LDO-Bypass affecting i.MX WDT, enabled in U-Boot

[ehson]

Hello folks,

We're using 4.14 GA release for i.MX6Q SabreSD rev 1.2. Our use-case dictates enabling the WDT in U-Boot, and ping it in userspace via systemd. So we do by enabling IMX_WATCHDOG, HW_WATCHDOG and setting WATCHDOG_TIMEOUT_MSECS to, say, 120000 in U-Boot. We also disable CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED in Linux.

In default DTB, imx6q-sabresd.dtb, ldo-bypass mode is configured. We've observed erratic behavior of WDT when it was triggered in U-Boot. Even with systemd servicing the watchdog through WDIOC_KEEPALIVE on /dev/watchdog, POR happens for some reason. It is like as if servicing WDOG_WSR by Linux driver imx2_wdt.c has no effect at all.

We've also verified that this behavior is not seen when using LDO, or WDT was not enabled in U-Boot. The only difference in the value of WCR, when using correctly working internal LDO mode, is debug-enable-bit. It is set by U-Boot (Why does U-Boot driver set that?) and is write-once-per-reset, but is seen cleared when we check it in PMIC mode. Linux driver doesn't touch this as far as I've looked.

Above is our reason to believe that ldo-bypass is doing something funny in U-Boot, thus causing WDT to go haywire.

[ehson]

Thanks igorpadykov‌ for your explanation. I didn't know that imx6q has a second wdog block purposed this way on SabreSD. Just curious, that in ldo-enabled mode, shouldn't a WDT triggered reset read WDT as reset cause in src_regs->srsr?

Having the cause as POR makes sense for wdog1, which feeds PWRON on PMIC via WDOG_B, but I'm getting the same behavior for wdog2.

[igorpadykov · Accepted solution]

Hi Ahsan

yes there may be differencies between wdog reset between ldo and ldo-bypass

options. Reason for example , if just before reboot  it is under lowest cpu freq setpoint,

but such low voltage (VDD_ARM_IN) can not make system boot up at default 800MHZ .

So solution is to use WDOG_B reset PMIC as it is implemented in Sabre SD schematic

spf-27392 p.21 U507.

Schematics (1)
Design files, including hardware schematics, Gerbers, and OrCAD files.
SABRE Platform for Smart Devices | NXP 

Best regards
igor
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[ehson]

Also, u-boot-imx_2018.03 always reports POR as the reset cause, even if WDT did it. This probably belongs to a separate post, but is not our major concern at the moment.