[PATCH 3/4] clk: Provide always-on clock support

On Mon, Mar 2, 2015 at 2:06 PM, Lee Jones [off-list ref] wrote:

On Sat, 28 Feb 2015, Jassi Brar wrote:

quoted

On 28 February 2015 at 02:44, Lee Jones [off-list ref] wrote:

quoted

Lots of platforms contain clocks which if turned off would prove fatal.
The only way to recover from these catastrophic failures is to restart
the board(s).  Now, when a clock is registered with the framework it is
compared against a list of provided always-on clock names which must be
kept ungated.  If it matches, we enable the existing CLK_IGNORE_UNUSED
flag, which will prevent the common clk framework from attempting to
gate it during the clk_disable_unused() procedure.

If a clock is critical on a certain board, it could be got+enabled
during early boot so there is always a user.

I tried this.  There was push-back from the DT maintainers.

  http://lists.infradead.org/pipermail/linux-arm-kernel/2015-February/324417.html

Thanks, I wasn't aware of the history.

quoted

To be able to do that from DT, maybe add a new, say, CLK_ALWAYS_ON
flag could be made to initialize the clock with one phantom user
already. Or just reuse the CLK_IGNORE_UNUSED?

How is that different to what this set is doing?

The phantom user - that's there but none can see it.

How about?

+       of_property_for_each_string(np, "clock-always-on", prop, clkname) {
+               clk = __clk_lookup(clkname);
+               if (!clk)
+                       continue;
+
+               clk->core->enable_count = 1;
+               clk->core->prepare_count = 1;
+       }

-jassi