[PATCH v3] i2c: at91: Send bus clear command if SCL or SDA is down

[Codrin Ciubotariu]

After a transfer timeout, some faulty I2C slave devices might hold down
the SCL or the SDA pins. We can generate a bus clear command, hoping that
the slave might release the pins.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@xxxxxxxxxxxxx>
Acked-by: Ludovic Desroches <ludovic.desroches@xxxxxxxxxxxxx>
---

Changes in v3:
 - fixed kbuild warning;

Changes in v2:
 - added '.has_clear_cmd' struct member to specify which IPs support the
   clear command; for now, only SAMA5D2 supports it;
 - added Ludovic's V1 ack since there were no major changes;

 drivers/i2c/busses/i2c-at91-core.c   |  8 ++++++++
 drivers/i2c/busses/i2c-at91-master.c | 22 ++++++++++++++++++++++
 drivers/i2c/busses/i2c-at91.h        |  7 ++++++-
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/drivers/i2c/busses/i2c-at91-core.c b/drivers/i2c/busses/i2c-at91-core.c
index 435c7d7377a3..cb07489e698f 100644
--- a/drivers/i2c/busses/i2c-at91-core.c
+++ b/drivers/i2c/busses/i2c-at91-core.c
@@ -68,6 +68,7 @@ static struct at91_twi_pdata at91rm9200_config = {
 	.has_unre_flag = true,
 	.has_alt_cmd = false,
 	.has_hold_field = false,
+	.has_clear_cmd = false,
 };
 
 static struct at91_twi_pdata at91sam9261_config = {
@@ -76,6 +77,7 @@ static struct at91_twi_pdata at91sam9261_config = {
 	.has_unre_flag = false,
 	.has_alt_cmd = false,
 	.has_hold_field = false,
+	.has_clear_cmd = false,
 };
 
 static struct at91_twi_pdata at91sam9260_config = {
@@ -84,6 +86,7 @@ static struct at91_twi_pdata at91sam9260_config = {
 	.has_unre_flag = false,
 	.has_alt_cmd = false,
 	.has_hold_field = false,
+	.has_clear_cmd = false,
 };
 
 static struct at91_twi_pdata at91sam9g20_config = {
@@ -92,6 +95,7 @@ static struct at91_twi_pdata at91sam9g20_config = {
 	.has_unre_flag = false,
 	.has_alt_cmd = false,
 	.has_hold_field = false,
+	.has_clear_cmd = false,
 };
 
 static struct at91_twi_pdata at91sam9g10_config = {
@@ -100,6 +104,7 @@ static struct at91_twi_pdata at91sam9g10_config = {
 	.has_unre_flag = false,
 	.has_alt_cmd = false,
 	.has_hold_field = false,
+	.has_clear_cmd = false,
 };
 
 static const struct platform_device_id at91_twi_devtypes[] = {
@@ -130,6 +135,7 @@ static struct at91_twi_pdata at91sam9x5_config = {
 	.has_unre_flag = false,
 	.has_alt_cmd = false,
 	.has_hold_field = false,
+	.has_clear_cmd = false,
 };
 
 static struct at91_twi_pdata sama5d4_config = {
@@ -138,6 +144,7 @@ static struct at91_twi_pdata sama5d4_config = {
 	.has_unre_flag = false,
 	.has_alt_cmd = false,
 	.has_hold_field = true,
+	.has_clear_cmd = false,
 };
 
 static struct at91_twi_pdata sama5d2_config = {
@@ -146,6 +153,7 @@ static struct at91_twi_pdata sama5d2_config = {
 	.has_unre_flag = true,
 	.has_alt_cmd = true,
 	.has_hold_field = true,
+	.has_clear_cmd = true,
 };
 
 static const struct of_device_id atmel_twi_dt_ids[] = {
diff --git a/drivers/i2c/busses/i2c-at91-master.c b/drivers/i2c/busses/i2c-at91-master.c
index a3fcc35ffd3b..12d4fa946a82 100644
--- a/drivers/i2c/busses/i2c-at91-master.c
+++ b/drivers/i2c/busses/i2c-at91-master.c
@@ -440,6 +440,7 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
 	unsigned long time_left;
 	bool has_unre_flag = dev->pdata->has_unre_flag;
 	bool has_alt_cmd = dev->pdata->has_alt_cmd;
+	bool has_clear_cmd = dev->pdata->has_clear_cmd;
 
