[PATCH v4 2/2] drm/bridge: ti-sn65dsi86: Implement the pwm_chip

From: Bjorn Andersson
Date: Tue Jun 22 2021 - 23:28:04 EST

The SN65DSI86 provides the ability to supply a PWM signal on GPIO 4,
with the primary purpose of controlling the backlight of the attached
panel. Add an implementation that exposes this using the standard PWM
framework, to allow e.g. pwm-backlight to expose this to the user.

Signed-off-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
---

Changes since v3:
- Use proper division operations for u64 divisions
- Calculate duty cycle per backlight / (scale + 1)
- Only mux in PWM when state->enabled
- Include linux/bitfield.h (for FIELD_{GET,PUT} on arm)
- Cap max period to avoid overflows

Changes since v2:
- Corrected calculation of scale, to include a 1 instead of 1/NSEC_TO_SEC and
rounded the period up in get_state, to make sure its idempotent
- Changed duty_cycle calculation to make sure it idempotent over my tested period
- Documented "Limitations"
- Documented muxing operation after pm_runtime_get_sync()

drivers/gpu/drm/bridge/ti-sn65dsi86.c | 344 +++++++++++++++++++++++++-
1 file changed, 343 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
index 5d712c8c3c3b..a5a0fa38a0ac 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
@@ -4,7 +4,9 @@
* datasheet: https://www.ti.com/lit/ds/symlink/sn65dsi86.pdf
*/

+#include <linux/atomic.h>
#include <linux/auxiliary_bus.h>
+#include <linux/bitfield.h>
#include <linux/bits.h>
#include <linux/clk.h>
#include <linux/debugfs.h>
@@ -15,6 +17,7 @@
#include <linux/module.h>
#include <linux/of_graph.h>
#include <linux/pm_runtime.h>
+#include <linux/pwm.h>
#include <linux/regmap.h>
#include <linux/regulator/consumer.h>

@@ -91,6 +94,13 @@
#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 BACKLIGHT_SCALE_MAX 0xFFFF
+#define SN_BACKLIGHT_REG 0xA3
+#define SN_PWM_EN_INV_REG 0xA5
+#define SN_PWM_INV_MASK BIT(0)
+#define SN_PWM_EN_MASK BIT(1)
#define SN_AUX_CMD_STATUS_REG 0xF4
#define AUX_IRQ_STATUS_AUX_RPLY_TOUT BIT(3)
#define AUX_IRQ_STATUS_AUX_SHORT BIT(5)
@@ -113,11 +123,14 @@

#define SN_LINK_TRAINING_TRIES 10

+#define SN_PWM_GPIO_IDX 3 /* 4th GPIO */
+
/**
* struct ti_sn65dsi86 - Platform data for ti-sn65dsi86 driver.
* @bridge_aux: AUX-bus sub device for MIPI-to-eDP bridge functionality.
* @gpio_aux: AUX-bus sub device for GPIO controller functionality.
* @aux_aux: AUX-bus sub device for eDP AUX channel functionality.
+ * @pwm_aux: AUX-bus sub device for PWM controller functionality.
*
* @dev: Pointer to the top level (i2c) device.
* @regmap: Regmap for accessing i2c.
@@ -145,11 +158,17 @@
* bitmap so we can do atomic ops on it without an extra
* lock so concurrent users of our 4 GPIOs don't stomp on
* each other's read-modify-write.
+ *
+ * @pchip: pwm_chip if the PWM is exposed.
+ * @pwm_enabled: Used to track if the PWM signal is currently enabled.
+ * @pwm_refclk_freq: Cache for the reference clock input to the PWM.
+ * @pwm_pin_busy: Track if GPIO4 is currently requested for GPIO or PWM.
*/
struct ti_sn65dsi86 {
struct auxiliary_device bridge_aux;
struct auxiliary_device gpio_aux;
struct auxiliary_device aux_aux;
+ struct auxiliary_device pwm_aux;

struct device *dev;
struct regmap *regmap;
@@ -172,6 +191,12 @@ struct ti_sn65dsi86 {
struct gpio_chip gchip;
DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS);
#endif
+#if defined(CONFIG_PWM)
+ struct pwm_chip pchip;
+ bool pwm_enabled;
+ unsigned int pwm_refclk_freq;
+ atomic_t pwm_pin_busy;
+#endif
};

static const struct regmap_range ti_sn65dsi86_volatile_ranges[] = {
@@ -190,6 +215,25 @@ static const struct regmap_config ti_sn65dsi86_regmap_config = {
.cache_type = REGCACHE_NONE,
};

