[PATCH 0/2] drm/bridge: ti-sn65dsi86: Support backlight controls

[PATCH 0/2] drm/bridge: ti-sn65dsi86: Support backlight controls

[Bjorn Andersson]

The TI SN65DSI86 support driving a backlight driver using a signal
exposed on GPIO4, implement this as a backlight device.

Bjorn Andersson (2):
  dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells
  drm/bridge: ti-sn65dsi86: Expose backlight controls

 .../bindings/display/bridge/ti,sn65dsi86.yaml |   9 +-
 drivers/gpu/drm/bridge/Kconfig                |   1 +
 drivers/gpu/drm/bridge/ti-sn65dsi86.c         | 143 +++++++++++++++++-
 3 files changed, 146 insertions(+), 7 deletions(-)

-- 
2.28.0

[PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells

[Bjorn Andersson]

While the signal on GPIO4 to drive the backlight controller indeed is
pulse width modulated its purpose is specifically to control the
brightness of a backlight.

Drop the #pwm-cells and instead expose a new property to configure the
granularity of the backlight PWM signal.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
---
 .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
index f8622bd0f61e..e380218b4646 100644
--- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
+++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
@@ -66,9 +66,12 @@ properties:
       1-based to match the datasheet.  See ../../gpio/gpio.txt for more
       information.
 
-  '#pwm-cells':
-    const: 1
-    description: See ../../pwm/pwm.yaml for description of the cell formats.
+  ti,backlight-scale:
+    description:
+      The granularity of brightness for the PWM signal provided on GPIO4, if
+      this property is specified.
+    minimum: 0
+    maximum: 65535
 
   ports:
     type: object
-- 
2.28.0

[PATCH 2/2] drm/bridge: ti-sn65dsi86: Expose backlight controls

[Bjorn Andersson]

The TI SN65DSI86 can be configured to generate a PWM pulse on GPIO4,
to be used to drive a backlight driver.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
---
 drivers/gpu/drm/bridge/Kconfig        |   1 +
 drivers/gpu/drm/bridge/ti-sn65dsi86.c | 143 +++++++++++++++++++++++++-
 2 files changed, 140 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
index 43271c21d3fc..eea310bd88e1 100644
--- a/drivers/gpu/drm/bridge/Kconfig
+++ b/drivers/gpu/drm/bridge/Kconfig
@@ -195,6 +195,7 @@ config DRM_TI_SN65DSI86
 	select REGMAP_I2C
 	select DRM_PANEL
 	select DRM_MIPI_DSI
+	select BACKLIGHT_CLASS_DEVICE
 	help
 	  Texas Instruments SN65DSI86 DSI to eDP Bridge driver
 
diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
index 5b6e19ecbc84..41e24d0dbd18 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
@@ -68,6 +68,7 @@
 #define  SN_GPIO_MUX_OUTPUT			1
 #define  SN_GPIO_MUX_SPECIAL			2
 #define  SN_GPIO_MUX_MASK			0x3
+#define  SN_GPIO_MUX_SHIFT(gpio)		((gpio) * 2)
 #define SN_AUX_WDATA_REG(x)			(0x64 + (x))
 #define SN_AUX_ADDR_19_16_REG			0x74
 #define SN_AUX_ADDR_15_8_REG			0x75
@@ -86,6 +87,12 @@
 #define SN_ML_TX_MODE_REG			0x96
 #define  ML_TX_MAIN_LINK_OFF			0
 #define  ML_TX_NORMAL_MODE			BIT(0)
+#define SN_PWM_PRE_DIV_REG			0xA0
+#define SN_BACKLIGHT_SCALE_REG			0xA1
+#define SN_BACKLIGHT_REG			0xA3
+#define SN_PWM_CTL_REG				0xA5
+#define  SN_PWM_ENABLE				BIT(1)
+#define  SN_PWM_INVERT				BIT(0)
 #define SN_AUX_CMD_STATUS_REG			0xF4
 #define  AUX_IRQ_STATUS_AUX_RPLY_TOUT		BIT(3)
 #define  AUX_IRQ_STATUS_AUX_SHORT		BIT(5)
@@ -155,6 +162,8 @@ struct ti_sn_bridge {
 	struct gpio_chip		gchip;
 	DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS);
 #endif
+	u32				brightness;
+	u32				max_brightness;
 };
 
 static const struct regmap_range ti_sn_bridge_volatile_ranges[] = {
@@ -173,6 +182,18 @@ static const struct regmap_config ti_sn_bridge_regmap_config = {
 	.cache_type = REGCACHE_NONE,
 };
 
+static void ti_sn_bridge_read_u16(struct ti_sn_bridge *pdata,
+				  unsigned int reg, u16 *val)
+{
+	unsigned int high;
+	unsigned int low;
+
+	regmap_read(pdata->regmap, reg, &low);
+	regmap_read(pdata->regmap, reg + 1, &high);
+
+	*val = high << 8 | low;
+}
+
 static void ti_sn_bridge_write_u16(struct ti_sn_bridge *pdata,
 				   unsigned int reg, u16 val)
 {
@@ -180,6 +201,50 @@ static void ti_sn_bridge_write_u16(struct ti_sn_bridge *pdata,
 	regmap_write(pdata->regmap, reg + 1, val >> 8);
 }
 
