On 3/12/22 06:04, Stephen Boyd wrote:
Quoting Marek Vasut (2022-03-09 12:54:35)
On 2/21/22 01:58, Marek Vasut wrote:
On 2/17/22 23:23, Stephen Boyd wrote:
I see that there isn't any more 'clock-critical' in the kernel's dts so
I wonder if we would be able to get rid of that function or at least
hollow it out and see if anyone complains. Either way, what is the
actual problem trying to be solved? If the crystal oscillator isn't used
anywhere in the kernel why are we registering it with the clk framework?
The problem is the other way around -- the SoC clock IPs often have a
couple of general purpose clock routed to various SoC IO pins, those
clock can be used for any purpose, and those are already registered with
kernel clock framework. Some devices save on BoM and use those general
purpose clock to supply clock networks which are otherwise not
interacting with the kernel, like some CPLD for example. Since from the
kernel point of view, those clock are unused, the kernel can turn those
clock OFF and that will make the entire device fail.
So this critical-clocks property permits marking clock which must not
ever be turned OFF accordingly.
How can we proceed here ?
Why are we registering the clks with the framework on device that are
saving on BoM and using them outside of the kernel. What is the use of
kernel memory for struct clk_core that aren't ever used?
Those clock may be used to supply a device in DT on another hardware
using the same SoC.
Take e.g. this random git grep result:
arch/arm/boot/dts/imx7d-remarkable2.dts
/ {
wifi_pwrseq {
...
clocks = <&clks IMX7D_CLKO2_ROOT_DIV>;
...
};
};
This IMX7D_CLKO2_ROOT_DIV is one such general purpose clock output. In
the aforementioned case, it is used to supply 32 kHz clock to a WiFi
chip, i.e. it has a consumer in DT. These clock are registered by the
platform clock driver:
drivers/clk/imx/clk-imx7d.c
But those clock can also be used to supply e.g. CPLD which has no other
connection to the SoC but the clock. That is where it needs this
critical-clocks property. Because then there is no consumer in DT. So
the kernel will now think the clock are not used and will turn them off
after boot, thus e.g. crashing such platform.
So in the later case, the DT would contain the following to avoid the crash:
&clks {
critical-clocks = <IMX7D_CLKO2_ROOT_DIV>;
};