+static int ti_sn65dsi86_read_u16(struct ti_sn65dsi86 *pdata,
+ unsigned int reg, u16 *val)
+{
+ unsigned int tmp;
+ int ret;
+
+ ret = regmap_read(pdata->regmap, reg, &tmp);
+ if (ret)
+ return ret;
+ *val = tmp;
+
+ ret = regmap_read(pdata->regmap, reg + 1, &tmp);
+ if (ret)
+ return ret;
+ *val |= tmp << 8;
+
+ return 0;
+}
+
static void ti_sn65dsi86_write_u16(struct ti_sn65dsi86 *pdata,
unsigned int reg, u16 val)
{
@@ -253,6 +297,14 @@ static void ti_sn_bridge_set_refclk_freq(struct ti_sn65dsi86 *pdata)

regmap_update_bits(pdata->regmap, SN_DPPLL_SRC_REG, REFCLK_FREQ_MASK,
REFCLK_FREQ(i));
+
+#if defined(CONFIG_PWM)
+ /*
+ * The PWM refclk is based on the value written to SN_DPPLL_SRC_REG,
+ * regardless of its actual sourcing.
+ */
+ pdata->pwm_refclk_freq = ti_sn_bridge_refclk_lut[i];
+#endif
}

static void ti_sn65dsi86_enable_comms(struct ti_sn65dsi86 *pdata)
@@ -1044,6 +1096,266 @@ static int ti_sn_bridge_parse_dsi_host(struct ti_sn65dsi86 *pdata)
return 0;
}

