Re: [PATCH] clk: add new Kconfig to control default behavior of disabling unused clocks

[Abel Vesa]

On 26-03-17 14:15:50, Maxime Ripard wrote:
> On Tue, Mar 17, 2026 at 02:14:25PM +0200, Abel Vesa wrote:
> > On 26-03-17 08:30:24, Maxime Ripard wrote:
> > > Hi,
> > > 
> > > On Mon, Mar 16, 2026 at 06:33:45PM -0400, Brian Masney wrote:
> > > > At the 2023 Linux Plumbers Conference in Richmond VA, there was a
> > > > discussion about how large number of systems need to boot with
> > > > clk_ignore_unused. Per the discussions at the conference, the existing
> > > > behavior in the clk core is broken, and there is a desire to completely
> > > > remove this functionality.
> > > 
> > > Broken how?
> > > 
> > > clk_ignore_unused is to a point where it's seriously cargo-culted and
> > > documented as a silver bullet, when in reality it's just a debug tool
> > > for broken drivers, and the driver must be fixed.
> > > 
> > > But nobody is actually fixing it.
> > > 
> > > See
> > > https://fedoraproject.org/wiki/Changes/Automatic_DTB_selection_for_aarch64_EFI_systems#How_To_Test
> > > for example. The affected clock could be marked as CLK_IS_CRITICAL, and
> > > fedora wouldn't have to package anything, change anything, etc. But no,
> > > the problem is clk_ignore_unused.
> > 
> > Nope. Don't ever mark clocks as critical unless system crashes without
> > them.
> > 
> > Here is an example or why clocks cannot be marked as critical but need
> > to be kept by the clk_ignore_unused: display driver probes later.
> > If you mark it as critical you just made the clock stay enabled even
> > when display is off.
> > 
> > And this is just one example.
> 
> Then use CLK_IGNORE_UNUSED.

But there is no way of knowing in advance to which clocks should this
flag be applied. As I mentioned on this thread already, we will be
playing whack-a-mole with clocks for a long time before we get this
right.

The solution has been already discussed for a long time now and it is:
drop the clk_ignore_unused late_initcall entirely and then make a
generic sync_state callback that the clock providers can use (or they
could implement one themselves). This way, until sync_state is reached
for a specific clock provider driver, all unused clocks remain as is.