soc: qcom: geni: geni_se_clk_freq_match() should always accept multiples

The geni_se_clk_freq_match() has some strange semantics.  Specifically
it is defined with two modes:
1. It can find a clock that's an exact multiple of the requested rate
2. It can find a non-exact match but it can't handle multiples then

...but callers should always be able to handle a clock that is a
multiple of the requested clock so mode #2 doesn't really make sense.
Let's change the semantics so that the non-exact match can also accept
multiples and then change the code to handle that.

The only caller of this code is the unlanded SPI driver [1] which
currently passes "exact = True", thus it should be safe to change the
semantics in this way.  ...and, in fact, the SPI driver should likely
be modified to pass "exact = False" (with the new semantics) since
that will allow it to work with SPI devices that request a clock rate
that doesn't exactly match a rate we can make.

[1] https://lkml.kernel.org/r/1535107336-2214-1-git-send-email-dkota@codeaurora.org

Fixes: eddac5af ("soc: qcom: Add GENI based QUP Wrapper driver")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>

drivers/soc/qcom/qcom-geni-se.c

@@ -544,16 +544,17 @@ EXPORT_SYMBOL(geni_se_clk_tbl_get);
  * @se:		Pointer to the concerned serial engine.
  * @req_freq:	Requested clock frequency.
  * @index:	Index of the resultant frequency in the table.
- * @res_freq:	Resultant frequency which matches or is closer to the
- *		requested frequency.
+ * @res_freq:	Resultant frequency of the source clock.
  * @exact:	Flag to indicate exact multiple requirement of the requested
  *		frequency.
  *
- * This function is called by the protocol drivers to determine the matching
- * or exact multiple of the requested frequency, as provided by the serial
- * engine clock in order to meet the performance requirements. If there is
- * no matching or exact multiple of the requested frequency found, then it
- * selects the closest floor frequency, if exact flag is not set.
+ * This function is called by the protocol drivers to determine the best match
+ * of the requested frequency as provided by the serial engine clock in order
+ * to meet the performance requirements.
+ *
+ * If we return success:
+ * - if @exact is true  then @res_freq / <an_integer> == @req_freq
+ * - if @exact is false then @res_freq / <an_integer> <= @req_freq
  *
  * Return: 0 on success, standard Linux error codes on failure.
  */
@@ -564,6 +565,9 @@ int geni_se_clk_freq_match(struct geni_se *se, unsigned long req_freq,
 	unsigned long *tbl;
 	int num_clk_levels;
 	int i;
+	unsigned long best_delta;
+	unsigned long new_delta;
+	unsigned int divider;
 
 	num_clk_levels = geni_se_clk_tbl_get(se, &tbl);
 	if (num_clk_levels < 0)
@@ -572,18 +576,21 @@ int geni_se_clk_freq_match(struct geni_se *se, unsigned long req_freq,
 	if (num_clk_levels == 0)
 		return -EINVAL;
 
-	*res_freq = 0;
+	best_delta = ULONG_MAX;
 	for (i = 0; i < num_clk_levels; i++) {
-		if (!(tbl[i] % req_freq)) {
+		divider = DIV_ROUND_UP(tbl[i], req_freq);
+		new_delta = req_freq - tbl[i] / divider;
+		if (new_delta < best_delta) {
+			/* We have a new best! */
 			*index = i;
 			*res_freq = tbl[i];
-			return 0;
-		}
 
-		if (!(*res_freq) || ((tbl[i] > *res_freq) &&
-				     (tbl[i] < req_freq))) {
-			*index = i;
-			*res_freq = tbl[i];
+			/* If the new best is exact then we're done */
+			if (new_delta == 0)
+				return 0;
+
+			/* Record how close we got */
+			best_delta = new_delta;
 		}
 	}