+static int ti_sn_backlight_update(struct ti_sn_bridge *pdata)
+{
+	unsigned int pre_div;
+
+	if (!pdata->max_brightness)
+		return 0;
+
+	/* Enable PWM on GPIO4 */
+	regmap_update_bits(pdata->regmap, SN_GPIO_CTRL_REG,
+			   SN_GPIO_MUX_MASK << SN_GPIO_MUX_SHIFT(4 - 1),
+			   SN_GPIO_MUX_SPECIAL << SN_GPIO_MUX_SHIFT(4 - 1));
+
+	if (pdata->brightness) {
+		/* Set max brightness */
+		ti_sn_bridge_write_u16(pdata, SN_BACKLIGHT_SCALE_REG, pdata->max_brightness);
+
+		/* Set brightness */
+		ti_sn_bridge_write_u16(pdata, SN_BACKLIGHT_REG, pdata->brightness);
+
+		/*
+		 * The PWM frequency is derived from the refclk as:
+		 * PWM_FREQ = REFCLK_FREQ / (PWM_PRE_DIV * BACKLIGHT_SCALE + 1)
+		 *
+		 * A hand wavy estimate based on 12MHz refclk and 500Hz desired
+		 * PWM frequency gives us a pre_div resulting in a PWM
+		 * frequency of between 500 and 1600Hz, depending on the actual
+		 * refclk rate.
+		 *
+		 * One is added to avoid high BACKLIGHT_SCALE values to produce
+		 * a pre_div of 0 - which cancels out the large BACKLIGHT_SCALE
+		 * value.
+		 */
+		pre_div = 12000000 / (500 * pdata->max_brightness) + 1;
+		regmap_write(pdata->regmap, SN_PWM_PRE_DIV_REG, pre_div);
+
+		/* Enable PWM */
+		regmap_update_bits(pdata->regmap, SN_PWM_CTL_REG, SN_PWM_ENABLE, SN_PWM_ENABLE);
+	} else {
+		regmap_update_bits(pdata->regmap, SN_PWM_CTL_REG, SN_PWM_ENABLE, 0);
+	}
+
+	return 0;
+}
+
 static int __maybe_unused ti_sn_bridge_resume(struct device *dev)
 {
 	struct ti_sn_bridge *pdata = dev_get_drvdata(dev);
@@ -193,7 +258,7 @@ static int __maybe_unused ti_sn_bridge_resume(struct device *dev)
 
 	gpiod_set_value(pdata->enable_gpio, 1);
 
-	return ret;
+	return ti_sn_backlight_update(pdata);
 }
 
 static int __maybe_unused ti_sn_bridge_suspend(struct device *dev)
@@ -1010,7 +1075,7 @@ static int ti_sn_bridge_gpio_direction_input(struct gpio_chip *chip,
 					     unsigned int offset)
 {
 	struct ti_sn_bridge *pdata = gpiochip_get_data(chip);
-	int shift = offset * 2;
+	int shift = SN_GPIO_MUX_SHIFT(offset);
 	int ret;
 
 	if (!test_and_clear_bit(offset, pdata->gchip_output))
@@ -1038,7 +1103,7 @@ static int ti_sn_bridge_gpio_direction_output(struct gpio_chip *chip,
 					      unsigned int offset, int val)
 {
 	struct ti_sn_bridge *pdata = gpiochip_get_data(chip);
-	int shift = offset * 2;
+	int shift = SN_GPIO_MUX_SHIFT(offset);
 	int ret;
 
 	if (test_and_set_bit(offset, pdata->gchip_output))
@@ -1073,12 +1138,17 @@ static const char * const ti_sn_bridge_gpio_names[SN_NUM_GPIOS] = {
 
 static int ti_sn_setup_gpio_controller(struct ti_sn_bridge *pdata)
 {
+	int ngpio = SN_NUM_GPIOS;
 	int ret;
 
 	/* Only init if someone is going to use us as a GPIO controller */
 	if (!of_property_read_bool(pdata->dev->of_node, "gpio-controller"))
 		return 0;
 
+	/* If GPIO4 is used for backlight, reduce number of gpios */
+	if (pdata->max_brightness)
+		ngpio--;
+
 	pdata->gchip.label = dev_name(pdata->dev);
 	pdata->gchip.parent = pdata->dev;
 	pdata->gchip.owner = THIS_MODULE;
@@ -1092,7 +1162,7 @@ static int ti_sn_setup_gpio_controller(struct ti_sn_bridge *pdata)
 	pdata->gchip.set = ti_sn_bridge_gpio_set;
 	pdata->gchip.can_sleep = true;
 	pdata->gchip.names = ti_sn_bridge_gpio_names;
-	pdata->gchip.ngpio = SN_NUM_GPIOS;
+	pdata->gchip.ngpio = ngpio;
 	pdata->gchip.base = -1;
 	ret = devm_gpiochip_add_data(pdata->dev, &pdata->gchip, pdata);
 	if (ret)
@@ -1159,6 +1229,65 @@ static void ti_sn_bridge_parse_lanes(struct ti_sn_bridge *pdata,
 	pdata->ln_polrs = ln_polrs;
 }
 
+static int ti_sn_backlight_update_status(struct backlight_device *bl)
+{
+	struct ti_sn_bridge *pdata = bl_get_data(bl);
+	int brightness = bl->props.brightness;
+
+	if (bl->props.power != FB_BLANK_UNBLANK ||
+	    bl->props.fb_blank != FB_BLANK_UNBLANK ||
+	    bl->props.state & BL_CORE_FBBLANK) {
+		pdata->brightness = 0;
+	}
+
+	pdata->brightness = brightness;
+
+	return ti_sn_backlight_update(pdata);
+}
+
+static int ti_sn_backlight_get_brightness(struct backlight_device *bl)
+{
+	struct ti_sn_bridge *pdata = bl_get_data(bl);
+	u16 val;
+
+	ti_sn_bridge_read_u16(pdata, SN_BACKLIGHT_REG, &val);
+
+	return val;
+}
+
+const struct backlight_ops ti_sn_backlight_ops = {
+	.update_status = ti_sn_backlight_update_status,
+	.get_brightness = ti_sn_backlight_get_brightness,
+};
+
+static int ti_sn_backlight_init(struct ti_sn_bridge *pdata)
+{
+	struct backlight_properties props = {};
+	struct backlight_device	*bl;
+	struct device *dev = pdata->dev;
+	struct device_node *np = dev->of_node;
+	int ret;
+
+	ret = of_property_read_u32(np, "ti,backlight-scale", &pdata->max_brightness);
+	if (ret == -EINVAL) {
+		return 0;
+	} else if (ret || pdata->max_brightness >= 0xffff) {
+		DRM_ERROR("invalid max-brightness\n");
+		return -EINVAL;
+	}
+
+	props.type = BACKLIGHT_RAW;
+	props.max_brightness = pdata->max_brightness;
+	bl = devm_backlight_device_register(dev, "sn65dsi86", dev, pdata,
+					    &ti_sn_backlight_ops, &props);
+	if (IS_ERR(bl)) {
+		DRM_ERROR("failed to register backlight device\n");
+		return PTR_ERR(bl);
+	}
+
+	return 0;
+}
+
 static int ti_sn_bridge_probe(struct i2c_client *client,
 			      const struct i2c_device_id *id)
 {
@@ -1224,6 +1353,12 @@ static int ti_sn_bridge_probe(struct i2c_client *client,
 
 	pm_runtime_enable(pdata->dev);
 
