drm/nouveau/fan: rewrite the fan tachometer driver to get more precision, faster

The previous driver waited for 250ms to accumulate data. This version times a
complete fan rotation and extrapolates to RPM.

The fan rotational speed should now be read in less than 250ms (worst case)
and usually in less 50ms.

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

diff --git a/drivers/gpu/drm/nouveau/core/subdev/therm/fan.c b/drivers/gpu/drm/nouveau/core/subdev/therm/fan.c
index 9ad6e16..b7339b5 100644 (file)
--- a/drivers/gpu/drm/nouveau/core/subdev/therm/fan.c
+++ b/drivers/gpu/drm/nouveau/core/subdev/therm/fan.c
@@ -105,30 +105,37 @@ nouveau_therm_fan_sense(struct nouveau_therm *therm)
        struct nouveau_gpio *gpio = nouveau_gpio(therm);
        struct dcb_gpio_func func;
        u32 cycles, cur, prev;
-       u64 start;
+       u64 start, end, tach;
 
        if (gpio->find(gpio, 0, DCB_GPIO_FAN_SENSE, 0xff, &func))
                return -ENODEV;
 
-       /* Monitor the GPIO input 0x3b for 250ms.
+       /* Time a complete rotation and extrapolate to RPM:
         * When the fan spins, it changes the value of GPIO FAN_SENSE.
-        * We get 4 changes (0 -> 1 -> 0 -> 1 -> [...]) per complete rotation.
+        * We get 4 changes (0 -> 1 -> 0 -> 1) per complete rotation.
         */
        start = ptimer->read(ptimer);
        prev = gpio->get(gpio, 0, func.func, func.line);
        cycles = 0;
        do {
+               usleep_range(500, 1000); /* supports 0 < rpm < 7500 */
+
                cur = gpio->get(gpio, 0, func.func, func.line);
                if (prev != cur) {
+                       if (!start)
+                               start = ptimer->read(ptimer);
                        cycles++;
                        prev = cur;
                }
-
-               usleep_range(500, 1000); /* supports 0 < rpm < 7500 */
-       } while (ptimer->read(ptimer) - start < 250000000);
-
-       /* interpolate to get rpm */
-       return cycles / 4 * 4 * 60;
+       } while (cycles < 5 && ptimer->read(ptimer) - start < 250000000);
+       end = ptimer->read(ptimer);
+
+       if (cycles == 5) {
+               tach = (u64)60000000000;
+               do_div(tach, (end - start));
+               return tach;
+       } else
+               return 0;
 }
 
 static void