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

[Saravana Kannan]

On Wed, Apr 28, 2021 at 3:29 PM Sebastian Reichel
<sebastian.reichel@xxxxxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Sebastian Reichel <sebastian.reichel@xxxxxxxxxxxxx>
> ---
>  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)

Reviewed-by: Saravana Kannan <saravanak@xxxxxxxxxx>

-Saravana