+	ret = ti_sn_backlight_init(pdata);
+	if (ret) {
+		pm_runtime_disable(pdata->dev);
+		return ret;
+	}
+
 	ret = ti_sn_setup_gpio_controller(pdata);
 	if (ret) {
 		pm_runtime_disable(pdata->dev);
-- 
2.28.0

Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells

[Steev Klimaszewski]

On 9/30/20 5:35 PM, Bjorn Andersson wrote:
> While the signal on GPIO4 to drive the backlight controller indeed is
> pulse width modulated its purpose is specifically to control the
> brightness of a backlight.
>
> Drop the #pwm-cells and instead expose a new property to configure the
> granularity of the backlight PWM signal.
>
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
>  .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> index f8622bd0f61e..e380218b4646 100644
> --- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> +++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> @@ -66,9 +66,12 @@ properties:
>        1-based to match the datasheet.  See ../../gpio/gpio.txt for more
>        information.
>  
> -  '#pwm-cells':
> -    const: 1
> -    description: See ../../pwm/pwm.yaml for description of the cell formats.
> +  ti,backlight-scale:
> +    description:
> +      The granularity of brightness for the PWM signal provided on GPIO4, if
> +      this property is specified.
> +    minimum: 0
> +    maximum: 65535
>  
>    ports:
>      type: object


Tested-By: Steev Klimaszewski <steev@kali.org>

Re: [PATCH 2/2] drm/bridge: ti-sn65dsi86: Expose backlight controls

[Steev Klimaszewski]

