发生异常的现象：

msm8953 lcd在快速亮灭的情况下背光概率性休眠不灭；测量高通pwm，发现正常的时候pwm的管脚LCM_BL_PWM为低电平，失败的时候为高电平；

根据原理图：

mpp是什么？
mpp是基于电源pmic的管脚，也叫做多功能管脚；MPP的全称是Multi Purpose Pin；可以做电源、gpio、ADC、PWM、SINK等功能。

背光的控制方式：

1. LCD控制IC支持动态背光控制功能（CABC）通过解析图像的直方图动态改变输出PWM的占空比从而动态调节LCD的背光，在不改变图像显示效果的情况下降低功耗，PMIC根据CABC的占空比控制背光输出电压；
2. 背光控制部分不经过PMIC而是通过一颗单独的带有boost转换功能的LED驱动器如LM3630A，该芯片通过PWM调节亮度。

我们使用的就是第一种方式；

通过soc->pmi8950(内部pwm)->mpp3的方式去控制。

lcd背光控制调用流程：

首先，我们用的是mipi接口，所以lcd显示驱动是在mdss_dsi.c中，pwm驱动控制是在pwm-qpnp.c文件中(kernel\msm-3.18\drivers\pwm);

在mdss_dsi.c文件中，具体在哪里调用到背光函数呢？

根据打印log，可以知道背光控制函数mdss_dsi_panel_bl_ctrl；

mdss_dsi_panel_bl_ctrl这个函数是在mdss_dsi_panel.c文件中；

调用顺序如下:
mdss_dsi_ctrl_probe -- >
　　mdss_dsi_config_panel -- >
　　　　mdss_dsi_panel_init -- >
　　　　　ctrl_pdata->panel_data.set_backlight = mdss_dsi_panel_bl_ctrl;

根据mdss_dsi_panel_bl_ctrl函数：

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;}
}

我们进入mdss_dsi_panel_bklt_pwm函数来看看：

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;}....
}

进入pwm_disable函数，这里有调用了一个回调函数：

/*** 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);}
}

搜索之后，可以在qpnp_pwm_disable（pwm-qpnp.c文件中找到相应的函数和函数集）:

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);
}

来到qpnp_lpg_configure_pwm_state(chip, QPNP_PWM_DISABLE);这个函数中来：

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;elsevalue = 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);elsevalue = 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会保存着上一次pwm的高低电平的值，

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时候的电平值；所以只要把这段语句去掉，在快速闪灭屏的时候，灭屏就不会出现背光不灭的情况，这是因为寄存器没有写好前，就保存亮屏的高电平值；

LCD背光驱动

在qpnp_lpg_init进入probe函数中，spmi驱动是什么呢？参考这篇文章：
SPMI理解
其实简单理解spmi就是一个通讯协议；

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;注册相应的回调函数；

patch地址

patch地址