[PATCH v4 3/4] clk: fractional-divider: Introduce POWER_OF_TWO_PS flag

Andy Shevchenko
2021-08-12T17:00:38Z

The newly introduced POWER_OF_TWO_PS flag, when set, makes the flow
to skip the assumption that the caller will use an additional 2^scale
prescaler to get the desired clock rate.

Reported-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
v4: rebased on top of latest CLK codebase

drivers/acpi/acpi_lpss.c | 4 ++--
drivers/clk/clk-fractional-divider.c | 10 ++++++----
drivers/mfd/intel-lpss.c | 3 ++-
include/linux/clk-provider.h | 11 +++++++++--
4 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/drivers/acpi/acpi_lpss.c b/drivers/acpi/acpi_lpss.c
index 7f163074e4e4..30b1f511c2af 100644
--- a/drivers/acpi/acpi_lpss.c
+++ b/drivers/acpi/acpi_lpss.c
@@ -436,8 +436,8 @@ static int register_device_clock(struct acpi_device *adev,
if (!clk_name)
return -ENOMEM;
clk = clk_register_fractional_divider(NULL, clk_name, parent,
- 0, prv_base,
- 1, 15, 16, 15, 0, NULL);
+ CLK_FRAC_DIVIDER_POWER_OF_TWO_PS,
+ prv_base, 1, 15, 16, 15, 0, NULL);
parent = clk_name;

clk_name = kasprintf(GFP_KERNEL, "%s-update", devname);
diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c
index 535d299af646..6a3ed82fdae9 100644
--- a/drivers/clk/clk-fractional-divider.c
+++ b/drivers/clk/clk-fractional-divider.c
@@ -76,16 +76,18 @@ void clk_fractional_divider_general_approximation(struct clk_hw *hw,
unsigned long *m, unsigned long *n)
{
struct clk_fractional_divider *fd = to_clk_fd(hw);
- unsigned long scale;

/*
* Get rate closer to *parent_rate to guarantee there is no overflow
* for m and n. In the result it will be the nearest rate left shifted
* by (scale - fd->nwidth) bits.
*/
- scale = fls_long(*parent_rate / rate - 1);
- if (scale > fd->nwidth)
- rate <<= scale - fd->nwidth;
+ if (fd->flags & CLK_FRAC_DIVIDER_POWER_OF_TWO_PS) {
+ unsigned long scale = fls_long(*parent_rate / rate - 1);
+
+ if (scale > fd->nwidth)
+ rate <<= scale - fd->nwidth;
+ }

rational_best_approximation(rate, *parent_rate,
GENMASK(fd->mwidth - 1, 0), GENMASK(fd->nwidth - 1, 0),
diff --git a/drivers/mfd/intel-lpss.c b/drivers/mfd/intel-lpss.c
index a9bf10bee796..0e15afc39f54 100644
--- a/drivers/mfd/intel-lpss.c
+++ b/drivers/mfd/intel-lpss.c
@@ -301,7 +301,8 @@ static int intel_lpss_register_clock_divider(struct intel_lpss *lpss,

snprintf(name, sizeof(name), "%s-div", devname);
tmp = clk_register_fractional_divider(NULL, name, __clk_get_name(tmp),
- 0, lpss->priv, 1, 15, 16, 15, 0,
+ CLK_FRAC_DIVIDER_POWER_OF_TWO_PS,
+ lpss->priv, 1, 15, 16, 15, 0,
NULL);
if (IS_ERR(tmp))
return PTR_ERR(tmp);
diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
index d8609aef2394..f59c875271a0 100644
--- a/include/linux/clk-provider.h
+++ b/include/linux/clk-provider.h
@@ -1001,6 +1001,12 @@ struct clk_hw *devm_clk_hw_register_fixed_factor(struct device *dev,
* CLK_FRAC_DIVIDER_BIG_ENDIAN - By default little endian register accesses are
* used for the divider register. Setting this flag makes the register
* accesses big endian.
+ * CLK_FRAC_DIVIDER_POWER_OF_TWO_PS - By default the resulting fraction might
+ * be saturated and the caller will get quite far from the good enough
+ * approximation. Instead the caller may require, by setting this flag,
+ * to shift left by a few bits in case, when the asked one is quite small
+ * to satisfy the desired range of denominator. It assumes that on the
+ * caller's side the power-of-two capable prescaler exists.
*/
struct clk_fractional_divider {
struct clk_hw hw;
@@ -1020,8 +1026,9 @@ struct clk_fractional_divider {

#define to_clk_fd(_hw) container_of(_hw, struct clk_fractional_divider, hw)

-#define CLK_FRAC_DIVIDER_ZERO_BASED BIT(0)
-#define CLK_FRAC_DIVIDER_BIG_ENDIAN BIT(1)
+#define CLK_FRAC_DIVIDER_ZERO_BASED BIT(0)
+#define CLK_FRAC_DIVIDER_BIG_ENDIAN BIT(1)
+#define CLK_FRAC_DIVIDER_POWER_OF_TWO_PS BIT(2)

struct clk *clk_register_fractional_divider(struct device *dev,
const char *name, const char *parent_name, unsigned long flags,
--
2.30.2