+#if defined(CONFIG_PWM)
+static int ti_sn_pwm_pin_request(struct ti_sn65dsi86 *pdata)
+{
+ return atomic_xchg(&pdata->pwm_pin_busy, 1) ? -EBUSY : 0;
+}
+
+static void ti_sn_pwm_pin_release(struct ti_sn65dsi86 *pdata)
+{
+ atomic_set(&pdata->pwm_pin_busy, 0);
+}
+
+static struct ti_sn65dsi86 *pwm_chip_to_ti_sn_bridge(struct pwm_chip *chip)
+{
+ return container_of(chip, struct ti_sn65dsi86, pchip);
+}
+
+static int ti_sn_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
+{
+ struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
+
+ return ti_sn_pwm_pin_request(pdata);
+}
+
+static void ti_sn_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
+{
+ struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
+
+ ti_sn_pwm_pin_release(pdata);
+}
+
+/*
+ * Limitations:
+ * - The PWM signal is not driven when the chip is powered down, or in its
+ * reset state and the driver does not implement the "suspend state"
+ * described in the documentation. In order to save power, state->enabled is
+ * interpreted as denoting if the signal is expected to be valid, and is used to keep
+ * the determine if the chip needs to be kept powered.
+ * - Changing both period and duty_cycle is not done atomically, so the output
+ * might briefly be a mix of the two settings.
+ */
+static int ti_sn_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
+ const struct pwm_state *state)
+{
+ struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
+ unsigned int pwm_en_inv;
+ unsigned int backlight;
+ unsigned int pre_div;
+ unsigned int scale;
+ u64 period_max;
+ u64 actual;
+ u64 period;
+ int ret;
+
+ if (!pdata->pwm_enabled) {
+ ret = pm_runtime_get_sync(pdata->dev);
+ if (ret < 0)
+ return ret;
+ }
+
+ if (state->enabled) {
+ if (!pdata->pwm_enabled) {
+ /*
+ * The chip might have been powered down while we
+ * didn't hold a PM runtime reference, so mux in the
+ * PWM function on the GPIO pin again.
+ */
+ ret = regmap_update_bits(pdata->regmap, SN_GPIO_CTRL_REG,
+ SN_GPIO_MUX_MASK << (2 * SN_PWM_GPIO_IDX),
+ SN_GPIO_MUX_SPECIAL << (2 * SN_PWM_GPIO_IDX));
+ if (ret) {
+ dev_err(pdata->dev, "failed to mux in PWM function\n");
+ goto out;
+ }
+ }
+
+ /*
+ * Per the datasheet the PWM frequency is given by:
+ *
+ * PWM_FREQ = REFCLK_FREQ / (PWM_PRE_DIV * BACKLIGHT_SCALE + 1)
+ *
+ * which can be rewritten:
+ *
+ * T_pwm * REFCLK_FREQ - 1 = PWM_PRE_DIV * BACKLIGHT_SCALE
+ *
+ * In order to keep BACKLIGHT_SCALE within its 16 bits,
+ * PWM_PRE_DIV must be:
+ *
+ * PWM_PRE_DIV >= (T_pwm * REFCLK_FREQ - 1) / BACKLIGHT_SCALE_MAX;
+ *
+ * To simplify the search and optimize the resolution of the
+ * PWM, the lowest possible PWM_PRE_DIV is used. Finally the
+ * scale is calculated as:
+ *
+ * BACKLIGHT_SCALE = (T_pwm * REFCLK_FREQ - 1) / PWM_PRE_DIV
+ *
+ * Here T_pwm is represented in seconds, so appropriate scaling
+ * to nanoseconds is necessary.
+ */
+
+ /* Minimum T_pwm is (0 * 0 + 1) / REFCLK_FREQ */
+ if (state->period <= NSEC_PER_SEC / pdata->pwm_refclk_freq) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ /*
+ * Maximum T_pwm is (255 * 65535 + 1) / * REFCLK_FREQ
+ * Limit period to this to avoid overflows
+ */
+ period_max = div_u64((u64)NSEC_PER_SEC * (255 * 65535 + 1), pdata->pwm_refclk_freq);
+ if (period > period_max)
+ period = period_max;
+ else
+ period = state->period;
+
+ pre_div = DIV64_U64_ROUND_UP((period * pdata->pwm_refclk_freq - NSEC_PER_SEC),
+ ((u64)NSEC_PER_SEC * BACKLIGHT_SCALE_MAX));
+ scale = div64_u64(period * pdata->pwm_refclk_freq - NSEC_PER_SEC,
+ (u64)NSEC_PER_SEC * pre_div);
+
+ /*
+ * The documentation has the duty ratio given as:
+ *
+ * duty BACKLIGHT
