spi: rspi: Increase bit rate accuracy on RZ/A

rspi_rz_set_config_register() favors high values of "brdv" over high
values of "spbr".  As "brdv" is not a plain divider, but controls a
power-of-two divider, this may cause the selection of non-optimal
divider values.  E.g. on RSK+RZA1, when 3.8 MHz is requested, the actual
configured bit rate is 2.08 MHz (spbr = 1, brdv = 3), while 3.7 MHz
would be possible (spbr = 8, brdv = 0).

Fix this by only resorting to higher "brdv" values when really needed.
This makes the driver always pick optimal divider values on RZ/A.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200819125904.20938-4-geert+renesas@glider.beSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-rspi.c

@@ -298,15 +298,13 @@ static int rspi_rz_set_config_register(struct rspi_data *rspi, int access_size)
 	rspi_write8(rspi, rspi->sppcr, RSPI_SPPCR);
 
 	clksrc = clk_get_rate(rspi->clk);
-	while (brdv < 3) {
-		if (rspi->speed_hz >= clksrc/4) /* 4=(CLK/2)/2 */
-			break;
+	spbr = DIV_ROUND_UP(clksrc, 2 * rspi->speed_hz) - 1;
+	while (spbr > 255 && brdv < 3) {
 		brdv++;
-		clksrc /= 2;
+		spbr = DIV_ROUND_UP(spbr + 1, 2) - 1;
 	}
 
 	/* Sets transfer bit rate */
-	spbr = DIV_ROUND_UP(clksrc, 2 * rspi->speed_hz) - 1;
 	rspi_write8(rspi, clamp(spbr, 0, 255), RSPI_SPBR);
 	rspi->spcmd |= SPCMD_BRDV(brdv);