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From:   Federico Vaga <federico.vaga@cern.ch>
To:     Peter Korsgaard <peter@korsgaard.com>, <linux-i2c@vger.kernel.org>
CC:     <linux-kernel@vger.kernel.org>, <federico.vaga@cern.ch>
Subject: [PATCH V2 1/5] i2c:ocores: stop transfer on timeout
Date:   Mon, 29 Oct 2018 15:50:47 +0100
Message-ID: <20181029145051.31984-2-federico.vaga@cern.ch>
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Detecting a timeout is ok, but we also need to assert a STOP command on
the bus in order to prevent it from generating interrupts when there are
no on going transfers.

Example: very long transmission.

1. ocores_xfer: START a transfer
2. ocores_isr : handle byte by byte the transfer
3. ocores_xfer: goes in timeout [[bugfix here]]
4. ocores_xfer: return to I2C subsystem and to the I2C driver
5. I2C driver : it may clean up the i2c_msg memory
6. ocores_isr : receives another interrupt (pending bytes to be
                transferred) but the i2c_msg memory is invalid now

So, since the transfer was too long, we have to detect the timeout and
STOP the transfer.

Another point is that we have a critical region here. When handling the
timeout condition we may have a running IRQ handler. For this reason I
introduce a spinlock.

In order to make easier to understan locking I have:
- added a new function to handle timeout
- modified the current ocores_process() function in order to be protected
  by the new spinlock
Like this it is obvious at first sight that this locking serializes
the execution of ocores_process() and ocores_process_timeout()

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
---
 drivers/i2c/busses/i2c-ocores.c | 54 ++++++++++++++++++++++++++++++++++-------
 1 file changed, 45 insertions(+), 9 deletions(-)

diff --git a/drivers/i2c/busses/i2c-ocores.c b/drivers/i2c/busses/i2c-ocores.c
index 87f9caa..aa85202 100644
--- a/drivers/i2c/busses/i2c-ocores.c
+++ b/drivers/i2c/busses/i2c-ocores.c
@@ -25,7 +25,12 @@
 #include <linux/slab.h>
 #include <linux/io.h>
 #include <linux/log2.h>
+#include <linux/spinlock.h>
 
+/**
+ * @process_lock: protect I2C transfer process.
+ *     ocores_process() and ocores_process_timeout() can't run in parallel.
+ */
 struct ocores_i2c {
 	void __iomem *base;
 	u32 reg_shift;
@@ -36,6 +41,7 @@ struct ocores_i2c {
 	int pos;
 	int nmsgs;
 	int state; /* see STATE_ */
+	spinlock_t process_lock;
 	struct clk *clk;
 	int ip_clock_khz;
 	int bus_clock_khz;
@@ -141,19 +147,26 @@ static void ocores_process(struct ocores_i2c *i2c)
 {
 	struct i2c_msg *msg = i2c->msg;
 	u8 stat = oc_getreg(i2c, OCI2C_STATUS);
+	unsigned long flags;
+
+	/*
+	 * If we spin here is because we are in timeout, so we are going
+	 * to be in STATE_ERROR. See ocores_process_timeout()
+	 */
+	spin_lock_irqsave(&i2c->process_lock, flags);
 
 	if ((i2c->state == STATE_DONE) || (i2c->state == STATE_ERROR)) {
 		/* stop has been sent */
 		oc_setreg(i2c, OCI2C_CMD, OCI2C_CMD_IACK);
 		wake_up(&i2c->wait);
-		return;
+		goto out;
 	}
 
 	/* error? */
 	if (stat & OCI2C_STAT_ARBLOST) {
 		i2c->state = STATE_ERROR;
 		oc_setreg(i2c, OCI2C_CMD, OCI2C_CMD_STOP);
-		return;
+		goto out;
 	}
 
 	if ((i2c->state == STATE_START) || (i2c->state == STATE_WRITE)) {
@@ -163,7 +176,7 @@ static void ocores_process(struct ocores_i2c *i2c)
 		if (stat & OCI2C_STAT_NACK) {
 			i2c->state = STATE_ERROR;
 			oc_setreg(i2c, OCI2C_CMD, OCI2C_CMD_STOP);
-			return;
+			goto out;
 		}
 	} else
 		msg->buf[i2c->pos++] = oc_getreg(i2c, OCI2C_DATA);
@@ -184,14 +197,14 @@ static void ocores_process(struct ocores_i2c *i2c)
 
 				oc_setreg(i2c, OCI2C_DATA, addr);
 				oc_setreg(i2c, OCI2C_CMD,  OCI2C_CMD_START);
-				return;
+				goto out;
 			} else
 				i2c->state = (msg->flags & I2C_M_RD)
 					? STATE_READ : STATE_WRITE;
 		} else {
 			i2c->state = STATE_DONE;
 			oc_setreg(i2c, OCI2C_CMD, OCI2C_CMD_STOP);
-			return;
+			goto out;
 		}
 	}
 
@@ -202,6 +215,9 @@ static void ocores_process(struct ocores_i2c *i2c)
 		oc_setreg(i2c, OCI2C_DATA, msg->buf[i2c->pos++]);
 		oc_setreg(i2c, OCI2C_CMD, OCI2C_CMD_WRITE);
 	}
+
+out:
+	spin_unlock_irqrestore(&i2c->process_lock, flags);
 }
 
 static irqreturn_t ocores_isr(int irq, void *dev_id)
@@ -213,9 +229,24 @@ static irqreturn_t ocores_isr(int irq, void *dev_id)
 	return IRQ_HANDLED;
 }
 
+/**
+ * Process timeout event
+ * @i2c: ocores I2C device instance
+ */
+static void ocores_process_timeout(struct ocores_i2c *i2c)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&i2c->process_lock, flags);
+	i2c->state = STATE_ERROR;
+	oc_setreg(i2c, OCI2C_CMD, OCI2C_CMD_STOP);
+	spin_unlock_irqrestore(&i2c->process_lock, flags);
+}
+
 static int ocores_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
 {
 	struct ocores_i2c *i2c = i2c_get_adapdata(adap);
+	int ret;
 
 	i2c->msg = msgs;
 	i2c->pos = 0;
@@ -225,11 +256,14 @@ static int ocores_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
 	oc_setreg(i2c, OCI2C_DATA, i2c_8bit_addr_from_msg(i2c->msg));
 	oc_setreg(i2c, OCI2C_CMD, OCI2C_CMD_START);
 
-	if (wait_event_timeout(i2c->wait, (i2c->state == STATE_ERROR) ||
-			       (i2c->state == STATE_DONE), HZ))
-		return (i2c->state == STATE_DONE) ? num : -EIO;
-	else
+	ret = wait_event_timeout(i2c->wait, (i2c->state == STATE_ERROR) ||
+				 (i2c->state == STATE_DONE), HZ);
+	if (ret == 0) {
+		ocores_process_timeout(i2c);
 		return -ETIMEDOUT;
+	}
+
+	return (i2c->state == STATE_DONE) ? num : -EIO;
 }
 
 static int ocores_init(struct device *dev, struct ocores_i2c *i2c)
@@ -422,6 +456,8 @@ static int ocores_i2c_probe(struct platform_device *pdev)
 	if (!i2c)
 		return -ENOMEM;
 
+	spin_lock_init(&i2c->process_lock);
+
 	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	i2c->base = devm_ioremap_resource(&pdev->dev, res);
 	if (IS_ERR(i2c->base))
-- 
2.15.0