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>

1 files changed, 16 insertions, 9 deletions

diff --git a/drivers/gpu/drm/nouveau/core/subdev/therm/fan.c b/drivers/gpu/drm/nouveau/core/subdev/therm/fan.c
index 9ad6e166dd75..b7339b52559e 100644
--- 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