clk: fractional-divider: Introduce POWER_OF_TWO_PS flag

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>
Link: https://lore.kernel.org/r/20210812170025.67074-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/acpi/acpi_lpss.c

@@ -434,8 +434,8 @@ 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,
+						      CLK_FRAC_DIVIDER_POWER_OF_TWO_PS,
-						      1, 15, 16, 15, 0, NULL);
+						      prv_base, 1, 15, 16, 15, 0, NULL);
 		parent = clk_name;
 
 		clk_name = kasprintf(GFP_KERNEL, "%s-update", devname);

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 (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),

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);

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;
@@ -1022,6 +1028,7 @@ struct clk_fractional_divider {
 
 #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,