[PATCHv1 1/6] rtc: m41t80: add support for protected clock

[Rob Herring]

On Tue, Feb 23, 2021 at 02:26:57AM +0100, Sebastian Reichel wrote:
> Hi,
> 
> On Mon, Feb 22, 2021 at 10:26:26PM +0100, Alexandre Belloni wrote:
> > On 22/02/2021 22:20:47+0100, Alexandre Belloni wrote:
> > > On 22/02/2021 18:12:42+0100, Sebastian Reichel wrote:
> > > > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The
> > > > modules SQW clock output defaults to 32768 Hz. This behaviour is
> > > > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed,
> > > > the clock is disabled and all i.MX6 functionality depending on
> > > > the 32 KHz clock has undefined behaviour. On systems using hardware
> > > > watchdog it seems to likely trigger a lot earlier than configured.
> > > > 
> > > > The proper solution would be to describe this dependency in DT,
> > > > but that will result in a deadlock. The kernel will see, that
> > > > i.MX6 system clock needs the RTC clock and do probe deferral.
> > > > But the i.MX6 I2C module never becomes usable without the i.MX6
> > > > CKIL clock and thus the RTC's clock will not be probed. So from
> > > > the kernel's perspective this is a chicken-and-egg problem.
> > > > 
> > > 
> > > Reading the previous paragraph, I was going to suggest describing the
> > > dependency and wondering whether this would cause a circular dependency.
> > > I guess this will keep being an issue for clocks on an I2C or SPI bus...
> 
> Yes, it is a circular dependency on this particular system on
> module. It only works because the RTC enables the clock by
> default. The i.MX6 CKIL is expected to be always enabled.

I think you should describe the circular clocking and then provide a way 
to break the dependency. It's a somewhat common issue.

Rob