[PATCH v3 0/4] i2c: imx: prevent rescheduling in non-dma mode

[PATCH v3 0/4] i2c: imx: prevent rescheduling in non-dma mode

[Stefan Eichenberger]

While running tests on an i.MX8M Mini connected to a TI ADS1015 ADC, we
found that the ADC would stop responding to i2c requests because it
would timeout after the bus was idle for 25ms. This timeout could be
traced back to the rescheduling events in the i2c-imx driver. The
problem is that if the system is under heavy load, the schedule call and
the wait_event_timeout may be rescheduled too late to reach the 25ms
timeout. The same problem may occur with other SMBus devices. Therefore,
this patchset removes the scheduling calls for non-DMA mode by handling
the interrupt events directly in the ISR instead of scheduling a task to
handle the events.

This patch will introduce some bigger changes because the logic for
handling events in the ISR had to be rewritten. Therefore we have tested
the following combinations:
- i.MX8M Mini with dma
- i.MX8M Mini without dma
- i.MX8M Plus with dma
- i.MX8M Plus without dma
- i.MX7D with dma
- i.MX7D without dma
- i.MX7D atomic mode

Because we do not have any devices that use the SMBus block transfer
mode, we were not able to test it. 

The ideas are based on the RFC:
https://lore.kernel.org/all/20240531142437.74831-1-eichest@gmail.com/
However, the handling of events in the ISR is new, because further
testing showed that it was not enough to simply remove the schedule
call.

Changes since v2:
- Add Acked-by tags from Oleksij
- Renamed i2c_imx_start_read to i2c_imx_prepare_read
- I did not add a Fixes tag because the issues from Flavio have a
  different root cause and are not fixed by this patchset

Changes since v1:
- Add Reviewed-by tags from Frank
- Add new patch to use readb_relaxed and writeb_relaxed (Frank)
- Update commit message for patch 1 with some clarifications (Frank)

Stefan Eichenberger (4):
  i2c: imx: only poll for bus busy in multi master mode
  i2c: imx: separate atomic, dma and non-dma use case
  i2c: imx: use readb_relaxed and writeb_relaxed
  i2c: imx: prevent rescheduling in non dma mode

 drivers/i2c/busses/i2c-imx.c | 357 ++++++++++++++++++++++++++++++-----
 1 file changed, 305 insertions(+), 52 deletions(-)

-- 
2.43.0

[PATCH v3 1/4] i2c: imx: only poll for bus busy in multi master mode

[Stefan Eichenberger]

From: Stefan Eichenberger <stefan.eichenberger@toradex.com>

According to the i.MX8M Mini reference manual chapter "16.1.4.2
Generation of Start" it is only necessary to poll for bus busy and
arbitration lost in multi master mode. This helps to avoid rescheduling
while the i2c bus is busy and avoids SMBus devices to timeout.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/i2c/busses/i2c-imx.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 3842e527116b7..1add946e3bc20 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -216,6 +216,8 @@ struct imx_i2c_struct {
 	struct i2c_client	*slave;
 	enum i2c_slave_event last_slave_event;
 
+	bool			multi_master;
+
 	/* For checking slave events. */
 	spinlock_t     slave_lock;
 	struct hrtimer slave_timer;
@@ -481,6 +483,9 @@ static int i2c_imx_bus_busy(struct imx_i2c_struct *i2c_imx, int for_busy, bool a
 	unsigned long orig_jiffies = jiffies;
 	unsigned int temp;
 
+	if (!i2c_imx->multi_master)
+		return 0;
+
 	while (1) {
 		temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2SR);
 
@@ -540,8 +545,8 @@ static int i2c_imx_trx_complete(struct imx_i2c_struct *i2c_imx, bool atomic)
 		return -ETIMEDOUT;
 	}
 
-	/* check for arbitration lost */
-	if (i2c_imx->i2csr & I2SR_IAL) {
+	/* In multi-master mode check for arbitration lost */
+	if (i2c_imx->multi_master && (i2c_imx->i2csr & I2SR_IAL)) {
 		dev_dbg(&i2c_imx->adapter.dev, "<%s> Arbitration lost\n", __func__);
 		i2c_imx_clear_irq(i2c_imx, I2SR_IAL);
 
@@ -1468,6 +1473,8 @@ static int i2c_imx_probe(struct platform_device *pdev)
 		goto rpm_disable;
 	}
 
