i2c: rcar: calculate divider instead of brute-forcing it

Instead of trying all values, we can actually compute it as the comment
suggests. It is unclear what the comment means with "involved", it works
nicely.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

diff --git a/drivers/i2c/busses/i2c-rcar.c b/drivers/i2c/busses/i2c-rcar.c
index 4bf47e35094f23b8ced0b8fe5df44b1ef48c6435..2585092bed52e10263e6ed7fcc1a13c03a0d84d1 100644 (file)
--- a/drivers/i2c/busses/i2c-rcar.c
+++ b/drivers/i2c/busses/i2c-rcar.c
@@ -303,24 +303,16 @@ static int rcar_i2c_clock_calculate(struct rcar_i2c_priv *priv)
        round = (round + 500) / 1000;
 
        /*
-        * SCL  = ick / (20 + SCGD * 8 + F[(ticf + tr + intd) * ick])
-        *
-        * Calculation result (= SCL) should be less than
-        * bus_speed for hardware safety
-        *
-        * We could use something along the lines of
-        *      div = ick / (bus_speed + 1) + 1;
-        *      scgd = (div - 20 - round + 7) / 8;
-        *      scl = ick / (20 + (scgd * 8) + round);
-        * (not fully verified) but that would get pretty involved
+        * SCL  = ick / (20 + 8 * SCGD + F[(ticf + tr + intd) * ick])
+        * 20 + 8 * SCGD + F[...] = ick / SCL
+        * SCGD = ((ick / SCL) - 20 - F[...]) / 8
+        * Result (= SCL) should be less than bus_speed for hardware safety
         */
-       for (scgd = 0; scgd < 0x40; scgd++) {
-               scl = ick / (20 + (scgd * 8) + round);
-               if (scl <= t.bus_freq_hz)
-                       break;
-       }
+       scgd = DIV_ROUND_UP(ick, t.bus_freq_hz ?: 1);
+       scgd = DIV_ROUND_UP(scgd - 20 - round, 8);
+       scl = ick / (20 + 8 * scgd + round);
 
-       if (scgd == 0x40)
+       if (scgd > 0x3f)
                goto err_no_val;
 
        dev_dbg(dev, "clk %u/%u(%lu), round %u, CDF: %u, SCGD: %u\n",