Re: [PATCH 4/6] ARM: i.MX8MP: skov: halt startup until power is good

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Marco,

On 4/14/25 12:56, Marco Felsch wrote:
> On 25-04-14, Ahmad Fatoum wrote:
>> Hello Marco,
>>
>> On 14.04.25 11:21, Marco Felsch wrote:
>>> On 25-04-14, Ahmad Fatoum wrote:
>>>> Hello Marco,
>>>>
>>>> On 14.04.25 10:40, Marco Felsch wrote:
>>>>> Hi,
>>>>>
>>>>> On 25-04-14, Ahmad Fatoum wrote:
>>>>>> The 24V regulator supplying the system can indicate via a GPIO imminent
>>>>>> voltage loss. The board's capacitors hold enough charge to power the system
>>>>>> a while longer in such a state, but eventually, unless external power is
>>>>>> restored, the brownout detection of the PMIC will kick in.
>>>>>>
>>>>>> For the span of time between voltage drop detection and PMIC brownout,
>>>>>> let's detect this situation and delay startup. This way, Linux can
>>>>>> detect the ongoing voltage loss, power down the eMMC, reboot into
>>>>>> barebox and barebox will delay boot as long as the problem persists.
>>>>>>
>>>>>> Co-developed-by: Kim Christensen <kch@skov.dk>
>>>>>> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
>>>>>> ---
>>>>>>  arch/arm/boards/skov-imx8mp/lowlevel.c | 60 ++++++++++++++++++++++++++
>>>>>>  1 file changed, 60 insertions(+)
>>>>>>
>>>>>> diff --git a/arch/arm/boards/skov-imx8mp/lowlevel.c b/arch/arm/boards/skov-imx8mp/lowlevel.c
>>>>>> index 637fc50b3f30..d7bd771f259d 100644
>>>>>> --- a/arch/arm/boards/skov-imx8mp/lowlevel.c
>>>>>> +++ b/arch/arm/boards/skov-imx8mp/lowlevel.c
>>>>>> @@ -20,6 +20,9 @@
>>>>>>  
>>>>>>  extern char __dtb_z_imx8mp_skov_start[];
>>>>>>  
>>>>>> +#define PGOOD_PAD_CTRL  MUX_PAD_CTRL(MX8MP_PAD_CTL_PUE | \
>>>>>> +				     MX8MP_PAD_CTL_PE)
>>>>>> +
>>>>>>  #define UART_PAD_CTRL   MUX_PAD_CTRL(MX8MP_PAD_CTL_DSE6 | \
>>>>>>  				     MX8MP_PAD_CTL_FSEL | \
>>>>>>  				     MX8MP_PAD_CTL_PUE | \
>>>>>> @@ -30,6 +33,12 @@ extern char __dtb_z_imx8mp_skov_start[];
>>>>>>  				     MX8MP_PAD_CTL_PUE | \
>>>>>>  				     MX8MP_PAD_CTL_PE)
>>>>>>  
>>>>>> +static inline void led_d1_toggle(bool *on)
>>>>>> +{
>>>>>> +	imx8m_gpio_direction_output(IOMEM(MX8MP_GPIO1_BASE_ADDR), 5, *on);
>>>>>> +	*on = !*on;
>>>>>> +}
>>>>>> +
>>>>>>  static void setup_uart(void)
>>>>>>  {
>>>>>>  	void __iomem *uart = IOMEM(MX8M_UART2_BASE_ADDR);
>>>>>> @@ -67,6 +76,55 @@ static struct pmic_config pca9450_cfg[] = {
>>>>>>  	{ PCA9450_BUCK2OUT_DVS0, 0x14 },
>>>>>>  };
>>>>>>  
>>>>>> +static inline bool power_good(void)
>>>>>> +{
>>>>>> +	/* IMX_SHDN_MF in schematics */
>>>>>> +	return imx8m_gpio_val(IOMEM(MX8MP_GPIO4_BASE_ADDR), 23);
>>>>>> +}
>>>>>> +
>>>>>> +static void wait_for_power_good(void)
>>>>>> +{
>>>>>> +	void __iomem *gpio4 = IOMEM(MX8MP_GPIO4_BASE_ADDR);
>>>>>> +	int timeout_ms = 0;
>>>>>> +	bool led_active = true;
>>>>>> +
>>>>>> +	imx8mp_setup_pad(MX8MP_PAD_SAI2_RXD0__GPIO4_IO23 | PGOOD_PAD_CTRL);
>>>>>> +	imx8m_gpio_direction_input(gpio4, 23);
