diff mbox series

[v2,2/2] clk: clarify that clk_set_rate() does updates from top to bottom

Message ID 20191226191224.3785282-3-martin.blumenstingl@googlemail.com (mailing list archive)
State Awaiting Upstream, archived
Headers show
Series clk: Meson8/8b/8m2: fix the mali clock flags | expand

Commit Message

Martin Blumenstingl Dec. 26, 2019, 7:12 p.m. UTC 
clk_set_rate() currently starts updating the rate for a clock at the
top-most affected clock and then walks down the tree to update the
bottom-most affected clock last.
This behavior is important for protected clocks where we can switch
between multiple parents to achieve the same output.

An example for this is the mali clock tree on Amlogic SoCs:
  mali_0_mux (must not change when enabled)
    mali_0_div (must not change when enabled)
     mali_0 (gate)
  mali_1_mux (must not change when enabled)
    mali_1_div (must not change when enabled)
      mali_1 (gate)
The final output can either use mali_0_gate or mali_1. To change the
final output we must switch to the "inactive" tree. Assuming mali_0 is
active, then we need to prepare mali_1 with the new desired rate and
finally switch the output to the mali_1 tree. This process will then
protect the mali_1 tree and at the same time unprotect the mali_0 tree.
The next call to clk_set_rate() will then switch from the mali_1 tree
back to mali_0.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
---
 include/linux/clk.h | 3 +++
 1 file changed, 3 insertions(+)

Comments

Stephen Boyd Jan. 6, 2020, 3:03 a.m. UTC | #1 
Quoting Martin Blumenstingl (2019-12-26 11:12:24)
> clk_set_rate() currently starts updating the rate for a clock at the
> top-most affected clock and then walks down the tree to update the
> bottom-most affected clock last.
> This behavior is important for protected clocks where we can switch
> between multiple parents to achieve the same output.
> 
> An example for this is the mali clock tree on Amlogic SoCs:
>   mali_0_mux (must not change when enabled)
>     mali_0_div (must not change when enabled)
>      mali_0 (gate)
>   mali_1_mux (must not change when enabled)
>     mali_1_div (must not change when enabled)
>       mali_1 (gate)
> The final output can either use mali_0_gate or mali_1. To change the
> final output we must switch to the "inactive" tree. Assuming mali_0 is
> active, then we need to prepare mali_1 with the new desired rate and
> finally switch the output to the mali_1 tree. This process will then
> protect the mali_1 tree and at the same time unprotect the mali_0 tree.
> The next call to clk_set_rate() will then switch from the mali_1 tree
> back to mali_0.
> 
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
> ---

Acked-by: Stephen Boyd <sboyd@kernel.org>
diff mbox series

Patch

diff --git a/include/linux/clk.h b/include/linux/clk.h
index 18b7b95a8253..7fd6a1febcf4 100644
--- a/include/linux/clk.h
+++ b/include/linux/clk.h
@@ -627,6 +627,9 @@  long clk_round_rate(struct clk *clk, unsigned long rate);
  * @clk: clock source
  * @rate: desired clock rate in Hz
  *
+ * Updating the rate starts at the top-most affected clock and then
+ * walks the tree down to the bottom-most clock that needs updating.
+ *
  * Returns success (0) or negative errno.
  */
 int clk_set_rate(struct clk *clk, unsigned long rate);