Re: [PATCH v11 2/8] mfd: bd70528: Support ROHM bd70528 PMIC - core

[Vaittinen, Matti]

On Wed, 2019-04-03 at 12:25 +0100, Lee Jones wrote:
> On Wed, 03 Apr 2019, Matti Vaittinen wrote:
> 
> > On Wed, Apr 03, 2019 at 10:30:15AM +0100, Lee Jones wrote:
> > > On Wed, 03 Apr 2019, Matti Vaittinen wrote:
> > > 
> > > > Hello Lee,
> > > > 
> > > > Thanks for taking a look on this again =) I agree with most of
> > > > the
> > > > comments and correct them at next version.
> > > > 
> > > > On Wed, Apr 03, 2019 at 08:31:52AM +0100, Lee Jones wrote:
> > > > > On Mon, 25 Mar 2019, Matti Vaittinen wrote:
> > > > > 
> > > > > > ROHM BD70528MWV is an ultra-low quiescent current general
> > > > > > purpose single-chip power management IC for battery-powered
> > > > > > portable devices.
> > > > > > 
> > > > > > Add MFD core which enables chip access for following
> > > > > > subdevices:
> > > > > > 	- regulators/LED drivers
> > > > > > 	- battery-charger
> > > > > > 	- gpios
> > > > > > 	- 32.768kHz clk
> > > > > > 	- RTC
> > > > > > 	- watchdog
> > > > > > 
> > > > > > Signed-off-by: Matti Vaittinen <
> > > > > > matti.vaittinen@xxxxxxxxxxxxxxxxx>
> > > > > > + * Mapping of main IRQ register bits to sub irq register
> > > > > > offsets so
> > > > > 
> > > > > "sub-IRQ"
> > > > > 
> > > > > > + * that we can access corect sub IRQ registers based on
> > > > > > bits that
> > > > > 
> > > > > "sub IRQ" is also fine, but please standardise.
> > > > > 
> > > > > I do prefer "sub-IRQ" though.
> > > > 
> > > > I'll go with "sub-IRQ" then
> > > > 
> > > > > > +
> > > > > > +#define WD_CTRL_MAGIC1 0x55
> > > > > > +#define WD_CTRL_MAGIC2 0xAA
> > > > > > +/**
> > > > > > + * bd70528_wdt_set - arm or disarm watchdog timer
> > > > > > + *
> > > > > > + * @data:	device data for the PMIC instance we want to
> > > > > > operate on
> > > > > > + * @enable:	new state of WDT. zero to disable, non
> > > > > > zero to enable
> > > > > > + * @old_state:	previous state of WDT will be filled
> > > > > > here
> > > > > > + *
> > > > > > + * Arm or disarm WDT on BD70528 PMIC. Expected to be
> > > > > > called only by
> > > > > > + * BD70528 RTC and BD70528 WDT drivers. The rtc_timer_lock
> > > > > > must be taken
> > > > > > + * by calling bd70528_wdt_lock before calling
> > > > > > bd70528_wdt_set.
> > > > > > + */
> > > > > > +int bd70528_wdt_set(struct rohm_regmap_dev *data, int
> > > > > > enable, int *old_state)
> > > > > 
> > > > > Why doesn't this reside in the watchdog driver?
> > > > 
> > > > If my memory serves me right we shortly discussed this already
> > > > during v8
> > > > review ;) Cant blame you though as I have seen some of the mail
> > > > traffic
> > > > going through your inbox :D
> > > > 
> > > > The motivation to have the functions exported from MFD is to
> > > > not create
> > > > sirect dependency between RTC and WDT. There may be cases where
> > > > we want
> > > > to leave either RTC or WDT out of compilation. MFD is always
> > > > needed so
> > > > the dependency from MFD to RTC/WDT does not harm.
> > > > 
> > > > (Here's some discussion necromancy if you are interested in re-
> > > > reading
> > > > how we did end up with this implementation:
> > > > https://lore.kernel.org/lkml/20190212091723.GZ20638@dell/)
> > > > 
> > > > I hope you are still Ok with having the WDT control functions
> > > > in MFD.
> > > 
> > > OOI, why does the RTC need to control the WDT?
> > 
> > I thought I had a comment about this somewhere in code... O_o Must
> > have
> > been in some development branch I had :/
> > 
> > Anyways, setting the RTC counter may cause watchdog to trigger. It
> > is not
> > further explained why but I would guess watchdog uses RTC counter
> > to check
> > if it should've been pinged already. So RTC needs to disable watch
> > dog for
> > the duration of hwclock setting and enable it again after the new
> > time is
> > set. I can add a comment about this to MFD driver if it helps :)
> 
> How does the user select between using the RTC and the WDT?
> 
> Or are the generally both enabled at the same time?
> 

Both RTC and WDT can be enabled at the same time. But they are not
required to be used. When WDT is enabled, it uses current RTC time as
'base' (and RTC time is running no matter if we have the RTC driver
here or not) - and time-out gets scheduled to specified amount of time
into future. (Same setting timeout into the future happens when WDT is
pinged).

When we set RTC, we disable WDT (if it was enabled), set clock and re-
enable WDT. This causes the previously used time-out value to be set to
WDT again. This works Ok because BD70528 does not support 'short ping
detection'. Only side-effect will be one 'prolonged' WDT feeding period
when RTC is set. (absolute time when RTC was set minus absolute time
when previous WD ping or enable was done) longer than reqular period.

So user should not need to care about this 'dependency'. Basically the
only possible problem I see is that someone could accidentally hang the
system with something that keeps setting the RTC time - this would then
prevent watch dog from doing the reset. This, I believe, is a corner
case which I don't consider now - and if this is considered to be an
issue then such a system may disable the RTC driver and do RTC setting
in a what-ever-manner sees practical.

I'm not sure if I answered to question you asked though =)

Br,
	Matti Vaittinen