[U-Boot] [PATCH 07/12] imx:mx6 Support LDO bypass

[Robin Gong]

On Thu, Feb 12, 2015 at 04:08:51PM -0800, Tim Harvey wrote:
> On Wed, Feb 11, 2015 at 7:47 AM, Tim Harvey <tharvey at gateworks.com> wrote:
> > On Wed, Feb 11, 2015 at 2:49 AM, Robin Gong <b38343 at freescale.com> wrote:
> > <snip>
> >>>
> >>> This is very board dependent. Here you are referring to a board that
> >>> has a reset input to the PMIC's from the IMX6's watchdog output. In
> >>> this case, this reset routing/pinmux would be needed regardless of
> >>> using ldo-bypass mode or not and that should just be a pinmux of the
> >>> pin your using for WDOG_B.
> >>>
> >> There are two types of reboot in our chip by watchdog : SRS/warm reset
> >> (Software Reset Signal) and WDOG_B(reset signal output to external pmic
> >> to trigger next power cycle). In fact, warm reset can work most cases except
> >> ldo-bypass mode and this is why we connect WDOG_B reset and ldo-bypass here:
> >> kernel may trigger warm reset at the lowest cpu freq setpoint, for example
> >> VDDARM 0.975v at 396Mhz ldo-bypass mode. i.mx6q will reboot with ldo-enable mode
> >> @792Mhz and the VDDARM_IN 0.975v can't support system boot.Thus, we need use
> >> WDOG_B pin to reset external pmic if using ldo-byapss mode.
> >
> > Hi Robin,
> >
> > I understand that configuring WDOG_B external reset must be used when
> > LDO bypass is used. This should be done in the kernel but you can also
> > set this pinmux up in uboot in the board-specific init.
> >
> > If your kernel is providing the PMIC driver and cpufreq driver that
> > alter the cpu core frequencies it must also be configuring pinmux so
> > that WDOG_B gets routed to the pin that connects to the PMIC so any
> > reset based on that wdog (SRS or timeout) issues a hard reset to the
> > PMIC. Failure to do so is a kernel bug. I believe this is done
> > properly in the Freescale kernel for the reference boards.
> >
> > If you are trying to take care of an issue caused by a watchdog reset
> > (SRS or timeout) not properly resetting a PMIC (ie perhaps a PCB
> > errata where signal doesn't go to the PMIC) then you should probably
> > simply set the PMIC rails where they need to be for the 800MHz
> > operation in the bootloader and not muck with the CPU frequency.
> > Honestly, if your PMIC rails are too low for 800MHz on powerup your
> > bootloader may not be stable anyway right? Note that the operating
> > setpoints have the same SOC voltage for both 792MHz and 396MHz, its
> > only the ARM voltage that is lower for 396 vs 792 and chances are your
> > not going to have an issue just in the bootloader at that point.
> >
> 
> Robin,
> 
> The issue your describing was interesting to me and I ran some tests
> and found that on a board with no reset to the PMIC, an IMX6Q CPU,
> ldo-bypass enabled, and the CPU freq set to 400MHz (such that VDD_ARM
> rail set to 0.960V for 400MHz operation) the chip does not even come
> out of reset (ie when SRS is set in the watchdog controller). So I
> don't really see any ability to work around this in bootloader
> software since you won't get there in this case.
Yes, ldo-bypass mode depends on reset PMIC while reboot happen. But even on
this feature supported boards, we'd better provide warm reset in ldo-enable
mode,for example, warm reset can keep printed log in DDR while system crash
in last time and trigger watchdog reset. Someone may use DCDC instead of PMIC,
or some board didn't connect WDOG_B reset with PMIC, have to use ldo-enable
mode.
> 
> Possible solutions I can think of for boards without a PMIC reset is
> to either blow the eFUSE so the chip comes up in 400MHz or in the
> kernel never allow the VDD_ARM or VDD_SOC rail to go below where they
> need to be for CPU startup (the only one that does is VDD_ARM) or
> before soft-reboot make sure the cpufreq is at 800MHz or above (which
> must be done at higher levels before single-cpu mode in
> machine_restart). This also does not deal with the case of a watchdog
> reset and/or a crash handler.
> 
Yes, absolutely. But we suggest connecting PMIC reset pin with WDOG_B or just
use ldo-enable mode if they are not very care of power number.
> Are you findings different?
> 
> Tim