[PATCH v2] i2c: imx: honour I2C_M_IGNORE_NAK
Haobin Jiang
lemonoutput at foxmail.com
Wed Sep 2 23:29:15 PDT 2026
The i.MX controller has no hardware "ignore NAK" enable bit and checks
each byte in software after transmitting it: i2c_imx_acked() on the
atomic paths, i2c_imx_isr_acked() on the ISR path. Any NAK is
therefore treated as a fatal error and aborts the transfer with -ENXIO.
There are legitimate cases where a target NAKs on purpose and the
transfer must continue anyway. The standard way to express this is
I2C_M_IGNORE_NAK, which the core passes through to the bus driver
unchanged: i2c_transfer_buffer_flags() performs no validation of the
message flags, and the i.MX driver has never advertised
I2C_FUNC_PROTOCOL_MANGLING. Today such a transfer transmits bytes
until the first NAK and then fails with -ENXIO, silently dropping the
flag. One in-tree caller that hits this on i.MX buses is the wake
sequence of the Atmel crypto drivers (drivers/crypto/atmel-i2c.c, used
by atmel-ecc.c for the ATECC508A/608A): the sleeping device NAKs the
wake token and the write must complete anyway; the ATECC508A is wired
to an i.MX I2C controller e.g. on the Gateworks GW5904.
Advertise I2C_FUNC_PROTOCOL_MANGLING and honour I2C_M_IGNORE_NAK at
every software ACK check: on the PIO write paths, which check the ACK
of the address and of every data byte, and on the PIO read paths, which
check the target's ACK of the address byte, mirroring i2c-img-scb.
With the flag set a NAK no longer aborts the transfer and the byte loop
continues so the remaining bytes are still transmitted. Every transfer
without the flag keeps the previous behaviour unchanged.
A NAK'd byte does not generate a further DMA request, so a DMA write to
a deliberately-NAKing target would stall until DMA_TIMEOUT instead of
completing. Force the PIO path for every message carrying
I2C_M_IGNORE_NAK, again mirroring i2c-img-scb, which implements the
flag in atomic mode only. The DMA write path therefore never sees the
flag, but it passes the flag through at the final ACK check like the
other paths.
I2C_FUNC_PROTOCOL_MANGLING is a blanket capability bit, so advertising
it while implementing only I2C_M_IGNORE_NAK would silently ignore the
other mangling flags. Reject I2C_M_NO_RD_ACK and I2C_M_REV_DIR_ADDR in
xfer_common() instead; no in-tree user of these flags transfers through
i2c-imx. I2C_M_NO_RD_ACK is not implementable at all on this
controller: it always clocks the ninth acknowledge bit and I2CR[TXAK]
only selects ACK versus NAK. I2C_M_STOP is deliberately not rejected:
the controller already terminates every transfer with a STOP and
separates messages with repeated STARTs, so leaving the flag unhandled
changes nothing on the bus, while rejecting it would break the ov2659
camera on imx6qdl-ds: I2C_CLIENT_SCCB is I2C_M_STOP | I2C_M_IGNORE_NAK
and the driver stamps client->flags on every message, including the
chip-ID reads done at probe time.
No retry is introduced: the meaning of a NAK is device-specific [1], so
it stays a device-driver decision. I2C_M_IGNORE_NAK is only honoured
when the device driver explicitly sets it, which is exactly how the
flag is meant to be used.
Link: https://patchwork.ozlabs.org/project/linux-i2c/patch/1467900229-5262-1-git-send-email-tharvey@gateworks.com/ # [1]
Link: https://lore.kernel.org/linux-i2c/4D46D571.5010907@armadeus.com/ # [2]
Link: https://patchwork.ozlabs.org/project/linux-i2c/patch/1378857490-30968-1-git-send-email-luka@openwrt.org/ # [3]
Signed-off-by: Haobin Jiang <lemonoutput at foxmail.com>
---
v1 -> v2:
- Honour I2C_M_IGNORE_NAK on the read path as well, at the checks
of the target's ACK of the address byte (i2c_imx_isr_read(),
i2c_imx_prepare_read()), mirroring i2c-img-scb, instead of
silently dropping the flag there. Read requests carrying the
flag cannot be rejected, because in-tree SCCB sensors such as
ov2659 set I2C_CLIENT_SCCB, i.e. I2C_M_STOP |
I2C_M_IGNORE_NAK, on their read messages too.
- Force the PIO path for every message with I2C_M_IGNORE_NAK: a
NAK'd byte does not generate a further DMA request, so a DMA
write to a deliberately-NAKing target would stall until
DMA_TIMEOUT (mirrors i2c-img-scb). i2c_imx_dma_write() can
therefore never see the flag, though it still passes it
through at the final ACK check like the other paths.