+ * ------- = ---------------------
+ * period BACKLIGHT_SCALE + 1
+ *
+ * Solve for BACKLIGHT gives us:
+ */
+ actual = DIV_ROUND_UP_ULL((u64)NSEC_PER_SEC * (pre_div * scale + 1),
+ pdata->pwm_refclk_freq);
+ backlight = div64_u64(state->duty_cycle * (scale + 1), actual);
+ if (backlight > scale)
+ backlight = scale;
+
+ ret = regmap_write(pdata->regmap, SN_PWM_PRE_DIV_REG, pre_div);
+ if (ret) {
+ dev_err(pdata->dev, "failed to update PWM_PRE_DIV\n");
+ goto out;
+ }
+
+ ti_sn65dsi86_write_u16(pdata, SN_BACKLIGHT_SCALE_REG, scale);
+ ti_sn65dsi86_write_u16(pdata, SN_BACKLIGHT_REG, backlight);
+ }
+
+ pwm_en_inv = FIELD_PREP(SN_PWM_EN_MASK, !!state->enabled) |
+ FIELD_PREP(SN_PWM_INV_MASK, state->polarity == PWM_POLARITY_INVERSED);
+ ret = regmap_write(pdata->regmap, SN_PWM_EN_INV_REG, pwm_en_inv);
+ if (ret) {
+ dev_err(pdata->dev, "failed to update PWM_EN/PWM_INV\n");
+ goto out;
+ }
+
+ pdata->pwm_enabled = !!state->enabled;
+out:
+
+ if (!pdata->pwm_enabled)
+ pm_runtime_put_sync(pdata->dev);
+
+ return ret;
+}
+
+static void ti_sn_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
+ struct pwm_state *state)
+{
+ struct ti_sn65dsi86 *pdata = pwm_chip_to_ti_sn_bridge(chip);
+ unsigned int pwm_en_inv;
+ unsigned int pre_div;
+ u16 backlight;
+ u16 scale;
+ int ret;
+
+ ret = regmap_read(pdata->regmap, SN_PWM_EN_INV_REG, &pwm_en_inv);
+ if (ret)
+ return;
+
+ ret = ti_sn65dsi86_read_u16(pdata, SN_BACKLIGHT_SCALE_REG, &scale);
+ if (ret)
+ return;
+
+ ret = ti_sn65dsi86_read_u16(pdata, SN_BACKLIGHT_REG, &backlight);
+ if (ret)
+ return;
+
+ ret = regmap_read(pdata->regmap, SN_PWM_PRE_DIV_REG, &pre_div);
+ if (ret)
+ return;
+
+ state->enabled = FIELD_GET(SN_PWM_EN_MASK, pwm_en_inv);
+ if (FIELD_GET(SN_PWM_INV_MASK, pwm_en_inv))
+ state->polarity = PWM_POLARITY_INVERSED;
+ else
+ state->polarity = PWM_POLARITY_NORMAL;
+
+ state->period = DIV_ROUND_UP_ULL((u64)NSEC_PER_SEC * (pre_div * scale + 1), pdata->pwm_refclk_freq);
+ state->duty_cycle = DIV_ROUND_UP_ULL(state->period * backlight, scale + 1);
+}
+
+static const struct pwm_ops ti_sn_pwm_ops = {
+ .request = ti_sn_pwm_request,
+ .free = ti_sn_pwm_free,
+ .apply = ti_sn_pwm_apply,
+ .get_state = ti_sn_pwm_get_state,
+ .owner = THIS_MODULE,
+};
+
+static int ti_sn_pwm_probe(struct auxiliary_device *adev,
+ const struct auxiliary_device_id *id)
+{
+ struct ti_sn65dsi86 *pdata = dev_get_drvdata(adev->dev.parent);
+
+ pdata->pchip.dev = pdata->dev;
+ pdata->pchip.ops = &ti_sn_pwm_ops;
+ pdata->pchip.npwm = 1;
+ pdata->pchip.of_xlate = of_pwm_single_xlate;
+ pdata->pchip.of_pwm_n_cells = 1;
+
+ return pwmchip_add(&pdata->pchip);
+}
+
+static void ti_sn_pwm_remove(struct auxiliary_device *adev)
+{
+ struct ti_sn65dsi86 *pdata = dev_get_drvdata(adev->dev.parent);
+
+ pwmchip_remove(&pdata->pchip);
+
+ if (pdata->pwm_enabled)
+ pm_runtime_put_sync(pdata->dev);
+}
+
+static const struct auxiliary_device_id ti_sn_pwm_id_table[] = {
+ { .name = "ti_sn65dsi86.pwm", },
+ {},
+};
+
+static struct auxiliary_driver ti_sn_pwm_driver = {
+ .name = "pwm",
+ .probe = ti_sn_pwm_probe,
+ .remove = ti_sn_pwm_remove,
+ .id_table = ti_sn_pwm_id_table,
+};
+
+static int __init ti_sn_pwm_register(void)
+{
+ return auxiliary_driver_register(&ti_sn_pwm_driver);
+}
+
+static void ti_sn_pwm_unregister(void)
+{
+ auxiliary_driver_unregister(&ti_sn_pwm_driver);
+}
+
+#else
+static inline int ti_sn_pwm_pin_request(struct ti_sn65dsi86 *pdata) { return 0; }
+static inline void ti_sn_pwm_pin_release(struct ti_sn65dsi86 *pdata) {}
+
+static inline int ti_sn_pwm_register(void) { return 0; }
+static inline void ti_sn_pwm_unregister(void) {}
+#endif
+
#if defined(CONFIG_OF_GPIO)