 	/*
 	 * WARNING: the TXCOMP bit in the Status Register is NOT a clear on
@@ -599,6 +600,27 @@ static int at91_do_twi_transfer(struct at91_twi_dev *dev)
 		at91_twi_write(dev, AT91_TWI_CR,
 			       AT91_TWI_THRCLR | AT91_TWI_LOCKCLR);
 	}
+
+	/*
+	 * After timeout, some faulty I2C slave devices might hold SCL/SDA down;
+	 * we can send a bus clear command, hoping that the pins will be
+	 * released
+	 */
+	if (has_clear_cmd &&
+	    (!(dev->transfer_status & AT91_TWI_SDA) ||
+	     !(dev->transfer_status & AT91_TWI_SCL))) {
+		dev_dbg(dev->dev,
+			"SDA/SCL are down; sending bus clear command\n");
+		if (dev->use_alt_cmd) {
+			unsigned int acr;
+
+			acr = at91_twi_read(dev, AT91_TWI_ACR);
+			acr &= ~AT91_TWI_ACR_DATAL_MASK;
+			at91_twi_write(dev, AT91_TWI_ACR, acr);
+		}
+		at91_twi_write(dev, AT91_TWI_CR, AT91_TWI_CLEAR);
+	}
+
 	return ret;
 }
 
diff --git a/drivers/i2c/busses/i2c-at91.h b/drivers/i2c/busses/i2c-at91.h
index 499b506f6128..0827c28a84db 100644
--- a/drivers/i2c/busses/i2c-at91.h
+++ b/drivers/i2c/busses/i2c-at91.h
@@ -36,6 +36,7 @@
 #define	AT91_TWI_SVDIS		BIT(5)	/* Slave Transfer Disable */
 #define	AT91_TWI_QUICK		BIT(6)	/* SMBus quick command */
 #define	AT91_TWI_SWRST		BIT(7)	/* Software Reset */
+#define	AT91_TWI_CLEAR		BIT(15) /* Bus clear command */
 #define	AT91_TWI_ACMEN		BIT(16) /* Alternative Command Mode Enable */
 #define	AT91_TWI_ACMDIS		BIT(17) /* Alternative Command Mode Disable */
 #define	AT91_TWI_THRCLR		BIT(24) /* Transmit Holding Register Clear */
@@ -69,6 +70,8 @@
 #define	AT91_TWI_NACK		BIT(8)	/* Not Acknowledged */
 #define	AT91_TWI_EOSACC		BIT(11)	/* End Of Slave Access */
 #define	AT91_TWI_LOCK		BIT(23) /* TWI Lock due to Frame Errors */
+#define	AT91_TWI_SCL		BIT(24) /* TWI SCL status */
+#define	AT91_TWI_SDA		BIT(25) /* TWI SDA status */
 
 #define	AT91_TWI_INT_MASK \
 	(AT91_TWI_TXCOMP | AT91_TWI_RXRDY | AT91_TWI_TXRDY | AT91_TWI_NACK \
@@ -81,7 +84,8 @@
 #define	AT91_TWI_THR		0x0034	/* Transmit Holding Register */
 
 #define	AT91_TWI_ACR		0x0040	/* Alternative Command Register */
-#define	AT91_TWI_ACR_DATAL(len)	((len) & 0xff)
+#define	AT91_TWI_ACR_DATAL_MASK	GENMASK(15, 0)
+#define	AT91_TWI_ACR_DATAL(len)	((len) & AT91_TWI_ACR_DATAL_MASK)
 #define	AT91_TWI_ACR_DIR	BIT(8)
 
 #define	AT91_TWI_FMR		0x0050	/* FIFO Mode Register */
@@ -108,6 +112,7 @@ struct at91_twi_pdata {
 	bool has_unre_flag;
 	bool has_alt_cmd;
 	bool has_hold_field;
+	bool has_clear_cmd;
 	struct at_dma_slave dma_slave;
 };
 
-- 
2.20.1