clk: ast2600: Add comment about combined clock + reset handling

Add a little description about how reset lines can be implicit with
clock enable/disable. This is mostly based on the commit message
from the original submission in 15ed8ce5f8.

Signed-off-by: Jeremy Kerr <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Joel Stanley <[email protected]>
Tested-by: Joel Stanley <[email protected]>
Signed-off-by: Stephen Boyd <[email protected]>

drivers/clk/clk-ast2600.c

@@ -73,6 +73,27 @@ static void __iomem *scu_g6_base;
 static u8 soc_rev;
 
 /*
+ * The majority of the clocks in the system are gates paired with a reset
+ * controller that holds the IP in reset; this is represented by the @reset_idx
+ * member of entries here.
+ *
+ * This borrows from clk_hw_register_gate, but registers two 'gates', one
+ * to control the clock enable register and the other to control the reset
+ * IP. This allows us to enforce the ordering:
+ *
+ * 1. Place IP in reset
+ * 2. Enable clock
+ * 3. Delay
+ * 4. Release reset
+ *
+ * Consequently, if reset_idx is set, reset control is implicit: the clock
+ * consumer does not need its own reset handling, as enabling the clock will
+ * also deassert reset.
+ *
+ * There are some gates that do not have an associated reset; these are
+ * handled by using -1 as the index for the reset, and the consumer must
+ * explictly assert/deassert reset lines as required.
+ *
  * Clocks marked with CLK_IS_CRITICAL:
  *
  *  ref0 and ref1 are essential for the SoC to operate