發生異常的現象：

msm8953 lcd在快速亮滅的情況下背光概率性休眠不滅；測量高通pwm，發現正常的時候pwm的管腳LCM_BL_PWM為低電平，失敗的時候為高電平；

根據原理圖：

mpp是什麽？
mpp是基於電源pmic的管腳，也叫做多功能管腳；MPP的全稱是Multi Purpose Pin；可以做電源、gpio、ADC、PWM、SINK等功能。

背光的控制方式：

1. LCD控制IC支持動態背光控制功能（CABC）通過解析圖像的直方圖動態改變輸出PWM的占空比從而動態調節LCD的背光，在不改變圖像顯示效果的情況下降低功耗，PMIC根據CABC的占空比控制背光輸出電壓；
2. 背光控制部分不經過PMIC而是通過一顆單獨的帶有boost轉換功能的LED驅動器如LM3630A，該芯片通過PWM調節亮度。

我們使用的就是第一種方式；

通過soc->pmi8950(內部pwm)->mpp3的方式去控制。

lcd背光控制調用流程：

首先，我們用的是mipi接口，所以lcd顯示驅動是在mdss_dsi.c中，pwm驅動控制是在pwm-qpnp.c文件中(kernel\msm-3.18\drivers\pwm);

在mdss_dsi.c文件中，具體在哪裏調用到背光函數呢？

根據打印log，可以知道背光控制函數mdss_dsi_panel_bl_ctrl；

mdss_dsi_panel_bl_ctrl這個函數是在mdss_dsi_panel.c文件中；

調用順序如下:
mdss_dsi_ctrl_probe

根據mdss_dsi_panel_bl_ctrl函數：

static void mdss_dsi_panel_bl_ctrl(struct mdss_panel_data *pdata,
                            u32 bl_level)
{
    ......

    /*
     * Some backlight controllers specify a minimum duty cycle
 

     * for the backlight brightness. If the brightness is less
     * than it, the controller can malfunction.
     */

    if ((bl_level < pdata->panel_info.bl_min) && (bl_level != 0))
        bl_level = pdata->panel_info.bl_min;

    switch (ctrl_pdata->bklt_ctrl) {
    case BL_WLED:
        led_trigger_event(bl_led_trigger, bl_level);
        break;
    case BL_PWM:
        mdss_dsi_panel_bklt_pwm(ctrl_pdata, bl_level);
        break;
    case BL_DCS_CMD:
        if (!mdss_dsi_sync_wait_enable(ctrl_pdata)) {
            mdss_dsi_panel_bklt_dcs(ctrl_pdata, bl_level);
            break;
        }
        /*
         * DCS commands to update backlight are usually sent at
         * the same time to both the controllers. However, if
         * sync_wait is enabled, we need to ensure that the
         * dcs commands are first sent to the non-trigger
         * controller so that when the commands are triggered,
         * both controllers receive it at the same time.
         */
        sctrl = mdss_dsi_get_other_ctrl(ctrl_pdata);
        if (mdss_dsi_sync_wait_trigger(ctrl_pdata)) {
            if (sctrl)
                mdss_dsi_panel_bklt_dcs(sctrl, bl_level);
            mdss_dsi_panel_bklt_dcs(ctrl_pdata, bl_level);
        } else {
            mdss_dsi_panel_bklt_dcs(ctrl_pdata, bl_level);
            if (sctrl)
                mdss_dsi_panel_bklt_dcs(sctrl, bl_level);
        }
        break;
    default:
        pr_err("%s: Unknown bl_ctrl configuration\n",
            __func__);
        break;
    }
}

我們進入mdss_dsi_panel_bklt_pwm函數來看看：

static void mdss_dsi_panel_bklt_pwm(struct mdss_dsi_ctrl_pdata *ctrl, int level)
{
    int ret;
    u32 duty;
    u32 period_ns;

    if (ctrl->pwm_bl == NULL) {
        pr_err("%s: no PWM\n", __func__);
        return;
    }

    if (level == 0) {
        if (ctrl->pwm_enabled) {
            ret = pwm_config_us(ctrl->pwm_bl, level,
                    ctrl->pwm_period);
            if (ret)
                pr_err("%s: pwm_config_us() failed err=%d.\n",
                        __func__, ret);
            pwm_disable(ctrl->pwm_bl);
        }
        ctrl->pwm_enabled = 0;
        return;
    }

    ....
}

進入pwm_disable函數，這裏有調用了一個回調函數：

/**
 * pwm_disable() - stop a PWM output toggling
 * @pwm: PWM device
 */
void pwm_disable(struct pwm_device *pwm)
{
    if (pwm && test_and_clear_bit(PWMF_ENABLED, &pwm->flags)) {
        pwm->chip->ops->disable(pwm->chip, pwm);
    }
}

搜索之後，可以在qpnp_pwm_disable（pwm-qpnp.c文件中找到相應的函數和函數集）:

static struct pwm_ops qpnp_pwm_ops = {
    .enable = qpnp_pwm_enable,
    .disable = qpnp_pwm_disable,
    .config = qpnp_pwm_config,
    .free = qpnp_pwm_free,
    .owner = THIS_MODULE,
};