On 9/30/20 5:35 PM, Bjorn Andersson wrote:
> The TI SN65DSI86 can be configured to generate a PWM pulse on GPIO4,
> to be used to drive a backlight driver.
>
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
>  drivers/gpu/drm/bridge/Kconfig        |   1 +
>  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 143 +++++++++++++++++++++++++-
>  2 files changed, 140 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
> index 43271c21d3fc..eea310bd88e1 100644
> --- a/drivers/gpu/drm/bridge/Kconfig
> +++ b/drivers/gpu/drm/bridge/Kconfig
> @@ -195,6 +195,7 @@ config DRM_TI_SN65DSI86
>  	select REGMAP_I2C
>  	select DRM_PANEL
>  	select DRM_MIPI_DSI
> +	select BACKLIGHT_CLASS_DEVICE
>  	help
>  	  Texas Instruments SN65DSI86 DSI to eDP Bridge driver
>  
> diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> index 5b6e19ecbc84..41e24d0dbd18 100644
> --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> @@ -68,6 +68,7 @@
>  #define  SN_GPIO_MUX_OUTPUT			1
>  #define  SN_GPIO_MUX_SPECIAL			2
>  #define  SN_GPIO_MUX_MASK			0x3
> +#define  SN_GPIO_MUX_SHIFT(gpio)		((gpio) * 2)
>  #define SN_AUX_WDATA_REG(x)			(0x64 + (x))
>  #define SN_AUX_ADDR_19_16_REG			0x74
>  #define SN_AUX_ADDR_15_8_REG			0x75
> @@ -86,6 +87,12 @@
>  #define SN_ML_TX_MODE_REG			0x96
>  #define  ML_TX_MAIN_LINK_OFF			0
>  #define  ML_TX_NORMAL_MODE			BIT(0)
> +#define SN_PWM_PRE_DIV_REG			0xA0
> +#define SN_BACKLIGHT_SCALE_REG			0xA1
> +#define SN_BACKLIGHT_REG			0xA3
> +#define SN_PWM_CTL_REG				0xA5
> +#define  SN_PWM_ENABLE				BIT(1)
> +#define  SN_PWM_INVERT				BIT(0)
>  #define SN_AUX_CMD_STATUS_REG			0xF4
>  #define  AUX_IRQ_STATUS_AUX_RPLY_TOUT		BIT(3)
>  #define  AUX_IRQ_STATUS_AUX_SHORT		BIT(5)
> @@ -155,6 +162,8 @@ struct ti_sn_bridge {
>  	struct gpio_chip		gchip;
>  	DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS);
>  #endif
> +	u32				brightness;
> +	u32				max_brightness;
>  };
>  
>  static const struct regmap_range ti_sn_bridge_volatile_ranges[] = {
> @@ -173,6 +182,18 @@ static const struct regmap_config ti_sn_bridge_regmap_config = {
>  	.cache_type = REGCACHE_NONE,
>  };
>  
> +static void ti_sn_bridge_read_u16(struct ti_sn_bridge *pdata,
> +				  unsigned int reg, u16 *val)
> +{
> +	unsigned int high;
> +	unsigned int low;
> +
> +	regmap_read(pdata->regmap, reg, &low);
> +	regmap_read(pdata->regmap, reg + 1, &high);
> +
> +	*val = high << 8 | low;
> +}
> +
>  static void ti_sn_bridge_write_u16(struct ti_sn_bridge *pdata,
>  				   unsigned int reg, u16 val)
>  {
> @@ -180,6 +201,50 @@ static void ti_sn_bridge_write_u16(struct ti_sn_bridge *pdata,
>  	regmap_write(pdata->regmap, reg + 1, val >> 8);
>  }
>  
> +static int ti_sn_backlight_update(struct ti_sn_bridge *pdata)
> +{
> +	unsigned int pre_div;
> +
> +	if (!pdata->max_brightness)
> +		return 0;
> +
> +	/* Enable PWM on GPIO4 */
> +	regmap_update_bits(pdata->regmap, SN_GPIO_CTRL_REG,
> +			   SN_GPIO_MUX_MASK << SN_GPIO_MUX_SHIFT(4 - 1),
> +			   SN_GPIO_MUX_SPECIAL << SN_GPIO_MUX_SHIFT(4 - 1));
> +
> +	if (pdata->brightness) {
> +		/* Set max brightness */
> +		ti_sn_bridge_write_u16(pdata, SN_BACKLIGHT_SCALE_REG, pdata->max_brightness);
> +
> +		/* Set brightness */
> +		ti_sn_bridge_write_u16(pdata, SN_BACKLIGHT_REG, pdata->brightness);
> +
> +		/*
> +		 * The PWM frequency is derived from the refclk as:
> +		 * PWM_FREQ = REFCLK_FREQ / (PWM_PRE_DIV * BACKLIGHT_SCALE + 1)
> +		 *
> +		 * A hand wavy estimate based on 12MHz refclk and 500Hz desired
> +		 * PWM frequency gives us a pre_div resulting in a PWM
> +		 * frequency of between 500 and 1600Hz, depending on the actual
> +		 * refclk rate.
> +		 *
> +		 * One is added to avoid high BACKLIGHT_SCALE values to produce
> +		 * a pre_div of 0 - which cancels out the large BACKLIGHT_SCALE
> +		 * value.
> +		 */
> +		pre_div = 12000000 / (500 * pdata->max_brightness) + 1;
> +		regmap_write(pdata->regmap, SN_PWM_PRE_DIV_REG, pre_div);
> +
> +		/* Enable PWM */
> +		regmap_update_bits(pdata->regmap, SN_PWM_CTL_REG, SN_PWM_ENABLE, SN_PWM_ENABLE);
> +	} else {
> +		regmap_update_bits(pdata->regmap, SN_PWM_CTL_REG, SN_PWM_ENABLE, 0);
> +	}
> +
> +	return 0;
> +}
> +
>  static int __maybe_unused ti_sn_bridge_resume(struct device *dev)
>  {
>  	struct ti_sn_bridge *pdata = dev_get_drvdata(dev);
> @@ -193,7 +258,7 @@ static int __maybe_unused ti_sn_bridge_resume(struct device *dev)
>  
>  	gpiod_set_value(pdata->enable_gpio, 1);
>  
> -	return ret;
> +	return ti_sn_backlight_update(pdata);
>  }
>  
>  static int __maybe_unused ti_sn_bridge_suspend(struct device *dev)
> @@ -1010,7 +1075,7 @@ static int ti_sn_bridge_gpio_direction_input(struct gpio_chip *chip,
>  					     unsigned int offset)
>  {
>  	struct ti_sn_bridge *pdata = gpiochip_get_data(chip);
> -	int shift = offset * 2;
> +	int shift = SN_GPIO_MUX_SHIFT(offset);
>  	int ret;
>  
>  	if (!test_and_clear_bit(offset, pdata->gchip_output))
> @@ -1038,7 +1103,7 @@ static int ti_sn_bridge_gpio_direction_output(struct gpio_chip *chip,
>  					      unsigned int offset, int val)
>  {
>  	struct ti_sn_bridge *pdata = gpiochip_get_data(chip);
> -	int shift = offset * 2;
> +	int shift = SN_GPIO_MUX_SHIFT(offset);
>  	int ret;
>  
>  	if (test_and_set_bit(offset, pdata->gchip_output))
> @@ -1073,12 +1138,17 @@ static const char * const ti_sn_bridge_gpio_names[SN_NUM_GPIOS] = {
>  
>  static int ti_sn_setup_gpio_controller(struct ti_sn_bridge *pdata)
>  {
> +	int ngpio = SN_NUM_GPIOS;
>  	int ret;
>  
>  	/* Only init if someone is going to use us as a GPIO controller */
>  	if (!of_property_read_bool(pdata->dev->of_node, "gpio-controller"))
>  		return 0;
>  
> +	/* If GPIO4 is used for backlight, reduce number of gpios */
> +	if (pdata->max_brightness)
> +		ngpio--;
> +
>  	pdata->gchip.label = dev_name(pdata->dev);
>  	pdata->gchip.parent = pdata->dev;
>  	pdata->gchip.owner = THIS_MODULE;
> @@ -1092,7 +1162,7 @@ static int ti_sn_setup_gpio_controller(struct ti_sn_bridge *pdata)
>  	pdata->gchip.set = ti_sn_bridge_gpio_set;
>  	pdata->gchip.can_sleep = true;
>  	pdata->gchip.names = ti_sn_bridge_gpio_names;
> -	pdata->gchip.ngpio = SN_NUM_GPIOS;
> +	pdata->gchip.ngpio = ngpio;
>  	pdata->gchip.base = -1;
>  	ret = devm_gpiochip_add_data(pdata->dev, &pdata->gchip, pdata);
>  	if (ret)
> @@ -1159,6 +1229,65 @@ static void ti_sn_bridge_parse_lanes(struct ti_sn_bridge *pdata,
>  	pdata->ln_polrs = ln_polrs;
>  }
>  
> +static int ti_sn_backlight_update_status(struct backlight_device *bl)
> +{
> +	struct ti_sn_bridge *pdata = bl_get_data(bl);
> +	int brightness = bl->props.brightness;
> +
> +	if (bl->props.power != FB_BLANK_UNBLANK ||
> +	    bl->props.fb_blank != FB_BLANK_UNBLANK ||
> +	    bl->props.state & BL_CORE_FBBLANK) {
> +		pdata->brightness = 0;
> +	}
> +
> +	pdata->brightness = brightness;
> +
> +	return ti_sn_backlight_update(pdata);
> +}
> +
> +static int ti_sn_backlight_get_brightness(struct backlight_device *bl)
> +{
> +	struct ti_sn_bridge *pdata = bl_get_data(bl);
> +	u16 val;
> +
> +	ti_sn_bridge_read_u16(pdata, SN_BACKLIGHT_REG, &val);
> +
> +	return val;
> +}
> +
> +const struct backlight_ops ti_sn_backlight_ops = {
> +	.update_status = ti_sn_backlight_update_status,
> +	.get_brightness = ti_sn_backlight_get_brightness,
> +};
> +
> +static int ti_sn_backlight_init(struct ti_sn_bridge *pdata)
> +{
> +	struct backlight_properties props = {};
> +	struct backlight_device	*bl;
> +	struct device *dev = pdata->dev;
> +	struct device_node *np = dev->of_node;
> +	int ret;
> +
> +	ret = of_property_read_u32(np, "ti,backlight-scale", &pdata->max_brightness);
> +	if (ret == -EINVAL) {
> +		return 0;
> +	} else if (ret || pdata->max_brightness >= 0xffff) {
> +		DRM_ERROR("invalid max-brightness\n");
> +		return -EINVAL;
> +	}
> +
> +	props.type = BACKLIGHT_RAW;
> +	props.max_brightness = pdata->max_brightness;
> +	bl = devm_backlight_device_register(dev, "sn65dsi86", dev, pdata,
> +					    &ti_sn_backlight_ops, &props);
> +	if (IS_ERR(bl)) {
> +		DRM_ERROR("failed to register backlight device\n");
> +		return PTR_ERR(bl);
> +	}
> +
> +	return 0;
> +}
> +
>  static int ti_sn_bridge_probe(struct i2c_client *client,
>  			      const struct i2c_device_id *id)
>  {
> @@ -1224,6 +1353,12 @@ static int ti_sn_bridge_probe(struct i2c_client *client,
>  
>  	pm_runtime_enable(pdata->dev);
>  
> +	ret = ti_sn_backlight_init(pdata);
> +	if (ret) {
> +		pm_runtime_disable(pdata->dev);
> +		return ret;
> +	}
> +
>  	ret = ti_sn_setup_gpio_controller(pdata);
>  	if (ret) {
>  		pm_runtime_disable(pdata->dev);