+	i2c_imx->multi_master = of_property_read_bool(pdev->dev.of_node, "multi-master");
+
 	/* Set up clock divider */
 	i2c_imx->bitrate = I2C_MAX_STANDARD_MODE_FREQ;
 	ret = of_property_read_u32(pdev->dev.of_node,
-- 
2.43.0

[PATCH v3 2/4] i2c: imx: separate atomic, dma and non-dma use case

[Stefan Eichenberger]

From: Stefan Eichenberger <stefan.eichenberger@toradex.com>

Separate the atomic, dma and non-dma use case as a preparation step for
moving the non-dma use case to the isr to avoid rescheduling while a
transfer is in progress.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/i2c/busses/i2c-imx.c | 107 +++++++++++++++++++++++------------
 1 file changed, 70 insertions(+), 37 deletions(-)

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 1add946e3bc20..2e86d363e186d 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -1011,6 +1011,43 @@ static int i2c_imx_dma_write(struct imx_i2c_struct *i2c_imx,
 	return i2c_imx_acked(i2c_imx);
 }
 
+static int i2c_imx_prepare_read(struct imx_i2c_struct *i2c_imx,
+			       struct i2c_msg *msgs, bool atomic,
+			       bool use_dma)
+{
+	int result;
+	unsigned int temp = 0;
+
+	/* write slave address */
+	imx_i2c_write_reg(i2c_8bit_addr_from_msg(msgs), i2c_imx, IMX_I2C_I2DR);
+	result = i2c_imx_trx_complete(i2c_imx, atomic);
+	if (result)
+		return result;
+	result = i2c_imx_acked(i2c_imx);
+	if (result)
+		return result;
+
+	dev_dbg(&i2c_imx->adapter.dev, "<%s> setup bus\n", __func__);
+
+	/* setup bus to read data */
+	temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
+	temp &= ~I2CR_MTX;
+
+	/*
+	 * Reset the I2CR_TXAK flag initially for SMBus block read since the
+	 * length is unknown
+	 */
+	if (msgs->len - 1)
+		temp &= ~I2CR_TXAK;
+	if (use_dma)
+		temp |= I2CR_DMAEN;
+
+	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
+	imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR); /* dummy read */
+
+	return 0;
+}
+
 static int i2c_imx_dma_read(struct imx_i2c_struct *i2c_imx,
 			struct i2c_msg *msgs, bool is_lastmsg)
 {
@@ -1021,6 +1058,11 @@ static int i2c_imx_dma_read(struct imx_i2c_struct *i2c_imx,
 	struct imx_i2c_dma *dma = i2c_imx->dma;
 	struct device *dev = &i2c_imx->adapter.dev;
 
+	result = i2c_imx_prepare_read(i2c_imx, msgs, false, true);
+	if (result)
+		return result;
+
+	dev_dbg(&i2c_imx->adapter.dev, "<%s> read data\n", __func__);
 
 	dma->chan_using = dma->chan_rx;
 	dma->dma_transfer_dir = DMA_DEV_TO_MEM;
@@ -1131,50 +1173,24 @@ static int i2c_imx_write(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 	return 0;
 }
 
+static int i2c_imx_atomic_write(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs)
+{
+	return i2c_imx_write(i2c_imx, msgs, true);
+}
+
 static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 			bool is_lastmsg, bool atomic)
 {
 	int i, result;
 	unsigned int temp;
 	int block_data = msgs->flags & I2C_M_RECV_LEN;
-	int use_dma = i2c_imx->dma && msgs->flags & I2C_M_DMA_SAFE &&
-		msgs->len >= DMA_THRESHOLD && !block_data;
 
-	dev_dbg(&i2c_imx->adapter.dev,
-		"<%s> write slave address: addr=0x%x\n",
-		__func__, i2c_8bit_addr_from_msg(msgs));
-
-	/* write slave address */
-	imx_i2c_write_reg(i2c_8bit_addr_from_msg(msgs), i2c_imx, IMX_I2C_I2DR);
-	result = i2c_imx_trx_complete(i2c_imx, atomic);
+	result = i2c_imx_prepare_read(i2c_imx, msgs, atomic, false);
 	if (result)
 		return result;
-	result = i2c_imx_acked(i2c_imx);
-	if (result)
-		return result;
-
-	dev_dbg(&i2c_imx->adapter.dev, "<%s> setup bus\n", __func__);
-
-	/* setup bus to read data */
-	temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
-	temp &= ~I2CR_MTX;
-
-	/*
-	 * Reset the I2CR_TXAK flag initially for SMBus block read since the
-	 * length is unknown
-	 */
-	if ((msgs->len - 1) || block_data)
-		temp &= ~I2CR_TXAK;
-	if (use_dma)
-		temp |= I2CR_DMAEN;
-	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
-	imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR); /* dummy read */
 
 	dev_dbg(&i2c_imx->adapter.dev, "<%s> read data\n", __func__);
 
