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

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Date:   Mon, 22 Feb 2021 22:26:26 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Sebastian Reichel <sebastian.reichel@...labora.com>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-rtc@...r.kernel.org, linux-mtd@...ts.infradead.org,
        kernel@...labora.com
Subject: Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock

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...
> 
> > Technically everything is fine by not touching anything, since
> > the RTC clock correctly enables the clock on reset (i.e. on
> > battery backup power loss) and also the bootloader enables it
> > in case a kernel without this support has been booted.
> > 
> > The 'protected-clocks' property is already in use for some clocks
> > that may not be touched because of firmware limitations and is
> > described in Documentation/devicetree/bindings/clock/clock-bindings.txt.
> > 
> > Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
> Acked-by: Alexandre Belloni <alexandre.belloni@...tlin.com>

Or maybe you expected me to apply the patch, how are the following
patches dependent on this one?


-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com