[PATCH v2 2/2] drm/bridge: ti-sn65dsi83: protect device resources on unplug

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

To support hot-unplug of this bridge we need to protect access to device
resources in case sn65dsi83_remove() happens concurrently to other code.

Some care is needed for the case when the unplug happens before
sn65dsi83_atomic_disable() has a chance to enter the critical section
(i.e. a successful drm_bridge_enter() call), which occurs whenever the
hardware is removed while the display is active. When that happens,
sn65dsi83_atomic_disable() in unable to release the resources taken by
sn65dsi83_atomic_pre_enable().

To ensure those resources are released exactly once on device removal:

 * move the code to release them to a dedicated function
 * register that function when the resources are taken in
   sn65dsi83_atomic_pre_enable()
 * if sn65dsi83_atomic_disable() happens before sn65dsi83_remove()
   (typical non-hot-unplug case):
   * sn65dsi83_atomic_disable() can enter the critical section
     (drm_bridge_enter() returns 0) -> it releases and executes the
     devres action
 * if sn65dsi83_atomic_disable() happens after sn65dsi83_remove()
   (typical hot-unplug case):
   * sn65dsi83_remove() -> drm_bridge_unplug() prevents
     sn65dsi83_atomic_disable() from entering the critical section
     (drm_bridge_enter() returns nonzero), so sn65dsi83_atomic_disable()
     cannot release and execute the devres action
   * the devres action is executed at the end of sn65dsi83_remove()

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

---

Changed in v2:
- Use a devres action instead of a flag
---
 drivers/gpu/drm/bridge/ti-sn65dsi83.c | 85 ++++++++++++++++++++++++++++-------
 1 file changed, 68 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
index 033c44326552ab167d4e8d9b74957c585e4c6fb7..92014366449237668b50cc884a67f52fcd5777c3 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
@@ -406,6 +406,10 @@ static void sn65dsi83_reset_work(struct work_struct *ws)
 {
 	struct sn65dsi83 *ctx = container_of(ws, struct sn65dsi83, reset_work);
 	int ret;
+	int idx;
+
+	if (!drm_bridge_enter(&ctx->bridge, &idx))
+		return;
 
 	/* Reset the pipe */
 	ret = sn65dsi83_reset_pipe(ctx);
@@ -415,12 +419,18 @@ static void sn65dsi83_reset_work(struct work_struct *ws)
 	}
 	if (ctx->irq)
 		enable_irq(ctx->irq);
+
+	drm_bridge_exit(idx);
 }
 
 static void sn65dsi83_handle_errors(struct sn65dsi83 *ctx)
 {
 	unsigned int irq_stat;
 	int ret;
+	int idx;
+
+	if (!drm_bridge_enter(&ctx->bridge, &idx))
+		return;
 
 	/*
 	 * Schedule a reset in case of:
@@ -441,6 +451,8 @@ static void sn65dsi83_handle_errors(struct sn65dsi83 *ctx)
 
 		schedule_work(&ctx->reset_work);
 	}
+
+	drm_bridge_exit(idx);
 }
 
 static void sn65dsi83_monitor_work(struct work_struct *work)
@@ -463,6 +475,40 @@ static void sn65dsi83_monitor_stop(struct sn65dsi83 *ctx)
 	cancel_delayed_work_sync(&ctx->monitor_work);
 }
 
+/*
+ * Release resources taken by sn65dsi83_atomic_pre_enable().
+ *
+ * Normally invoked by sn65dsi83_atomic_disable(). But if
+ * sn65dsi83_remove() is invoked while enabled, then drm_bridge_unplug()
+ * would defuse sn65dsi83_atomic_disable(). Thus a devres action is used to
+ * ensure exactly one code path (removal or atomic disable) invokes this
+ * code.
+ */
+static void sn65dsi83_release_resources(void *data)
+{
+	struct sn65dsi83 *ctx = (struct sn65dsi83 *)data;
+	int ret;
+
+	if (ctx->irq) {
+		/* Disable irq */
+		regmap_write(ctx->regmap, REG_IRQ_EN, 0x0);
+		regmap_write(ctx->regmap, REG_IRQ_GLOBAL, 0x0);
+	} else {
+		/* Stop the polling task */
+		sn65dsi83_monitor_stop(ctx);
+	}
+
+	/* Put the chip in reset, pull EN line low, and assure 10ms reset low timing. */
+	gpiod_set_value_cansleep(ctx->enable_gpio, 0);
+	usleep_range(10000, 11000);
+
+	ret = regulator_disable(ctx->vcc);
+	if (ret)
+		dev_err(ctx->dev, "Failed to disable vcc: %d\n", ret);
+
+	regcache_mark_dirty(ctx->regmap);
+}
+
 static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
 					struct drm_atomic_state *state)
 {
@@ -478,10 +524,15 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
 	__le16 le16val;
 	u16 val;
 	int ret;
+	int idx;
+
+	if (!drm_bridge_enter(bridge, &idx))
+		return;
 