-	if (use_dma)
-		return i2c_imx_dma_read(i2c_imx, msgs, is_lastmsg);
-
 	/* read data */
 	for (i = 0; i < msgs->len; i++) {
 		u8 len = 0;
@@ -1241,6 +1257,12 @@ static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 	return 0;
 }
 
+static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
+			bool is_lastmsg)
+{
+	return i2c_imx_read(i2c_imx, msgs, is_lastmsg, true);
+}
+
 static int i2c_imx_xfer_common(struct i2c_adapter *adapter,
 			       struct i2c_msg *msgs, int num, bool atomic)
 {
@@ -1248,6 +1270,7 @@ static int i2c_imx_xfer_common(struct i2c_adapter *adapter,
 	int result;
 	bool is_lastmsg = false;
 	struct imx_i2c_struct *i2c_imx = i2c_get_adapdata(adapter);
+	int use_dma = 0;
 
 	/* Start I2C transfer */
 	result = i2c_imx_start(i2c_imx, atomic);
@@ -1300,15 +1323,25 @@ static int i2c_imx_xfer_common(struct i2c_adapter *adapter,
 			(temp & I2SR_SRW ? 1 : 0), (temp & I2SR_IIF ? 1 : 0),
 			(temp & I2SR_RXAK ? 1 : 0));
 #endif
+
+		use_dma = i2c_imx->dma && msgs[i].len >= DMA_THRESHOLD &&
+			msgs[i].flags & I2C_M_DMA_SAFE;
 		if (msgs[i].flags & I2C_M_RD) {
-			result = i2c_imx_read(i2c_imx, &msgs[i], is_lastmsg, atomic);
+			int block_data = msgs->flags & I2C_M_RECV_LEN;
+
+			if (atomic)
+				result = i2c_imx_atomic_read(i2c_imx, &msgs[i], is_lastmsg);
+			else if (use_dma && !block_data)
+				result = i2c_imx_dma_read(i2c_imx, &msgs[i], is_lastmsg);
+			else
+				result = i2c_imx_read(i2c_imx, &msgs[i], is_lastmsg, false);
 		} else {
-			if (!atomic &&
-			    i2c_imx->dma && msgs[i].len >= DMA_THRESHOLD &&
-				msgs[i].flags & I2C_M_DMA_SAFE)
+			if (atomic)
+				result = i2c_imx_atomic_write(i2c_imx, &msgs[i]);
+			else if (use_dma)
 				result = i2c_imx_dma_write(i2c_imx, &msgs[i]);
 			else
-				result = i2c_imx_write(i2c_imx, &msgs[i], atomic);
+				result = i2c_imx_write(i2c_imx, &msgs[i], false);
 		}
 		if (result)
 			goto fail0;
-- 
2.43.0

[PATCH v3 3/4] i2c: imx: use readb_relaxed and writeb_relaxed

[Stefan Eichenberger]

From: Stefan Eichenberger <stefan.eichenberger@toradex.com>

Use the relaxed version of readb and writeb to reduce overhead. It is
safe to use the relaxed version because we either do not rely on dma
completion, or we use a dma callback to ensure that the dma transfer is
complete before we continue.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/i2c/busses/i2c-imx.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 2e86d363e186d..9917f3e1d06cf 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -313,13 +313,13 @@ static inline int is_vf610_i2c(struct imx_i2c_struct *i2c_imx)
 static inline void imx_i2c_write_reg(unsigned int val,
 		struct imx_i2c_struct *i2c_imx, unsigned int reg)
 {
-	writeb(val, i2c_imx->base + (reg << i2c_imx->hwdata->regshift));
+	writeb_relaxed(val, i2c_imx->base + (reg << i2c_imx->hwdata->regshift));
 }
 
 static inline unsigned char imx_i2c_read_reg(struct imx_i2c_struct *i2c_imx,
 		unsigned int reg)
 {
-	return readb(i2c_imx->base + (reg << i2c_imx->hwdata->regshift));
+	return readb_relaxed(i2c_imx->base + (reg << i2c_imx->hwdata->regshift));
 }
 
 static void i2c_imx_clear_irq(struct imx_i2c_struct *i2c_imx, unsigned int bits)
-- 
2.43.0

[PATCH v3 4/4] i2c: imx: prevent rescheduling in non dma mode

[Stefan Eichenberger]

From: Stefan Eichenberger <stefan.eichenberger@toradex.com>

We are experiencing a problem with the i.MX I2C controller when
communicating with SMBus devices. We are seeing devices time-out because
the time between sending/receiving two bytes is too long, and the SMBus
device returns to the idle state. This happens because the i.MX I2C
controller sends and receives byte by byte. When a byte is sent or
received, we get an interrupt and can send or receive the next byte.

