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From: Stephen Boyd <sboyd@kernel.org>
To: Sebastian Reichel <sebastian.reichel@collabora.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>,
	linux-clk@vger.kernel.org, linux-rtc@vger.kernel.org,
	Alessandro Zummo <a.zummo@towertech.it>,
	Russell King <linux@armlinux.org.uk>,
	Michael Turquette <mturquette@baylibre.com>,
	linux-kernel@vger.kernel.org, kernel@collabora.com
Subject: Re: [RFCv1] rtc: m41t80: disable clock provider support
Date: Tue, 12 Nov 2019 14:20:11 -0800
Message-ID: <20191112222012.157CC20674@mail.kernel.org> (raw)
In-Reply-To: <20191112151526.txl5rwpuiwjpopzx@earth.universe>

Quoting Sebastian Reichel (2019-11-12 07:15:26)
> Hi,
> 
> On Fri, Nov 08, 2019 at 10:53:33PM -0800, Stephen Boyd wrote:
> > Quoting Sebastian Reichel (2019-11-08 17:41:51)
> > > On Fri, Nov 08, 2019 at 04:24:48PM -0800, Stephen Boyd wrote:
> > > > 
> > > > Is this the chicken-egg scenario? I read this thread but I can't follow
> > > > along with what the problem is. Sorry.
> > > 
> > > Yes. The board has an I2C based RTC (m41t62), which provides a programmable 1
> > > Hz to 32 kHz square wave (SQW) output defaulting to 32 kHz. The board designers
> > > connected the RTC's SQW output to the i.MX6 CKIL clock input instead of adding
> > > another oscillator. The i.MX6 CCM acquires that clock in imx6q_clocks_init()
> > > (and assumes it is a fixed clock):
> > > 
> > > hws[IMX6QDL_CLK_CKIL] = imx6q_obtain_fixed_clk_hw(ccm_node, "ckil", 0);
> > 
> > Who uses the IMX6QDL_CLK_CKIL though? Grep on kernel sources shows me
> > nothing.
> 
> The manual specifies, that CKIL is synchronized with the main system
> clock. The resulting clock is used by all kind of IP cores inside
> the i.MX6, for example the SNVS RTC and watchdog. I couldn't find
> any registers to configure the CKIL pipeline and CKIL input is
> usually a fixed clock, so current implementation might be "broken"
> without anyone noticing. Checking a running i.MX6 system, that
> actually seems to be the case :(
> 
> $ cat /sys/kernel/debug/clk/ckil/clk_rate         
> 32768
> $ cat /sys/kernel/debug/clk/ckil/clk_enable_count 
> 0
> $ cat /sys/kernel/debug/clk/ckil/clk_prepare_count 
> 0
> $ cat /sys/kernel/debug/clk/ckil/clk_flags         
> CLK_IS_BASIC
> 
> I suppose an easy fix would be to mark that clock as critical and
> that would also keep the parent clocks enabled?

Yes. It sounds like some sort of low frequency timer clk. It probably
should always be left enabled with CLK_IS_CRITICAL then.


  reply index

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-08 17:01 Sebastian Reichel
2019-11-08 17:53 ` Alexandre Belloni
2019-11-08 22:34   ` Sebastian Reichel
2019-11-09  0:24     ` Stephen Boyd
2019-11-09  1:41       ` Sebastian Reichel
2019-11-09  6:53         ` Stephen Boyd
2019-11-12 15:15           ` Sebastian Reichel
2019-11-12 22:20             ` Stephen Boyd [this message]
2019-11-13 22:27               ` Sebastian Reichel

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