>>>>>> +
>>>>>> +	led_d1_toggle(&led_active);
>>>>>> +
>>>>>> +	if (power_good())
>>>>>> +		return;
>>>>>> +
>>>>>> +	pr_warn("\nDelaying boot until power stabilizes\n");
>>>>>> +
>>>>>> +	/* If we reach this, because Linux did a hw_protection_reboot, we don't
>>>>>> +	 * want to continue booting right away.
>>>>>> +	 *
>>>>>> +	 * Thus let's either wait for the condition to subscede or for voltage
>>>>>> +	 * to reach a low enough level for the PMIC to detect VSYS_UVLO going
>>>>>> +	 * lower than allowed
>>>>>> +	 */
>>>>>> +
>>>>>> +	while (1) {
>>>>>> +		if (power_good()) {
>>>>>> +			/* wait 10ms longer and check if it still good */
>>>>>> +			udelay(10000);
>>>>>> +			if (power_good()) {
>>>>>> +				pr_info("IMX_SHDN_MF stuck low for ~%ums.\n", timeout_ms);
>>>>>> +				break;
>>>>>> +			}
>>>>>> +		}
>>>>>> +		/* fast blink LED D1 */
>>>>>> +		if (timeout_ms % 100 == 0) {
>>>>>> +			pr_debug(".");
>>>>>> +			led_d1_toggle(&led_active);
>>>>>> +		}
>>>>>> +		udelay(1000);
>>>>>> +		timeout_ms++;
>>>>>> +	}
>>>>>> +}
>>>>>> +
>>>>>>  static void power_init_board(void)
>>>>>>  {
>>>>>>  	struct pbl_i2c *i2c;
>>>>>> @@ -77,6 +135,8 @@ static void power_init_board(void)
>>>>>>  	imx8mp_setup_pad(MX8MP_PAD_SAI3_TXD__GPIO5_IO01);
>>>>>>  	imx8m_gpio_direction_output(IOMEM(MX8MP_GPIO5_BASE_ADDR), 1, 0);
>>>>>>  
>>>>>> +	wait_for_power_good();
>>>>>
>>>>> out of curiosity, wouldn't it be more useful to do the check right
>>>>> before the system wants to boot the kernel and abort in that case?
>>>>
>>>> How so?
>>>>
>>>> When we have brownout, we usually never make it out of the PBL,
>>>
>>> Okay, since your backup charge is really low already.
>>>
>>>> so I prefer the SoC gets powered off during this loop instead
>>>> of during the RAM setup.
>>>
>>> I understand that to wait for the power to become good again, but what
>>> if the power is getting good and changes to bad again once you entered
>>> barebox? Do you get notified about such an event?
>>>
>>> IMHO the later point a bit more important since this ensures that the
>>> system was in power-good state before we the system starts.
>>
>> Checking just before booting is insufficient anyway, because power good
>> may fluctuate before the Linux driver starts taking care of the power
>> good GPIO. Given that we need to handle this anyway, I don't think
>> it makes much difference whether when we check the power good GPIO
>> during barebox runtime.
> 
> I know, but still it is good to check and to wait for power-good state
> before you cont. booting. Like you do for barebox: you wait till the
> power is good or die within the PBL.

I will take note of this, so I can revisit it with Oleksij in the future.

Thanks,
Ahmad

> 
> Regards,
>   Marco
> 
> 
>>
>> Cheers,
>> Ahmad
>>
>>>
>>> Regards,
>>>   Marco
>>>
>>
>>
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>>
> 

-- 
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