The current implementation sends a byte and then waits for an event
generated by the interrupt subroutine. After the event is received, the
next byte is sent and we wait again. This waiting allows the scheduler
to reschedule other tasks, with the disadvantage that we may not send
the next byte for a long time because the send task is not immediately
scheduled. For example, if the rescheduling takes more than 25ms, this
can cause SMBus devices to timeout and communication to fail.

This patch changes the behavior so that we do not reschedule the
send/receive task, but instead send or receive the next byte in the
interrupt subroutine. This prevents rescheduling and drastically reduces
the time between sending/receiving bytes. The cost in the interrupt
subroutine is relatively small, we check what state we are in and then
send/receive the next byte. Before we had to call wake_up, which is even
less expensive. However, we also had to do some scheduling, which
increased the overall cost compared to the new solution. The wake_up
function to wake up the send/receive task is now only called when an
error occurs or when the transfer is complete.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
 drivers/i2c/busses/i2c-imx.c | 257 ++++++++++++++++++++++++++++++++---
 1 file changed, 235 insertions(+), 22 deletions(-)

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 9917f3e1d06cf..c2a92d1857af1 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -197,6 +197,17 @@ struct imx_i2c_dma {
 	enum dma_data_direction dma_data_dir;
 };
 
+enum imx_i2c_state {
+	IMX_I2C_STATE_DONE,
+	IMX_I2C_STATE_FAILED,
+	IMX_I2C_STATE_WRITE,
+	IMX_I2C_STATE_DMA,
+	IMX_I2C_STATE_READ,
+	IMX_I2C_STATE_READ_CONTINUE,
+	IMX_I2C_STATE_READ_BLOCK_DATA,
+	IMX_I2C_STATE_READ_BLOCK_DATA_LEN,
+};
+
 struct imx_i2c_struct {
 	struct i2c_adapter	adapter;
 	struct clk		*clk;
@@ -216,6 +227,12 @@ struct imx_i2c_struct {
 	struct i2c_client	*slave;
 	enum i2c_slave_event last_slave_event;
 
+	struct i2c_msg		*msg;
+	unsigned int		msg_buf_idx;
+	int			isr_result;
+	bool			is_lastmsg;
+	enum imx_i2c_state	state;
+
 	bool			multi_master;
 
 	/* For checking slave events. */
@@ -908,11 +925,150 @@ static int i2c_imx_unreg_slave(struct i2c_client *client)
 	return ret;
 }
 
