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@xxxxxxxxxxxxx> Acked-by: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxx> > --- > Documentation/devicetree/bindings/rtc/rtc-m41t80.txt | 1 + > drivers/rtc/rtc-m41t80.c | 3 +++ > 2 files changed, 4 insertions(+) > > diff --git a/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt b/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt > index c746cb221210..ea4bbf5c4282 100644 > --- a/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt > +++ b/Documentation/devicetree/bindings/rtc/rtc-m41t80.txt > @@ -19,6 +19,7 @@ Optional properties: > - interrupts: rtc alarm interrupt. > - clock-output-names: From common clock binding to override the default output > clock name > +- protected-clocks: Bool, if set operating system should not handle clock. > - wakeup-source: Enables wake up of host system on alarm > > Example: > diff --git a/drivers/rtc/rtc-m41t80.c b/drivers/rtc/rtc-m41t80.c > index 160dcf68e64e..3296583853a8 100644 > --- a/drivers/rtc/rtc-m41t80.c > +++ b/drivers/rtc/rtc-m41t80.c > @@ -546,6 +546,9 @@ static struct clk *m41t80_sqw_register_clk(struct m41t80_data *m41t80) > struct clk_init_data init; > int ret; > > + if (of_property_read_bool(node, "protected-clocks")) > + return 0; > + > /* First disable the clock */ > ret = i2c_smbus_read_byte_data(client, M41T80_REG_ALARM_MON); > if (ret < 0) > -- > 2.30.0 > -- Alexandre Belloni, co-owner and COO, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
