drm/tegra: Enable the hotplug interrupt only when necessary

The hotplug handling needs access to the DRM device, which only appears
at ->init() time. Disable interrupts up until that time. Similarly, when
an output is removed, disable the hotplug interrupt again because the
DRM device (and with it the hotplug infrastructure) is going away.

Also make sure to only access the DRM device if it's available. Given
the above change for the hotplug interrupt this should really never
happen, but the extra check doesn't hurt either.
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/output.c

@@ -183,7 +183,8 @@ static irqreturn_t hpd_irq(int irq, void *data)
 {
 	struct tegra_output *output = data;
 
-	drm_helper_hpd_irq_event(output->connector.dev);
+	if (output->connector.dev)
+		drm_helper_hpd_irq_event(output->connector.dev);
 
 	return IRQ_HANDLED;
 }
@@ -255,6 +256,13 @@ int tegra_output_probe(struct tegra_output *output)
 		}
 
 		output->connector.polled = DRM_CONNECTOR_POLL_HPD;
+
+		/*
+		 * Disable the interrupt until the connector has been
+		 * initialized to avoid a race in the hotplug interrupt
+		 * handler.
+		 */
+		disable_irq(output->hpd_irq);
 	}
 
 	return 0;
@@ -320,10 +328,24 @@ int tegra_output_init(struct drm_device *drm, struct tegra_output *output)
 
 	output->encoder.possible_crtcs = 0x3;
 
+	/*
+	 * The connector is now registered and ready to receive hotplug events
+	 * so the hotplug interrupt can be enabled.
+	 */
+	if (gpio_is_valid(output->hpd_gpio))
+		enable_irq(output->hpd_irq);
+
 	return 0;
 }
 
 int tegra_output_exit(struct tegra_output *output)
 {
+	/*
+	 * The connector is going away, so the interrupt must be disabled to
+	 * prevent the hotplug interrupt handler from potentially crashing.
+	 */
+	if (gpio_is_valid(output->hpd_gpio))
+		disable_irq(output->hpd_irq);
+
 	return 0;
 }