+static inline int i2c_imx_isr_acked(struct imx_i2c_struct *i2c_imx)
+{
+	i2c_imx->isr_result = 0;
+
+	if (imx_i2c_read_reg(i2c_imx, IMX_I2C_I2SR) & I2SR_RXAK) {
+		i2c_imx->state = IMX_I2C_STATE_FAILED;
+		i2c_imx->isr_result = -ENXIO;
+		wake_up(&i2c_imx->queue);
+	}
+
+	return i2c_imx->isr_result;
+}
+
+static inline int i2c_imx_isr_write(struct imx_i2c_struct *i2c_imx)
+{
+	int result;
+
+	result = i2c_imx_isr_acked(i2c_imx);
+	if (result)
+		return result;
+
+	if (i2c_imx->msg->len == i2c_imx->msg_buf_idx)
+		return 0;
+
+	imx_i2c_write_reg(i2c_imx->msg->buf[i2c_imx->msg_buf_idx++], i2c_imx, IMX_I2C_I2DR);
+
+	return 1;
+}
+
+static inline int i2c_imx_isr_read(struct imx_i2c_struct *i2c_imx)
+{
+	int result;
+	unsigned int temp;
+
+	result = i2c_imx_isr_acked(i2c_imx);
+	if (result)
+		return result;
+
+	/* setup bus to read data */
+	temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
+	temp &= ~I2CR_MTX;
+	if (i2c_imx->msg->len - 1)
+		temp &= ~I2CR_TXAK;
+
+	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
+	imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR); /* dummy read */
+
+	return 0;
+}
+
+static inline void i2c_imx_isr_read_continue(struct imx_i2c_struct *i2c_imx)
+{
+	unsigned int temp;
+
+	if ((i2c_imx->msg->len - 1) == i2c_imx->msg_buf_idx) {
+		if (i2c_imx->is_lastmsg) {
+			/*
+			 * It must generate STOP before read I2DR to prevent
+			 * controller from generating another clock cycle
+			 */
+			temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
+			if (!(temp & I2CR_MSTA))
+				i2c_imx->stopped =  1;
+			temp &= ~(I2CR_MSTA | I2CR_MTX);
+			imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
+		} else {
+			/*
+			 * For i2c master receiver repeat restart operation like:
+			 * read -> repeat MSTA -> read/write
+			 * The controller must set MTX before read the last byte in
+			 * the first read operation, otherwise the first read cost
+			 * one extra clock cycle.
+			 */
+			temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
+			temp |= I2CR_MTX;
+			imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
+		}
+	} else if (i2c_imx->msg_buf_idx == (i2c_imx->msg->len - 2)) {
+		temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
+		temp |= I2CR_TXAK;
+		imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
+	}
+
+	i2c_imx->msg->buf[i2c_imx->msg_buf_idx++] = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR);
+}
+
+static inline void i2c_imx_isr_read_block_data_len(struct imx_i2c_struct *i2c_imx)
+{
+	u8 len = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR);
+
+	if ((len == 0) || (len > I2C_SMBUS_BLOCK_MAX)) {
+		i2c_imx->isr_result = -EPROTO;
+		i2c_imx->state = IMX_I2C_STATE_FAILED;
+		wake_up(&i2c_imx->queue);
+	}
+	i2c_imx->msg->len += len;
+}
+
 static irqreturn_t i2c_imx_master_isr(struct imx_i2c_struct *i2c_imx, unsigned int status)
 {
-	/* save status register */
-	i2c_imx->i2csr = status;
-	wake_up(&i2c_imx->queue);
+	switch (i2c_imx->state) {
+	case IMX_I2C_STATE_DMA:
+		i2c_imx->i2csr = status;
+		wake_up(&i2c_imx->queue);
+		break;
+
+	case IMX_I2C_STATE_READ:
+		if (i2c_imx_isr_read(i2c_imx))
+			break;
+		i2c_imx->state = IMX_I2C_STATE_READ_CONTINUE;
+		break;
+
+	case IMX_I2C_STATE_READ_CONTINUE:
+		i2c_imx_isr_read_continue(i2c_imx);
+		if (i2c_imx->msg_buf_idx == i2c_imx->msg->len) {
+			i2c_imx->state = IMX_I2C_STATE_DONE;
+			wake_up(&i2c_imx->queue);
+		}
+		break;
+
+	case IMX_I2C_STATE_READ_BLOCK_DATA:
+		if (i2c_imx_isr_read(i2c_imx))
+			break;
+		i2c_imx->state = IMX_I2C_STATE_READ_BLOCK_DATA_LEN;
+		break;
+
+	case IMX_I2C_STATE_READ_BLOCK_DATA_LEN:
+		i2c_imx_isr_read_block_data_len(i2c_imx);
+		i2c_imx->state = IMX_I2C_STATE_READ_CONTINUE;
+		break;
+
+	case IMX_I2C_STATE_WRITE:
+		if (i2c_imx_isr_write(i2c_imx))
+			break;
+		i2c_imx->state = IMX_I2C_STATE_DONE;
+		wake_up(&i2c_imx->queue);
+		break;
+
+	default:
+		i2c_imx->i2csr = status;
+		i2c_imx->state = IMX_I2C_STATE_FAILED;
+		i2c_imx->isr_result = -EINVAL;
+		wake_up(&i2c_imx->queue);
+	}
 
 	return IRQ_HANDLED;
 }
@@ -959,6 +1115,8 @@ static int i2c_imx_dma_write(struct imx_i2c_struct *i2c_imx,
 	struct imx_i2c_dma *dma = i2c_imx->dma;
 	struct device *dev = &i2c_imx->adapter.dev;
 
