removed when switching to
+	 * the atomic PWM API.
+	 */
+	pwm_apply_args(pb->pwm);
+
+	if (data->levels) {
+		/*
+		 * For the DT case, only when brightness levels is defined
+		 * data->levels is filled. For the non-DT case, data->levels
+		 * can come from platform data, however is not usual.
+		 */
+		for (i = 0; i <= data->max_brightness; i++) {
+			if (data->levels[i] > pb->scale)
+				pb->scale = data->levels[i];
+
+			pb->levels = data->levels;
+		}
+	} else if (!data->max_brightness) {
+		/*
+		 * If no brightness levels are provided and max_brightness is
+		 * not set, use the default brightness table. For the DT case,
+		 * max_brightness is set to 0 when brightness levels is not
+		 * specified. For the non-DT case, max_brightness is usually
+		 * set to some value.
+		 */
+
 		/* Get the PWM period (in nanoseconds) */
 		pwm_get_state(pb->pwm, &state);
 
@@ -550,21 +576,21 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 				"failed to setup default brightness table\n");
 			goto err_alloc;
 		}
-	}
 
-	for (i = 0; i <= data->max_brightness; i++) {
-		if (data->levels[i] > pb->scale)
-			pb->scale = data->levels[i];
+		for (i = 0; i <= data->max_brightness; i++) {
+			if (data->levels[i] > pb->scale)
+				pb->scale = data->levels[i];
 
-		pb->levels = data->levels;
+			pb->levels = data->levels;
+		}
+	} else {
+		/*
+		 * That only happens for the non-DT case, where platform data
+		 * sets the max_brightness value.
+		 */
+		pb->scale = data->max_brightness;
 	}
 
-	/*
-	 * FIXME: pwm_apply_args() should be removed when switching to
-	 * the atomic PWM API.
-	 */
-	pwm_apply_args(pb->pwm);
-
 	/*
 	 * The DT case will set the pwm_period_ns field to 0 and store the
 	 * period, parsed from the DT, in the PWM device. For the non-DT case,
-- 
2.18.0