static int tn_sn_bridge_of_xlate(struct gpio_chip *chip,
@@ -1176,10 +1488,26 @@ static int ti_sn_bridge_gpio_direction_output(struct gpio_chip *chip,
return ret;
}

+static int ti_sn_bridge_gpio_request(struct gpio_chip *chip, unsigned int offset)
+{
+ struct ti_sn65dsi86 *pdata = gpiochip_get_data(chip);
+
+ if (offset == SN_PWM_GPIO_IDX)
+ return ti_sn_pwm_pin_request(pdata);
+
+ return 0;
+}
+
+
static void ti_sn_bridge_gpio_free(struct gpio_chip *chip, unsigned int offset)
{
+ struct ti_sn65dsi86 *pdata = gpiochip_get_data(chip);
+
/* We won't keep pm_runtime if we're input, so switch there on free */
ti_sn_bridge_gpio_direction_input(chip, offset);
+
+ if (offset == SN_PWM_GPIO_IDX)
+ ti_sn_pwm_pin_release(pdata);
}

static const char * const ti_sn_bridge_gpio_names[SN_NUM_GPIOS] = {
@@ -1201,6 +1529,7 @@ static int ti_sn_gpio_probe(struct auxiliary_device *adev,
pdata->gchip.owner = THIS_MODULE;
pdata->gchip.of_xlate = tn_sn_bridge_of_xlate;
pdata->gchip.of_gpio_n_cells = 2;
+ pdata->gchip.request = ti_sn_bridge_gpio_request;
pdata->gchip.free = ti_sn_bridge_gpio_free;
pdata->gchip.get_direction = ti_sn_bridge_gpio_get_direction;
pdata->gchip.direction_input = ti_sn_bridge_gpio_direction_input;
@@ -1500,6 +1829,12 @@ static int ti_sn65dsi86_probe(struct i2c_client *client,
return ret;
}

+ if (IS_ENABLED(CONFIG_PWM)) {
+ ret = ti_sn65dsi86_add_aux_device(pdata, &pdata->pwm_aux, "pwm");
+ if (ret)
+ return ret;
+ }
+
/*
* NOTE: At the end of the AUX channel probe we'll add the aux device
* for the bridge. This is because the bridge can't be used until the
@@ -1543,10 +1878,14 @@ static int __init ti_sn65dsi86_init(void)
if (ret)
goto err_main_was_registered;

- ret = auxiliary_driver_register(&ti_sn_aux_driver);
+ ret = ti_sn_pwm_register();
if (ret)
goto err_gpio_was_registered;

+ ret = auxiliary_driver_register(&ti_sn_aux_driver);
+ if (ret)
+ goto err_pwm_was_registered;
+
ret = auxiliary_driver_register(&ti_sn_bridge_driver);
if (ret)
goto err_aux_was_registered;
@@ -1555,6 +1894,8 @@ static int __init ti_sn65dsi86_init(void)

err_aux_was_registered:
auxiliary_driver_unregister(&ti_sn_aux_driver);
+err_pwm_was_registered:
+ ti_sn_pwm_unregister();
err_gpio_was_registered:
ti_sn_gpio_unregister();
err_main_was_registered:
@@ -1568,6 +1909,7 @@ static void __exit ti_sn65dsi86_exit(void)
{
auxiliary_driver_unregister(&ti_sn_bridge_driver);
auxiliary_driver_unregister(&ti_sn_aux_driver);
+ ti_sn_pwm_unregister();
ti_sn_gpio_unregister();
i2c_del_driver(&ti_sn65dsi86_driver);
}
--
2.31.0