Tested-By: Steev Klimaszewski <steev@kali.org>

Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells

[Doug Anderson]

Hi,

On Wed, Sep 30, 2020 at 3:40 PM Bjorn Andersson
<bjorn.andersson@linaro.org> wrote:
>
> While the signal on GPIO4 to drive the backlight controller indeed is
> pulse width modulated its purpose is specifically to control the
> brightness of a backlight.

I'm a bit on the fence about this.  I guess you're doing this because
it avoids some -EPROBE_DEFER cycles in Linux?  It does seem to have a
few downsides, though.

1. It means a bit of re-inventing the wheel.  It's not a very big
wheel, though, I'll give you.  ...but it's still something.

2. I'm not sure why you'd want to, but in theory one could use this
PWM for some other purposes.  It really is just a generic PWM.  Your
change prevents that.



> Drop the #pwm-cells and instead expose a new property to configure the
> granularity of the backlight PWM signal.
>
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
>  .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> index f8622bd0f61e..e380218b4646 100644
> --- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> +++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> @@ -66,9 +66,12 @@ properties:
>        1-based to match the datasheet.  See ../../gpio/gpio.txt for more
>        information.
>
> -  '#pwm-cells':
> -    const: 1
> -    description: See ../../pwm/pwm.yaml for description of the cell formats.
> +  ti,backlight-scale:
> +    description:
> +      The granularity of brightness for the PWM signal provided on GPIO4, if
> +      this property is specified.
> +    minimum: 0
> +    maximum: 65535

A few issues here:

1. Maybe call this "num-steps" instead of backlight-scale.  That's
essentially what it is, right?  Saying how many discrete steps you're
allowing in your backlight?

2. IMO you need the PWM frequency specified, since it can actually
matter.  NOTE: once you have the PWM frequency specified, you could
imagine automatically figuring out what "num-steps" was.  Really you'd
want it to be the largest possible value you could achieve with your
hardware at the specified frequency.  There's no advantage (is there?)
of providing fewer steps to the backlight client.

3. Some backlights are specified inverted.  It looks like this maps
nicely to the bridge chip, which has a bit for it.  Probably nice to
expose this?

Of course, if we were just exposing the PWM directly to Linux we could
just use the PWM backlight driver and it'd all magically work.  ;-)


-Doug

Re: [PATCH 2/2] drm/bridge: ti-sn65dsi86: Expose backlight controls

[Doug Anderson]

Hi,

On Wed, Sep 30, 2020 at 3:40 PM Bjorn Andersson
<bjorn.andersson@linaro.org> wrote:
>
> The TI SN65DSI86 can be configured to generate a PWM pulse on GPIO4,
> to be used to drive a backlight driver.
>
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
>  drivers/gpu/drm/bridge/Kconfig        |   1 +
>  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 143 +++++++++++++++++++++++++-
>  2 files changed, 140 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
> index 43271c21d3fc..eea310bd88e1 100644
> --- a/drivers/gpu/drm/bridge/Kconfig
> +++ b/drivers/gpu/drm/bridge/Kconfig
> @@ -195,6 +195,7 @@ config DRM_TI_SN65DSI86
>         select REGMAP_I2C
>         select DRM_PANEL
>         select DRM_MIPI_DSI
> +       select BACKLIGHT_CLASS_DEVICE
>         help
>           Texas Instruments SN65DSI86 DSI to eDP Bridge driver
>
> diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> index 5b6e19ecbc84..41e24d0dbd18 100644
> --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> @@ -68,6 +68,7 @@
>  #define  SN_GPIO_MUX_OUTPUT                    1
>  #define  SN_GPIO_MUX_SPECIAL                   2
>  #define  SN_GPIO_MUX_MASK                      0x3
> +#define  SN_GPIO_MUX_SHIFT(gpio)               ((gpio) * 2)
>  #define SN_AUX_WDATA_REG(x)                    (0x64 + (x))
>  #define SN_AUX_ADDR_19_16_REG                  0x74
>  #define SN_AUX_ADDR_15_8_REG                   0x75
> @@ -86,6 +87,12 @@
>  #define SN_ML_TX_MODE_REG                      0x96
>  #define  ML_TX_MAIN_LINK_OFF                   0
>  #define  ML_TX_NORMAL_MODE                     BIT(0)
> +#define SN_PWM_PRE_DIV_REG                     0xA0
> +#define SN_BACKLIGHT_SCALE_REG                 0xA1
> +#define SN_BACKLIGHT_REG                       0xA3
> +#define SN_PWM_CTL_REG                         0xA5
> +#define  SN_PWM_ENABLE                         BIT(1)
> +#define  SN_PWM_INVERT                         BIT(0)
>  #define SN_AUX_CMD_STATUS_REG                  0xF4
>  #define  AUX_IRQ_STATUS_AUX_RPLY_TOUT          BIT(3)
>  #define  AUX_IRQ_STATUS_AUX_SHORT              BIT(5)
> @@ -155,6 +162,8 @@ struct ti_sn_bridge {
>         struct gpio_chip                gchip;
>         DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS);
>  #endif
> +       u32                             brightness;
> +       u32                             max_brightness;

You missed adding to the docstring for brightness and max_brightness.