+	i2c_imx->state = IMX_I2C_STATE_DMA;
+
 	dma->chan_using = dma->chan_tx;
 	dma->dma_transfer_dir = DMA_MEM_TO_DEV;
 	dma->dma_data_dir = DMA_TO_DEVICE;
@@ -1012,15 +1170,14 @@ static int i2c_imx_dma_write(struct imx_i2c_struct *i2c_imx,
 }
 
 static int i2c_imx_prepare_read(struct imx_i2c_struct *i2c_imx,
-			       struct i2c_msg *msgs, bool atomic,
-			       bool use_dma)
+			       struct i2c_msg *msgs, bool use_dma)
 {
 	int result;
 	unsigned int temp = 0;
 
 	/* write slave address */
 	imx_i2c_write_reg(i2c_8bit_addr_from_msg(msgs), i2c_imx, IMX_I2C_I2DR);
-	result = i2c_imx_trx_complete(i2c_imx, atomic);
+	result = i2c_imx_trx_complete(i2c_imx, !use_dma);
 	if (result)
 		return result;
 	result = i2c_imx_acked(i2c_imx);
@@ -1058,7 +1215,9 @@ static int i2c_imx_dma_read(struct imx_i2c_struct *i2c_imx,
 	struct imx_i2c_dma *dma = i2c_imx->dma;
 	struct device *dev = &i2c_imx->adapter.dev;
 
-	result = i2c_imx_prepare_read(i2c_imx, msgs, false, true);
+	i2c_imx->state = IMX_I2C_STATE_DMA;
+
+	result = i2c_imx_prepare_read(i2c_imx, msgs, true);
 	if (result)
 		return result;
 
@@ -1139,8 +1298,8 @@ static int i2c_imx_dma_read(struct imx_i2c_struct *i2c_imx,
 	return 0;
 }
 
-static int i2c_imx_write(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
-			 bool atomic)
+static int i2c_imx_atomic_write(struct imx_i2c_struct *i2c_imx,
+				struct i2c_msg *msgs)
 {
 	int i, result;
 
@@ -1149,7 +1308,7 @@ static int i2c_imx_write(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 
 	/* write slave address */
 	imx_i2c_write_reg(i2c_8bit_addr_from_msg(msgs), i2c_imx, IMX_I2C_I2DR);
-	result = i2c_imx_trx_complete(i2c_imx, atomic);
+	result = i2c_imx_trx_complete(i2c_imx, true);
 	if (result)
 		return result;
 	result = i2c_imx_acked(i2c_imx);
@@ -1163,7 +1322,7 @@ static int i2c_imx_write(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 			"<%s> write byte: B%d=0x%X\n",
 			__func__, i, msgs->buf[i]);
 		imx_i2c_write_reg(msgs->buf[i], i2c_imx, IMX_I2C_I2DR);
-		result = i2c_imx_trx_complete(i2c_imx, atomic);
+		result = i2c_imx_trx_complete(i2c_imx, true);
 		if (result)
 			return result;
 		result = i2c_imx_acked(i2c_imx);
@@ -1173,19 +1332,40 @@ static int i2c_imx_write(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 	return 0;
 }
 
-static int i2c_imx_atomic_write(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs)
+static int i2c_imx_write(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs)
 {
-	return i2c_imx_write(i2c_imx, msgs, true);
+	dev_dbg(&i2c_imx->adapter.dev, "<%s> write slave address: addr=0x%x\n",
+		__func__, i2c_8bit_addr_from_msg(msgs));
+
+	i2c_imx->state = IMX_I2C_STATE_WRITE;
+	i2c_imx->msg = msgs;
+	i2c_imx->msg_buf_idx = 0;
+	/* write slave address and start transmission */
+	imx_i2c_write_reg(i2c_8bit_addr_from_msg(msgs), i2c_imx, IMX_I2C_I2DR);
+	wait_event_timeout(i2c_imx->queue,
+			   i2c_imx->state == IMX_I2C_STATE_DONE ||
+			   i2c_imx->state == IMX_I2C_STATE_FAILED,
+			   (msgs->len + 1)*HZ / 10);
+	if (i2c_imx->state == IMX_I2C_STATE_FAILED) {
+		dev_err(&i2c_imx->adapter.dev, "<%s> write failed with %d\n",
+			__func__, i2c_imx->isr_result);
+		return i2c_imx->isr_result;
+	}
+	if (i2c_imx->state != IMX_I2C_STATE_DONE) {
+		dev_err(&i2c_imx->adapter.dev, "<%s> write timedout\n", __func__);
+		return -ETIMEDOUT;
+	}
+	return 0;
 }
 