/**
 * qpnp_pwm_disable - stop a PWM output toggling
 * @pwm_chip: the PWM chip
 * @pwm: the PWM device
 */
static void qpnp_pwm_disable(struct pwm_chip *pwm_chip,
        struct pwm_device *pwm)
{

    struct qpnp_pwm_chip    *chip = qpnp_pwm_from_pwm_chip(pwm_chip);
    unsigned long       flags;
    int rc = 0;

    spin_lock_irqsave(&chip->lpg_lock, flags);

    if (QPNP_IS_PWM_CONFIG_SELECTED(
        chip->qpnp_lpg_registers[QPNP_ENABLE_CONTROL]) ||
            chip->flags & QPNP_PWM_LUT_NOT_SUPPORTED)
        rc = qpnp_lpg_configure_pwm_state(chip,
                    QPNP_PWM_DISABLE);
    else if (!(chip->flags & QPNP_PWM_LUT_NOT_SUPPORTED))
        rc = qpnp_lpg_configure_lut_state(chip,
                    QPNP_LUT_DISABLE);

    if (!rc)
        chip->enabled = false;

    spin_unlock_irqrestore(&chip->lpg_lock, flags);

    if (rc)
        pr_err("Failed to disable PWM channel: %d\n",
                    chip->channel_id);
}

來到qpnp_lpg_configure_pwm_state(chip, QPNP_PWM_DISABLE);這個函數中來：

static int qpnp_lpg_configure_pwm_state(struct qpnp_pwm_chip *chip,
                    enum qpnp_pwm_state state)
{
    struct qpnp_lpg_config  *lpg_config = &chip->lpg_config;
    u8          value, mask;
    int         rc;
    bool            test_enable;

    if (chip->sub_type == QPNP_PWM_MODE_ONLY_SUB_TYPE) {
        if (state == QPNP_PWM_ENABLE)
            value = QPNP_ENABLE_PWM_MODE_ONLY_SUB_TYPE;
        else
            value = QPNP_DISABLE_PWM_MODE_ONLY_SUB_TYPE;

        mask = QPNP_PWM_MODE_ONLY_ENABLE_DISABLE_MASK_SUB_TYPE;
    } else {
        if (state == QPNP_PWM_ENABLE)
            value = qpnp_enable_pwm_mode(chip);
        else
            value = QPNP_DISABLE_PWM_MODE(chip);

        mask = QPNP_EN_PWM_HIGH_MASK | QPNP_EN_PWM_LO_MASK |
            QPNP_PWM_SRC_SELECT_MASK | QPNP_PWM_EN_RAMP_GEN_MASK;
        if (chip->sub_type != QPNP_LPG_S_CHAN_SUB_TYPE)
            mask |= QPNP_EN_PWM_OUTPUT_MASK;
    }

    if (chip->in_test_mode) {
        test_enable = (state == QPNP_PWM_ENABLE) ? 1 : 0;
        rc = qpnp_dtest_config(chip, test_enable);
        if (rc)
            pr_err("Failed to configure TEST mode\n");
    }

    pr_debug("pwm_enable_control: %d\n", value);
    rc = qpnp_lpg_save_and_write(value, mask,
        &chip->qpnp_lpg_registers[QPNP_ENABLE_CONTROL],
        SPMI_LPG_REG_ADDR(lpg_config->base_addr,
        QPNP_ENABLE_CONTROL), 1, chip);
    if (rc)
        goto out;

    /*
     * Due to LPG hardware bug, in the PWM mode, having enabled PWM,
     * We have to write PWM values one more time.
     */
    if (state == QPNP_PWM_ENABLE)
        return qpnp_lpg_save_pwm_value(chip);
out:
    return rc;
}

qpnp_lpg_save_pwm_value會保存著上一次pwm的高低電平的值，

rc = qpnp_lpg_save_and_write(value, mask,
            &chip->qpnp_lpg_registers[QPNP_PWM_VALUE_LSB],
            SPMI_LPG_REG_ADDR(lpg_config->base_addr,
            QPNP_PWM_VALUE_LSB), 1, chip);

保存了上一次亮屏的時候的電平值；所以只要把這段語句去掉，在快速閃滅屏的時候，滅屏就不會出現背光不滅的情況，這是因為寄存器沒有寫好前，就保存亮屏的高電平值；

LCD背光驅動

在qpnp_lpg_init進入probe函數中，spmi驅動是什麽呢？參考這篇文章：
SPMI理解
其實簡單理解spmi就是一個通訊協議；

static int qpnp_pwm_probe(struct spmi_device *spmi)
{
    struct qpnp_pwm_chip    *pwm_chip;
    int         rc;

    pwm_chip = kzalloc(sizeof(*pwm_chip), GFP_KERNEL);
    if (pwm_chip == NULL) {
        pr_err("kzalloc() failed.\n");
        return -ENOMEM;
    }

    spin_lock_init(&pwm_chip->lpg_lock);

    pwm_chip->spmi_dev = spmi;
    dev_set_drvdata(&spmi->dev, pwm_chip);

    rc = qpnp_parse_dt_config(spmi, pwm_chip);

    if (rc) {
        pr_err("Failed parsing DT parameters, rc=%d\n", rc);
        goto failed_config;
    }

    pwm_chip->chip.dev = &spmi->dev;
    pwm_chip->chip.ops = &qpnp_pwm_ops;    
    pwm_chip->chip.base = -1;
    pwm_chip->chip.npwm = 1;

    rc = pwmchip_add(&pwm_chip->chip);
    if (rc < 0) {
        pr_err("pwmchip_add() failed: %d\n", rc);
        goto failed_insert;
    }

    if (pwm_chip->channel_owner)
        pwm_chip->chip.pwms[0].label = pwm_chip->channel_owner;

    pr_debug("PWM device sid:%d channel:%d probed successfully\n",
        spmi->sid, pwm_chip->channel_id);
    return 0;

failed_insert:
    kfree(pwm_chip->lpg_config.lut_config.duty_pct_list);
failed_config:
    dev_set_drvdata(&spmi->dev, NULL);
    kfree(pwm_chip);
    return rc;
}

pwm_chip->chip.ops = &qpnp_pwm_ops;註冊相應的回調函數；

patch地址

patch地址

高通LCD的pwm背光驅動