Also: why do you need your own copy of these two values?  Couldn't you
just store the "struct backlight_device *" that came back from
"devm_backlight_device_register()" and then reference
bl->props.brightness / bl->props.max_brightness?


>  };
>
>  static const struct regmap_range ti_sn_bridge_volatile_ranges[] = {
> @@ -173,6 +182,18 @@ static const struct regmap_config ti_sn_bridge_regmap_config = {
>         .cache_type = REGCACHE_NONE,
>  };
>
> +static void ti_sn_bridge_read_u16(struct ti_sn_bridge *pdata,
> +                                 unsigned int reg, u16 *val)
> +{
> +       unsigned int high;
> +       unsigned int low;
> +
> +       regmap_read(pdata->regmap, reg, &low);
> +       regmap_read(pdata->regmap, reg + 1, &high);
> +
> +       *val = high << 8 | low;
> +}

Ideally you should be error checking your reads.  I know this driver
isn't very good about error checking the regmap reads in general, but
probably that should be fixed.  Certainly i2c-backed regmaps can have
failures and you will then do your math on whatever uninitialized
memory was on the stack.  That seems bad.

Presumably you'll then want to return the error code from this
function?  If for some reason you don't, your function should just
return the val instead of passing by reference.


>  static void ti_sn_bridge_write_u16(struct ti_sn_bridge *pdata,
>                                    unsigned int reg, u16 val)
>  {
> @@ -180,6 +201,50 @@ static void ti_sn_bridge_write_u16(struct ti_sn_bridge *pdata,
>         regmap_write(pdata->regmap, reg + 1, val >> 8);
>  }
>
> +static int ti_sn_backlight_update(struct ti_sn_bridge *pdata)
> +{
> +       unsigned int pre_div;
> +
> +       if (!pdata->max_brightness)
> +               return 0;
> +
> +       /* Enable PWM on GPIO4 */
> +       regmap_update_bits(pdata->regmap, SN_GPIO_CTRL_REG,
> +                          SN_GPIO_MUX_MASK << SN_GPIO_MUX_SHIFT(4 - 1),
> +                          SN_GPIO_MUX_SPECIAL << SN_GPIO_MUX_SHIFT(4 - 1));
> +
> +       if (pdata->brightness) {
> +               /* Set max brightness */
> +               ti_sn_bridge_write_u16(pdata, SN_BACKLIGHT_SCALE_REG, pdata->max_brightness);
> +
> +               /* Set brightness */
> +               ti_sn_bridge_write_u16(pdata, SN_BACKLIGHT_REG, pdata->brightness);
> +
> +               /*
> +                * The PWM frequency is derived from the refclk as:
> +                * PWM_FREQ = REFCLK_FREQ / (PWM_PRE_DIV * BACKLIGHT_SCALE + 1)
> +                *
> +                * A hand wavy estimate based on 12MHz refclk and 500Hz desired
> +                * PWM frequency gives us a pre_div resulting in a PWM
> +                * frequency of between 500 and 1600Hz, depending on the actual
> +                * refclk rate.
> +                *
> +                * One is added to avoid high BACKLIGHT_SCALE values to produce
> +                * a pre_div of 0 - which cancels out the large BACKLIGHT_SCALE
> +                * value.
> +                */
> +               pre_div = 12000000 / (500 * pdata->max_brightness) + 1;
> +               regmap_write(pdata->regmap, SN_PWM_PRE_DIV_REG, pre_div);

Different panels have different requirements for PWM frequency.  Some
may also have different duty-cycle to brightness curves that differ
based on the PWM frequency and it would be nice to make sure we know
what frequency we're at rather than getting something random-ish.  It
feels like you need to be less hand-wavy.  You should presumably
specify the desired frequency in the device tree and then do the math.


> +               /* Enable PWM */
> +               regmap_update_bits(pdata->regmap, SN_PWM_CTL_REG, SN_PWM_ENABLE, SN_PWM_ENABLE);
> +       } else {
> +               regmap_update_bits(pdata->regmap, SN_PWM_CTL_REG, SN_PWM_ENABLE, 0);
> +       }

While technically it works OK to conflate brightness = 0 with
backlight disabled (the PWM driver exposed by the Chrome OS EC does,
at least), I believe the API in Linux does make a difference.  Why not
match the Linux API.  If Linux says that the backlight should be at
brightness 50 but should be off, set the brightness to 50 and turn the
backlight off.  If it says set the brightness to 0 and turn it on,
honor it.

I believe (but haven't tested) one side effect of the way you're doing
is is that:

set_brightness(50)
blank()
unblank()
get_brightness()

...will return 0, not 50.  I believe (but haven't tested) that if you
don't implement get_brightness() it would fix things,


> +static int ti_sn_backlight_update_status(struct backlight_device *bl)
> +{
> +       struct ti_sn_bridge *pdata = bl_get_data(bl);
> +       int brightness = bl->props.brightness;
> +
> +       if (bl->props.power != FB_BLANK_UNBLANK ||
> +           bl->props.fb_blank != FB_BLANK_UNBLANK ||
> +           bl->props.state & BL_CORE_FBBLANK) {

backlight_is_blank() instead of open-coding?  ...or you somehow don't
want the extra test for "BL_CORE_SUSPENDED" ?


> +               pdata->brightness = 0;

As per comments in ti_sn_backlight_update(), IMO you want to keep
enabled / disabled state separate from brightness.


> +       }
> +
> +       pdata->brightness = brightness;
> +
> +       return ti_sn_backlight_update(pdata);
> +}

Just to be neat and tidy, I'd expect something in the above would do a
pm_runtime_get_sync() when the backlight first turns on and
pm_runtime_put() when the backlight goes blank.  Right now you're
relying on the fact that the backlight is usually turned on later in
the sequence, but it shouldn't hurt to add an extra pm_runtime
reference and means you're no longer relying on the implicitness.

Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells

[Rob Herring]

On Wed, 30 Sep 2020 17:35:31 -0500, Bjorn Andersson wrote:
> While the signal on GPIO4 to drive the backlight controller indeed is
> pulse width modulated its purpose is specifically to control the
> brightness of a backlight.
> 
> Drop the #pwm-cells and instead expose a new property to configure the
> granularity of the backlight PWM signal.
> 
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
>  .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 


My bot found errors running 'make dt_binding_check' on your patch:

/builds/robherring/linux-dt-review/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml: properties:ti,backlight-scale: {'description': 'The granularity of brightness for the PWM signal provided on GPIO4, if this property is specified.', 'minimum': 0, 'maximum': 65535} is not valid under any of the given schemas (Possible causes of the failure):
	/builds/robherring/linux-dt-review/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml: properties:ti,backlight-scale: 'not' is a required property

/builds/robherring/linux-dt-review/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml: ignoring, error in schema: properties: ti,backlight-scale
warning: no schema found in file: ./Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml


See https://patchwork.ozlabs.org/patch/1374751

If you already ran 'make dt_binding_check' and didn't see the above
error(s), then make sure dt-schema is up to date:

pip3 install git+https://github.com/devicetree-org/dt-schema.git@master --upgrade

Please check and re-submit.

Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells

[Bjorn Andersson]

On Fri 02 Oct 15:42 CDT 2020, Doug Anderson wrote:

> Hi,
> 
> On Wed, Sep 30, 2020 at 3:40 PM Bjorn Andersson
> <bjorn.andersson@linaro.org> wrote:
> >
> > While the signal on GPIO4 to drive the backlight controller indeed is
> > pulse width modulated its purpose is specifically to control the
> > brightness of a backlight.
> 
> I'm a bit on the fence about this.  I guess you're doing this because
> it avoids some -EPROBE_DEFER cycles in Linux?  It does seem to have a
> few downsides, though.
> 

No, the reason for exposing a backlight is that while the thing
certainly is a PWM signal, the description of it and the registers
available to control it surely seems "backlight" to me.

In particular No, the reason for exposing a backlight is that while
while the thing certainly is a PWM signal, the description of it and the
registers available to control it surely seems "backlight" to me.

> 1. It means a bit of re-inventing the wheel.  It's not a very big
> wheel, though, I'll give you.  ...but it's still something.
> 

The main problem I saw with exposing this as a PWM was the fact that we
have both period and frequency to control...

> 2. I'm not sure why you'd want to, but in theory one could use this
> PWM for some other purposes.  It really is just a generic PWM.  Your
> change prevents that.
> 

...and in the even that you use it as a "generic" PWM I'd expect that
the specified period is related to the frequency of the signal. But the
period is documented to be related to the number of brightness steps of
the panel.

> 
> 
> > Drop the #pwm-cells and instead expose a new property to configure the
> > granularity of the backlight PWM signal.
> >
> > Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> > ---
> >  .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > index f8622bd0f61e..e380218b4646 100644
> > --- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > +++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > @@ -66,9 +66,12 @@ properties:
> >        1-based to match the datasheet.  See ../../gpio/gpio.txt for more
> >        information.
> >
> > -  '#pwm-cells':
> > -    const: 1
> > -    description: See ../../pwm/pwm.yaml for description of the cell formats.
> > +  ti,backlight-scale:
> > +    description:
> > +      The granularity of brightness for the PWM signal provided on GPIO4, if
> > +      this property is specified.
> > +    minimum: 0
> > +    maximum: 65535
> 
> A few issues here:
> 
> 1. Maybe call this "num-steps" instead of backlight-scale.  That's
> essentially what it is, right?  Saying how many discrete steps you're
> allowing in your backlight?
> 

That would work, I had it as "max-brightness" for a while as well. But I
reverted to backlight-scale, because that's the name used in the
datasheet.

I'm fine with whatever color of the shed though :)

> 2. IMO you need the PWM frequency specified, since it can actually
> matter.  NOTE: once you have the PWM frequency specified, you could
> imagine automatically figuring out what "num-steps" was.  Really you'd
> want it to be the largest possible value you could achieve with your
> hardware at the specified frequency.  There's no advantage (is there?)
> of providing fewer steps to the backlight client.
> 

I guess there's no problem in having a "num-steps" that is unrelated to
the number of brightness steps of the panel - but I did distinguish them
because the datasheet clearly does so.

> 3. Some backlights are specified inverted.  It looks like this maps
> nicely to the bridge chip, which has a bit for it.  Probably nice to
> expose this?
> 

Yes, that should be covered.

> Of course, if we were just exposing the PWM directly to Linux we could
> just use the PWM backlight driver and it'd all magically work.  ;-)
> 

Please help me figure out how to properly expose this in the PWM api and
I'll be happy to respin it using this - as you say my wheel does look
pretty similar...

Regards,
Bjorn

Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells

[Doug Anderson]

Hi,

On Mon, Nov 2, 2020 at 9:08 AM Bjorn Andersson
<bjorn.andersson@linaro.org> wrote:
>
> On Fri 02 Oct 15:42 CDT 2020, Doug Anderson wrote:
>
> > Hi,
> >
> > On Wed, Sep 30, 2020 at 3:40 PM Bjorn Andersson
> > <bjorn.andersson@linaro.org> wrote:
> > >
> > > While the signal on GPIO4 to drive the backlight controller indeed is
> > > pulse width modulated its purpose is specifically to control the
> > > brightness of a backlight.
> >
> > I'm a bit on the fence about this.  I guess you're doing this because
> > it avoids some -EPROBE_DEFER cycles in Linux?  It does seem to have a
> > few downsides, though.
> >
>
> No, the reason for exposing a backlight is that while the thing
> certainly is a PWM signal, the description of it and the registers
> available to control it surely seems "backlight" to me.
>
> In particular No, the reason for exposing a backlight is that while
> while the thing certainly is a PWM signal, the description of it and the
> registers available to control it surely seems "backlight" to me.
>
> > 1. It means a bit of re-inventing the wheel.  It's not a very big
> > wheel, though, I'll give you.  ...but it's still something.
> >
>
> The main problem I saw with exposing this as a PWM was the fact that we
> have both period and frequency to control...
>
> > 2. I'm not sure why you'd want to, but in theory one could use this
> > PWM for some other purposes.  It really is just a generic PWM.  Your
> > change prevents that.
> >
>
> ...and in the even that you use it as a "generic" PWM I'd expect that
> the specified period is related to the frequency of the signal. But the
> period is documented to be related to the number of brightness steps of
> the panel.

