clk: allow a clk divider with max divisor when zero

This commit allows certain Broadcom STB clock dividers to be used with
clk-divider.c. It allows for a clock whose field value is the equal
to the divisor, execpt when the field value is zero, in which case the
divisor is 2^width. For example, consider a divisor clock with a two
bit field:

value divisor
0 4
1 1
2 2
3 3

Signed-off-by: Jim Quinlan <[e-mail]>
Signed-off-by: Michael Turquette <[e-mail]>

drivers/clk/clk-divider.c

···
 }
 
 static unsigned int _get_div(const struct clk_div_table *table,
-			     unsigned int val, unsigned long flags)
+			     unsigned int val, unsigned long flags, u8 width)
 {
 	if (flags & CLK_DIVIDER_ONE_BASED)
 		return val;
 	if (flags & CLK_DIVIDER_POWER_OF_TWO)
 		return 1 << val;
+	if (flags & CLK_DIVIDER_MAX_AT_ZERO)
+		return val ? val : div_mask(width) + 1;
 	if (table)
 		return _get_table_div(table, val);
 	return val + 1;
···
 }
 
 static unsigned int _get_val(const struct clk_div_table *table,
-			     unsigned int div, unsigned long flags)
+			     unsigned int div, unsigned long flags, u8 width)
 {
 	if (flags & CLK_DIVIDER_ONE_BASED)
 		return div;
 	if (flags & CLK_DIVIDER_POWER_OF_TWO)
 		return __ffs(div);
+	if (flags & CLK_DIVIDER_MAX_AT_ZERO)
+		return (div == div_mask(width) + 1) ? 0 : div;
 	if (table)
 		return  _get_table_val(table, div);
 	return div - 1;
···
 				  const struct clk_div_table *table,
 				  unsigned long flags)
 {
+	struct clk_divider *divider = to_clk_divider(hw);
 	unsigned int div;
 
-	div = _get_div(table, val, flags);
+	div = _get_div(table, val, flags, divider->width);
 	if (!div) {
 		WARN(!(flags & CLK_DIVIDER_ALLOW_ZERO),
 			"%s: Zero divisor and CLK_DIVIDER_ALLOW_ZERO not set\n",
···
 	if (divider->flags & CLK_DIVIDER_READ_ONLY) {
 		bestdiv = readl(divider->reg) >> divider->shift;
 		bestdiv &= div_mask(divider->width);
-		bestdiv = _get_div(divider->table, bestdiv, divider->flags);
+		bestdiv = _get_div(divider->table, bestdiv, divider->flags,
+			divider->width);
 		return DIV_ROUND_UP(*prate, bestdiv);
 	}
 
···
 	if (!_is_valid_div(table, div, flags))
 		return -EINVAL;
 
-	value = _get_val(table, div, flags);
+	value = _get_val(table, div, flags, width);
 
 	return min_t(unsigned int, value, div_mask(width));
 }

include/linux/clk-provider.h

···
  *	to the closest integer instead of the up one.
  * CLK_DIVIDER_READ_ONLY - The divider settings are preconfigured and should
  *	not be changed by the clock framework.
+ * CLK_DIVIDER_MAX_AT_ZERO - For dividers which are like CLK_DIVIDER_ONE_BASED
+ *	except when the value read from the register is zero, the divisor is
+ *	2^width of the field.
  */
 struct clk_divider {
 	struct clk_hw	hw;
···
 #define CLK_DIVIDER_HIWORD_MASK		BIT(3)
 #define CLK_DIVIDER_ROUND_CLOSEST	BIT(4)
 #define CLK_DIVIDER_READ_ONLY		BIT(5)
+#define CLK_DIVIDER_MAX_AT_ZERO		BIT(6)
 
 extern const struct clk_ops clk_divider_ops;