Re: [PATCHv2 1/5] rtc: m41t80: add support for fixed clock

[Sebastian Reichel <sebastian.reichel@collabora.com>]

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Hi Alexandre/Alessandro,

Can you merge this patch? It received Reviewed-by from Rob Herring
and Saravana Kannan.

It is required for the last patch adding a DT for a new board, but
there is no compile-time dependency. Thus it can just go through your
tree (just like the second patch has been merged through DRM
already).

Thanks,

-- Sebastian

On Thu, Apr 29, 2021 at 12:29:49AM +0200, 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. For example when using
> the hardware watchdog the system will likely do arbitrary reboots.
> 
> Referencing the m41t62 directly results 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.
> 
> 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 an something (e.g. an unpatched kernel) disabled this
> incorrectly.
> 
> A workaround for this issue is describing the square wave pin
> as fixed-clock, which is registered early and basically how
> this pin is used on the i.MX6.
> 
> Suggested-by: Saravana Kannan <saravanak@google.com>
> Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
> ---
>  Documentation/devicetree/bindings/rtc/rtc-m41t80.txt |  9 +++++++++
>  drivers/rtc/rtc-m41t80.c                             | 12 ++++++++++++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt b/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt
> index c746cb221210..cdd196b1e9bd 100644
> --- a/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt
> +++ b/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt
> @@ -21,10 +21,19 @@ Optional properties:
>                        clock name
>  - wakeup-source: Enables wake up of host system on alarm
>  
> +Optional child node:
> +- clock: Provide this if the square wave pin is used as boot-enabled fixed clock.
> +
>  Example:
>  	rtc@68 {
>  		compatible = "st,m41t80";
>  		reg = <0x68>;
>  		interrupt-parent = <&UIC0>;
>  		interrupts = <0x9 0x8>;
> +
> +		clock {
> +			compatible = "fixed-clock";
> +			#clock-cells = <0>;
> +			clock-frequency = <32768>;
> +		};
>  	};
> diff --git a/drivers/rtc/rtc-m41t80.c b/drivers/rtc/rtc-m41t80.c
> index 89128fc29ccc..b3ece42b6f90 100644
> --- a/drivers/rtc/rtc-m41t80.c
> +++ b/drivers/rtc/rtc-m41t80.c
> @@ -544,10 +544,22 @@ static struct clk *m41t80_sqw_register_clk(struct m41t80_data *m41t80)
>  {
>  	struct i2c_client *client = m41t80->client;
>  	struct device_node *node = client->dev.of_node;
> +	struct device_node *fixed_clock;
>  	struct clk *clk;
>  	struct clk_init_data init;
>  	int ret;
>  
> +	fixed_clock = of_get_child_by_name(node, "clock");
> +	if (fixed_clock) {
> +		/*
> +		 * skip registering square wave clock when a fixed
> +		 * clock has been registered. The fixed clock is
> +		 * registered automatically when being referenced.
> +		 */
> +		of_node_put(fixed_clock);
> +		return 0;
> +	}
> +
>  	/* First disable the clock */
>  	ret = i2c_smbus_read_byte_data(client, M41T80_REG_ALARM_MON);
>  	if (ret < 0)
> -- 
> 2.30.2
> 
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