I think the key here is that the "number of brightness steps of the
panel" isn't really a thing that's worried about.  At least in my
experience, you can pretty much just use as many steps as you can
represent based on your PWM hardware.  If a panel happens to map some
of those steps to the same brightness then it wouldn't be the end of
the world, but in experience it's not really such a digital thing.  If
you choose 4096 steps then you likely get 4096 different brightness
levels.  If you choose 256 steps then you get 256 different brightness
levels.  Once you have "more than enough" steps then everything's
pretty much fine.

Looking at one random panel (just to get an idea of numbers), I see
that it specifies:
* min PWM Freq: 200 Hz
* max PWM Freq: 10,000 Hz.

...and refclk is something between 12 MHz and 38.4 MHz, right?

The bridge chip datasheet says:

PWM_FREQ = REFCLK_FREQ / (PWM_PRE_DIV * BACKLIGHT_SCALE + 1)

So let's see what we can do.  I'm arguing that we want the client to
be able to specify the PWM frequency and duty cycle and we'll do the
job of picking the number of steps.  We'll try for the most steps we
can get (65535).

I guess we need to solve for PWM_PRE_DIV :

PWM_FREQ * (PWM_PRE_DIV * BACKLIGHT_SCALE + 1) = REFCLK_FREQ
PWM_PRE_DIV * BACKLIGHT_SCALE + 1 = REFCLK_FREQ / PWM_FREQ
PWM_PRE_DIV * BACKLIGHT_SCALE = REFCLK_FREQ / PWM_FREQ - 1
PWM_PRE_DIV = (REFCLK_FREQ / PWM_FREQ - 1) / BACKLIGHT_SCALE

...and solve for BACKLIGHT_SCALE:
BACKLIGHT_SCALE = (REFCLK_FREQ / PWM_FREQ - 1) / PWM_PRE_DIV


With 1000 Hz, 12 MHz refclk:

PWM_PRE_DIV = DIV_ROUND_UP(12000000 / 1000 - 1, 65535)
=> 1
BACKLIGHT_SCALE = (12000000 / 1000 - 1) / 1
=> 11999

With 1000 Hz, 38.4 MHz refclk:
PWM_PRE_DIV = DIV_ROUND_UP(38400000 / 1000 - 1, 65535)
=> 1
BACKLIGHT_SCALE = (38400000 / 1000 - 1) / 1
=> 38399

With 200 Hz, 38.4 MHz refclk:
PWM_PRE_DIV = DIV_ROUND_UP(38400000 / 200 - 1, 65535)
=> 3
BACKLIGHT_SCALE = (38400000 / 200 - 1) / 3
=> 63999

Now that you have BACKLIGHT_SCALE specified, then when someone tries
to give you a duty cycle you just map it to the closest value you can
make.  Obviously you won't be able to perfectly make every exact duty
cycle / period that a client requests, but that's true of all PWMs out
there.

The nice thing here is that (assuming my math is right) we should be
getting nearly exactly the frequency that the client requested and
that (in my mind) is what matters.  You also get as many steps as
possible which means that (with the PWM backlight API) you'll be able
to get as close as possible to whatever a user requests.


> > > Drop the #pwm-cells and instead expose a new property to configure the
> > > granularity of the backlight PWM signal.
> > >
> > > Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> > > ---
> > >  .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 9 ++++++---
> > >  1 file changed, 6 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > > index f8622bd0f61e..e380218b4646 100644
> > > --- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > > +++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> > > @@ -66,9 +66,12 @@ properties:
> > >        1-based to match the datasheet.  See ../../gpio/gpio.txt for more
> > >        information.
> > >
> > > -  '#pwm-cells':
> > > -    const: 1
> > > -    description: See ../../pwm/pwm.yaml for description of the cell formats.
> > > +  ti,backlight-scale:
> > > +    description:
> > > +      The granularity of brightness for the PWM signal provided on GPIO4, if
> > > +      this property is specified.
> > > +    minimum: 0
> > > +    maximum: 65535
> >
> > A few issues here:
> >
> > 1. Maybe call this "num-steps" instead of backlight-scale.  That's
> > essentially what it is, right?  Saying how many discrete steps you're
> > allowing in your backlight?
> >
>
> That would work, I had it as "max-brightness" for a while as well. But I
> reverted to backlight-scale, because that's the name used in the
> datasheet.
>
> I'm fine with whatever color of the shed though :)
>
> > 2. IMO you need the PWM frequency specified, since it can actually
> > matter.  NOTE: once you have the PWM frequency specified, you could
> > imagine automatically figuring out what "num-steps" was.  Really you'd
> > want it to be the largest possible value you could achieve with your
> > hardware at the specified frequency.  There's no advantage (is there?)
> > of providing fewer steps to the backlight client.
> >
>
> I guess there's no problem in having a "num-steps" that is unrelated to
> the number of brightness steps of the panel - but I did distinguish them
> because the datasheet clearly does so.

I think the datasheet talks about the number of steps that you will be
able to make, but that doesn't mean it has to be what's exposed to
clients of this driver, right?


> > 3. Some backlights are specified inverted.  It looks like this maps
> > nicely to the bridge chip, which has a bit for it.  Probably nice to
> > expose this?
> >
>
> Yes, that should be covered.
>
> > Of course, if we were just exposing the PWM directly to Linux we could
> > just use the PWM backlight driver and it'd all magically work.  ;-)
> >
>
> Please help me figure out how to properly expose this in the PWM api and
> I'll be happy to respin it using this - as you say my wheel does look
> pretty similar...

Hopefully the above seems sane to you?

-Doug