-static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
-			bool is_lastmsg, bool atomic)
+static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx,
+			       struct i2c_msg *msgs, bool is_lastmsg)
 {
 	int i, result;
 	unsigned int temp;
 	int block_data = msgs->flags & I2C_M_RECV_LEN;
 
-	result = i2c_imx_prepare_read(i2c_imx, msgs, atomic, false);
+	result = i2c_imx_prepare_read(i2c_imx, msgs, false);
 	if (result)
 		return result;
 
@@ -1195,7 +1375,7 @@ static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 	for (i = 0; i < msgs->len; i++) {
 		u8 len = 0;
 
-		result = i2c_imx_trx_complete(i2c_imx, atomic);
+		result = i2c_imx_trx_complete(i2c_imx, true);
 		if (result)
 			return result;
 		/*
@@ -1226,7 +1406,7 @@ static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 				temp &= ~(I2CR_MSTA | I2CR_MTX);
 				imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
 				if (!i2c_imx->stopped)
-					i2c_imx_bus_busy(i2c_imx, 0, atomic);
+					i2c_imx_bus_busy(i2c_imx, 0, true);
 			} else {
 				/*
 				 * For i2c master receiver repeat restart operation like:
@@ -1257,10 +1437,43 @@ static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 	return 0;
 }
 
-static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
+static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,
 			bool is_lastmsg)
 {
-	return i2c_imx_read(i2c_imx, msgs, is_lastmsg, true);
+	int block_data = msgs->flags & I2C_M_RECV_LEN;
+
+	dev_dbg(&i2c_imx->adapter.dev,
+		"<%s> write slave address: addr=0x%x\n",
+		__func__, i2c_8bit_addr_from_msg(msgs));
+
+	i2c_imx->is_lastmsg = is_lastmsg;
+
+	if (block_data)
+		i2c_imx->state = IMX_I2C_STATE_READ_BLOCK_DATA;
+	else
+		i2c_imx->state = IMX_I2C_STATE_READ;
+	i2c_imx->msg = msgs;
+	i2c_imx->msg_buf_idx = 0;
+
+	/* write slave address */
+	imx_i2c_write_reg(i2c_8bit_addr_from_msg(msgs), i2c_imx, IMX_I2C_I2DR);
+	wait_event_timeout(i2c_imx->queue,
+			   i2c_imx->state == IMX_I2C_STATE_DONE ||
+			   i2c_imx->state == IMX_I2C_STATE_FAILED,
+			   (msgs->len + 1)*HZ / 10);
+	if (i2c_imx->state == IMX_I2C_STATE_FAILED) {
+		dev_err(&i2c_imx->adapter.dev, "<%s> write failed with %d\n",
+			__func__, i2c_imx->isr_result);
+		return i2c_imx->isr_result;
+	}
+	if (i2c_imx->state != IMX_I2C_STATE_DONE) {
+		dev_err(&i2c_imx->adapter.dev, "<%s> write timedout\n", __func__);
+		return -ETIMEDOUT;
+	}
+	if (!i2c_imx->stopped)
+		return i2c_imx_bus_busy(i2c_imx, 0, false);
+
+	return 0;
 }
 
 static int i2c_imx_xfer_common(struct i2c_adapter *adapter,
@@ -1334,14 +1547,14 @@ static int i2c_imx_xfer_common(struct i2c_adapter *adapter,
 			else if (use_dma && !block_data)
 				result = i2c_imx_dma_read(i2c_imx, &msgs[i], is_lastmsg);
 			else
-				result = i2c_imx_read(i2c_imx, &msgs[i], is_lastmsg, false);
+				result = i2c_imx_read(i2c_imx, &msgs[i], is_lastmsg);
 		} else {
 			if (atomic)
 				result = i2c_imx_atomic_write(i2c_imx, &msgs[i]);
 			else if (use_dma)
 				result = i2c_imx_dma_write(i2c_imx, &msgs[i]);
 			else
-				result = i2c_imx_write(i2c_imx, &msgs[i], false);
+				result = i2c_imx_write(i2c_imx, &msgs[i]);
 		}
 		if (result)
 			goto fail0;
-- 
2.43.0

Re: [PATCH v3 4/4] i2c: imx: prevent rescheduling in non dma mode

[Andi Shyti]

Hi Stefan,

One final ask...

