linux-kernel - Re: [RFCv1] rtc: m41t80: disable clock provider support

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Message-Id: <20191109002449.534B6207FA@mail.kernel.org>
Date:   Fri, 08 Nov 2019 16:24:48 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Sebastian Reichel <sebastian.reichel@...labora.com>
Cc:     linux-clk@...r.kernel.org, linux-rtc@...r.kernel.org,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Russell King <linux@...linux.org.uk>,
        Michael Turquette <mturquette@...libre.com>,
        linux-kernel@...r.kernel.org, kernel@...labora.com
Subject: Re: [RFCv1] rtc: m41t80: disable clock provider support

Quoting Sebastian Reichel (2019-11-08 14:34:15)
> Hi,
> 
> On Fri, Nov 08, 2019 at 06:53:29PM +0100, Alexandre Belloni wrote:
> > On 08/11/2019 18:01:35+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 have undefined behaviour (e.g. the hardware watchdog
> > > run to fast or slow).
> > > 
> > > The normal solution would be to properly describe the clock tree
> > > in DT, but from the kernel's perspective this is a chicken-and-egg
> > > problem: CKIL is required very early, but the clock is only provided
> > > after the I2C RTC has been probed.
> > > 
> > > Technically everything is fine by not touching anything, so this
> > > works around the issue by disabling the clock handling from the
> > > RTC driver. I guess the proper solution would be to simply mark the
> > > clock as always-enabled, but this does not seem to be supported by
> > > the clock framework.
> > > 
> > 
> > You need to have a consumer so this clock is not disabled by the CCF
> > after seeing nobody uses it.
> 
> That's why I was wondering if we can have something like regulator's
> always-enabled for clocks.

There's a flag CLK_IS_CRITICAL that providers can set.

> 
> > If you need it early, you can have a look at rtc-sun6i.c but I
> > would like that to not become a recurrent pattern, especially for
> > discrete RTCs.
> 
> I don't just need it early. The issue is, that CKIL is the 32khz
> low frequency clock fed into the i.MX6. It is initialized by the
> clock manager, so I need it before any of the SoC clocks are
> registered. Without the SoC clocks, the I2C bus cannot be probed
> and thus the RTC driver cannot be probed.
> 

Is this the chicken-egg scenario? I read this thread but I can't follow
along with what the problem is. Sorry.