docs: clk: add documentation to log which clocks have been disabled

The existing clk documentation has a section that talks about the
clk_ignore_unused kernel parameter. Add additional documentation that
describes how to log which clocks the kernel disables on bootup. This
will log messages like the following to the console on bootup:

    [    1.268115] clk: Disabling unused clocks
    [    1.272167] clk_disable: gcc_usb_clkref_en
    [    1.276389] clk_disable: gcc_usb30_sec_sleep_clk
    [    1.281131] clk_disable: gcc_usb30_prim_sleep_clk
    ...

Signed-off-by: Brian Masney <bmasney@redhat.com>
Link: https://lore.kernel.org/r/20230411192153.289688-1-bmasney@redhat.com
[jc: turned parameters into a literal block]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/driver-api/clk.rst b/Documentation/driver-api/clk.rst
index 3cad45d14187..93bab5336dfd 100644
--- a/Documentation/driver-api/clk.rst
+++ b/Documentation/driver-api/clk.rst
@@ -258,6 +258,11 @@ clocks properly but rely on them being on from the bootloader, bypassing
 the disabling means that the driver will remain functional while the issues
 are sorted out.
 
+You can see which clocks have been disabled by booting your kernel with these
+parameters::
+
+ tp_printk trace_event=clk:clk_disable
+
 To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
 kernel.