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

[Sebastian Reichel <sre@kernel.org>]

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Hi,

On Sat, Mar 06, 2021 at 11:56:45AM -0800, Rob Herring wrote:
> On Tue, Feb 23, 2021 at 02:26:57AM +0100, Sebastian Reichel wrote:
> > 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.

This is very much not trivial. The clock is required during early
initialization of the i.MX. At this point we are far from probing
I2C drivers and without the I2C driver the clock is not registered.
The current i.MX code expects the system clocks to be fixed clocks,
since they must be enabled before any code is executed (incl.
bootloader) and must never be disabled. From a HW design point of
view it does not make sense to have a SW controllable clock for it,
since it just adds extra cost. I believe for QMX6 it is only SW
controllable, because that avoids the need for an extra crystal.

So how is the clock framework supposed to know, that it can ignore
the clock during registration? I see the following options:

1. My solution is the simplest one. Keep i.MX clock code the same
   (it assumes a fixed-clock being used for CKIL) and avoid
   registering RTC clock. This basically means the RTC is considered
   to be a fixed-clock on this system, which is what the HW designers
   seemed to have in mind (vendor kernel for the QMX6 is old enough
   (4.9.x) to not to have CLK feature in the RTC driver. Vendor
   U-Boot also does not touch the RTC. Booting mainline kernel once
   bricks QMX6 boards until RTC battery is removed, so one could
   actually argue addition of the CLK feature in 1373e77b4f10 (4.13)
   is a regression). Currently Qualcomm device uses "protected-clocks"
   for FW controlled clocks where Linux would crash the system by
   trying to access them. IMHO the RTC is similar, since disabling
   or modifying its frequency on QMX6 results in undefined behaviour
   and possibly system crash.

2. Make i.MX clock code use the RTC as CKIL clock provider, but
   ignore it somehow. I see three sub-options:

2.1. Add a property 'boot-enabled' to the RTC node, so that the
     clock framework is aware of clock being enabled. This can
     be used to satisfy clock dependencies somehow.

2.2. The RTC device is not probed without I2C bus, but the driver
     could also register a fake clock purely based on DT
     information by adding some early init hook and take over
     the clock once the I2C part is being probed. I think this
     is a bad idea regarding maintainability of the driver.
     Also for systems not using the RTC clock, the early clock
     registration is basically wrong: If the kernel disables
     the RTC it will stay disabled across boots if the RTC has
     a backup battery. Basically we cannot imply anything from
     the RTC compatible value alone.

2.3 The i.MX core code could request CKIL with some flag, that
    it's fine to have an unresolvable clock and just expect it
    to be boot-enabled. The rationale would be, that CKIL must
    be always-enabled.

> It's a somewhat common issue.

It is? This only works, because one can treat the RTC's clock
output like a fixed clock by not messing around with it.

-- Sebastian

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