On Mon, Sep 02, 2024 at 09:42:04AM GMT, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <stefan.eichenberger@toradex.com>
> 
> We are experiencing a problem with the i.MX I2C controller when
> communicating with SMBus devices. We are seeing devices time-out because
> the time between sending/receiving two bytes is too long, and the SMBus
> device returns to the idle state. This happens because the i.MX I2C
> controller sends and receives byte by byte. When a byte is sent or
> received, we get an interrupt and can send or receive the next byte.
> 
> The current implementation sends a byte and then waits for an event
> generated by the interrupt subroutine. After the event is received, the
> next byte is sent and we wait again. This waiting allows the scheduler
> to reschedule other tasks, with the disadvantage that we may not send
> the next byte for a long time because the send task is not immediately
> scheduled. For example, if the rescheduling takes more than 25ms, this
> can cause SMBus devices to timeout and communication to fail.
> 
> This patch changes the behavior so that we do not reschedule the
> send/receive task, but instead send or receive the next byte in the
> interrupt subroutine. This prevents rescheduling and drastically reduces
> the time between sending/receiving bytes. The cost in the interrupt
> subroutine is relatively small, we check what state we are in and then
> send/receive the next byte. Before we had to call wake_up, which is even
> less expensive. However, we also had to do some scheduling, which
> increased the overall cost compared to the new solution. The wake_up
> function to wake up the send/receive task is now only called when an
> error occurs or when the transfer is complete.
> 
> Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
> Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>

The code is fine and looks good to me. The commit log is also
very descriptive. However, there isn’t a single line of comment
in the code.

If someone else encounters this code without understanding your
specific context, they might find it too complex and attempt to
simplify it or revert to the previous implementation.

Please add comments to describe the state machine you
implemented, the reasoning behind it (as you explained in the
commit log), and make it understandable for those who haven’t
reviewed your patches.

Thanks,
Andi

Re: [PATCH v3 4/4] i2c: imx: prevent rescheduling in non dma mode

[Andi Shyti]

Hi Stefan,

On Wed, Sep 04, 2024 at 01:27:55PM GMT, Stefan Eichenberger wrote:
> On Tuesday 3 September 2024, Andi Shyti <andi.shyti@kernel.org> wrote:
> 
>     Hi Stefan,
> 
>     One final ask...
> 
>     On Mon, Sep 02, 2024 at 09:42:04AM GMT, Stefan Eichenberger wrote:
>     > From: Stefan Eichenberger <stefan.eichenberger@toradex.com>
>     >
>     > We are experiencing a problem with the i.MX I2C controller when
>     > communicating with SMBus devices. We are seeing devices time-out because
>     > the time between sending/receiving two bytes is too long, and the SMBus
>     > device returns to the idle state. This happens because the i.MX I2C
>     > controller sends and receives byte by byte. When a byte is sent or
>     > received, we get an interrupt and can send or receive the next byte.
>     >
>     > The current implementation sends a byte and then waits for an event
>     > generated by the interrupt subroutine. After the event is received, the
>     > next byte is sent and we wait again. This waiting allows the scheduler
>     > to reschedule other tasks, with the disadvantage that we may not send
>     > the next byte for a long time because the send task is not immediately
>     > scheduled. For example, if the rescheduling takes more than 25ms, this
>     > can cause SMBus devices to timeout and communication to fail.
>     >
>     > This patch changes the behavior so that we do not reschedule the
>     > send/receive task, but instead send or receive the next byte in the
>     > interrupt subroutine. This prevents rescheduling and drastically reduces
>     > the time between sending/receiving bytes. The cost in the interrupt
>     > subroutine is relatively small, we check what state we are in and then
>     > send/receive the next byte. Before we had to call wake_up, which is even
>     > less expensive. However, we also had to do some scheduling, which
>     > increased the overall cost compared to the new solution. The wake_up
>     > function to wake up the send/receive task is now only called when an
>     > error occurs or when the transfer is complete.
>     >
>     > Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
>     > Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
> 
>     The code is fine and looks good to me. The commit log is also
>     very descriptive. However, there isn’t a single line of comment
>     in the code.
> 
>     If someone else encounters this code without understanding your
>     specific context, they might find it too complex and attempt to
>     simplify it or revert to the previous implementation.
> 
>     Please add comments to describe the state machine you
>     implemented, the reasoning behind it (as you explained in the
>     commit log), and make it understandable for those who haven’t
>     reviewed your patches.
> 
> 
> Thanks a lot for the feedback. I only have limited access to my computer
> the next two-three weeks but will add comments as soon as I'm back. 

Other than the comment, there are also some checkpatch errors.
Please, do fix those errors, add the code and then I will take
your patches.

Sorry about that,
Andi