 	ret = regulator_enable(ctx->vcc);
 	if (ret) {
 		dev_err(ctx->dev, "Failed to enable vcc: %d\n", ret);
+		drm_bridge_exit(idx);
 		return;
 	}
 
@@ -625,6 +676,8 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
 		dev_err(ctx->dev, "failed to lock PLL, ret=%i\n", ret);
 		/* On failure, disable PLL again and exit. */
 		regmap_write(ctx->regmap, REG_RC_PLL_EN, 0x00);
+		devm_add_action(ctx->dev, sn65dsi83_release_resources, ctx);
+		drm_bridge_exit(idx);
 		return;
 	}
 
@@ -633,6 +686,9 @@ static void sn65dsi83_atomic_pre_enable(struct drm_bridge *bridge,
 
 	/* Wait for 10ms after soft reset as specified in datasheet */
 	usleep_range(10000, 12000);
+
+	devm_add_action(ctx->dev, sn65dsi83_release_resources, ctx);
+	drm_bridge_exit(idx);
 }
 
 static void sn65dsi83_atomic_enable(struct drm_bridge *bridge,
@@ -640,6 +696,10 @@ static void sn65dsi83_atomic_enable(struct drm_bridge *bridge,
 {
 	struct sn65dsi83 *ctx = bridge_to_sn65dsi83(bridge);
 	unsigned int pval;
+	int idx;
+
+	if (!drm_bridge_enter(bridge, &idx))
+		return;
 
 	/* Clear all errors that got asserted during initialization. */
 	regmap_read(ctx->regmap, REG_IRQ_STAT, &pval);
@@ -659,32 +719,22 @@ static void sn65dsi83_atomic_enable(struct drm_bridge *bridge,
 		/* Use the polling task */
 		sn65dsi83_monitor_start(ctx);
 	}
+
+	drm_bridge_exit(idx);
 }
 
 static void sn65dsi83_atomic_disable(struct drm_bridge *bridge,
 				     struct drm_atomic_state *state)
 {
 	struct sn65dsi83 *ctx = bridge_to_sn65dsi83(bridge);
-	int ret;
-
-	if (ctx->irq) {
-		/* Disable irq */
-		regmap_write(ctx->regmap, REG_IRQ_EN, 0x0);
-		regmap_write(ctx->regmap, REG_IRQ_GLOBAL, 0x0);
-	} else {
-		/* Stop the polling task */
-		sn65dsi83_monitor_stop(ctx);
-	}
+	int idx;
 
-	/* Put the chip in reset, pull EN line low, and assure 10ms reset low timing. */
-	gpiod_set_value_cansleep(ctx->enable_gpio, 0);
-	usleep_range(10000, 11000);
+	if (!drm_bridge_enter(bridge, &idx))
+		return;
 
-	ret = regulator_disable(ctx->vcc);
-	if (ret)
-		dev_err(ctx->dev, "Failed to disable vcc: %d\n", ret);
+	devm_release_action(ctx->dev, sn65dsi83_release_resources, ctx);
 
-	regcache_mark_dirty(ctx->regmap);
+	drm_bridge_exit(idx);
 }
 
 static enum drm_mode_status
@@ -1005,6 +1055,7 @@ static void sn65dsi83_remove(struct i2c_client *client)
 {
 	struct sn65dsi83 *ctx = i2c_get_clientdata(client);
 
+	drm_bridge_unplug(&ctx->bridge);
 	drm_bridge_remove(&ctx->bridge);
 }
 

-- 
2.51.0