ARM: OMAP2: Clock: New OMAP2/3 DPLL rate rounding algorithm

This patch adds a new rate rounding algorithm for DPLL clocks on the
OMAP2/3 architecture.

For a desired DPLL target rate, there may be several
multiplier/divider (M, N) values which will generate a sufficiently
close rate.  Lower N values result in greater power economy.  However,
lower N values can cause the difference between the rounded rate and
the target rate ("rate error") to be larger than it would be with a
higher N.  This can cause downstream devices to run more slowly than
they otherwise would.

This DPLL rate rounding algorithm:

- attempts to find the lowest possible N (DPLL divider) to reach the
  target_rate (since, according to Richard Woodruff <r-woodruff@ti.com>,
  lower N values save more power than higher N values).

- allows developers to set an upper bound on the error between the
  rounded rate and the desired target rate ("rate tolerance"), so an
  appropriate balance between rate fidelity and power savings can be
  set.  This maximum rate error tolerance is set via
  omap2_set_dpll_rate_tolerance().

- never returns a rounded rate higher than the target rate.

The rate rounding algorithm caches the last rounded M, N, and rate
computation to avoid rounding the rate twice for each clk_set_rate()
call.  (This patch does not yet implement set_rate for DPLLs; that
follows in a future patch.)

The algorithm trades execution speed for rate accuracy.  It will find
the (M, N) set that results in the least rate error, within a
specified rate tolerance.  It does this by evaluating each divider
setting - on OMAP3, this involves 128 steps.  Another approach to DPLL
rate rounding would be to bail out as soon as a valid rate is found
within the rate tolerance, which would trade rate accuracy for
execution speed.  Alternate implementations welcome.

This code is not yet used by the OMAP24XX DPLL clock, since it
is currently defined as a composite clock, fusing the DPLL M,N and the
M2 output divider.  This patch also renames the existing OMAP24xx DPLL
programming functions to highlight that they program both the DPLL and
the DPLL's output multiplier.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/include/asm-arm/arch-omap/clock.h b/include/asm-arm/arch-omap/clock.h
index 22daa5d64d35..4c7b3514f71a 100644
--- a/include/asm-arm/arch-omap/clock.h
+++ b/include/asm-arm/arch-omap/clock.h
@@ -33,6 +33,13 @@ struct dpll_data {
         void __iomem            *mult_div1_reg;
         u32                     mult_mask;
         u32                     div1_mask;
+        u16                     last_rounded_m;
+        u8                      last_rounded_n;
+        unsigned long           last_rounded_rate;
+        unsigned int            rate_tolerance;
+        u16                     max_multiplier;
+        u8                      max_divider;
+        u32                     max_tolerance;
 #  if defined(CONFIG_ARCH_OMAP3)
         u8                      modes;
         void __iomem            *control_reg;
@@ -71,7 +78,7 @@ struct clk {
         void __iomem            *clksel_reg;
         u32                     clksel_mask;
         const struct clksel     *clksel;
-        const struct dpll_data  *dpll_data;
+        struct dpll_data        *dpll_data;
 #else
         __u8                    rate_offset;
         __u8                    src_offset;