- Reject I2C_M_NO_RD_ACK and I2C_M_REV_DIR_ADDR in
xfer_common() instead of advertising blanket support and
silently ignoring them. I2C_M_STOP is not rejected: the
controller already emits a STOP after the final message and
repeated STARTs between messages, and rejecting it would break
the ov2659 camera on imx6qdl-ds.
- Correct the description of the status quo: the core does not
reject I2C_M_IGNORE_NAK; i2c_transfer_buffer_flags() passes the
message flags through unchanged, so the current driver sends
bytes until the first NAK and then fails with -ENXIO.
---
drivers/i2c/busses/i2c-imx.c | 44 ++++++++++++++++++++++++++++--------
1 file changed, 35 insertions(+), 9 deletions(-)
diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 28313d0fad37..5b22a497a791 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -612,9 +612,12 @@ static int i2c_imx_trx_complete(struct imx_i2c_struct *i2c_imx, bool atomic)
return 0;
}
-static int i2c_imx_acked(struct imx_i2c_struct *i2c_imx)
+static int i2c_imx_acked(struct imx_i2c_struct *i2c_imx, bool ignore_nak)
{
if (imx_i2c_read_reg(i2c_imx, IMX_I2C_I2SR) & I2SR_RXAK) {
+ if (ignore_nak)
+ return 0;
+
dev_dbg(&i2c_imx->adapter.dev, "<%s> No ACK\n", __func__);
return -ENXIO; /* No ACK */
}
@@ -968,11 +971,15 @@ 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)
+static inline int i2c_imx_isr_acked(struct imx_i2c_struct *i2c_imx,
+ bool ignore_nak)
{
i2c_imx->isr_result = 0;
if (imx_i2c_read_reg(i2c_imx, IMX_I2C_I2SR) & I2SR_RXAK) {
+ if (ignore_nak)
+ return 0;
+
i2c_imx->state = IMX_I2C_STATE_FAILED;
i2c_imx->isr_result = -ENXIO;
wake_up(&i2c_imx->queue);
@@ -985,7 +992,7 @@ static inline int i2c_imx_isr_write(struct imx_i2c_struct *i2c_imx)
{
int result;
- result = i2c_imx_isr_acked(i2c_imx);
+ result = i2c_imx_isr_acked(i2c_imx, i2c_imx->msg->flags & I2C_M_IGNORE_NAK);
if (result)
return result;
@@ -1002,7 +1009,7 @@ 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);
+ result = i2c_imx_isr_acked(i2c_imx, i2c_imx->msg->flags & I2C_M_IGNORE_NAK);
if (result)
return result;
@@ -1213,7 +1220,8 @@ static int i2c_imx_dma_write(struct imx_i2c_struct *i2c_imx,
if (result)
return result;
- return i2c_imx_acked(i2c_imx);
+ /* I2C_M_IGNORE_NAK messages always take the PIO path. */
+ return i2c_imx_acked(i2c_imx, msgs->flags & I2C_M_IGNORE_NAK);
}
static int i2c_imx_prepare_read(struct imx_i2c_struct *i2c_imx,
@@ -1227,7 +1235,7 @@ static int i2c_imx_prepare_read(struct imx_i2c_struct *i2c_imx,
result = i2c_imx_trx_complete(i2c_imx, !use_dma);
if (result)
return result;
- result = i2c_imx_acked(i2c_imx);
+ result = i2c_imx_acked(i2c_imx, msgs->flags & I2C_M_IGNORE_NAK);
if (result)
return result;
@@ -1358,7 +1366,7 @@ static int i2c_imx_atomic_write(struct imx_i2c_struct *i2c_imx,
result = i2c_imx_trx_complete(i2c_imx, true);
if (result)
return result;
- result = i2c_imx_acked(i2c_imx);
+ result = i2c_imx_acked(i2c_imx, msgs->flags & I2C_M_IGNORE_NAK);
if (result)
return result;
dev_dbg(&i2c_imx->adapter.dev, "<%s> write data\n", __func__);
@@ -1372,7 +1380,7 @@ static int i2c_imx_atomic_write(struct imx_i2c_struct *i2c_imx,
result = i2c_imx_trx_complete(i2c_imx, true);
if (result)
return result;
- result = i2c_imx_acked(i2c_imx);
+ result = i2c_imx_acked(i2c_imx, msgs->flags & I2C_M_IGNORE_NAK);
if (result)
return result;
}
@@ -1549,6 +1557,16 @@ static int i2c_imx_xfer_common(struct i2c_adapter *adapter,
bool is_lastmsg = false;
struct imx_i2c_struct *i2c_imx = i2c_get_adapdata(adapter);
int use_dma = 0;
+ u16 mangling = I2C_M_NO_RD_ACK | I2C_M_REV_DIR_ADDR;
+
+ /*
+ * Only I2C_M_IGNORE_NAK is implemented; reject the unsupported
+ * mangling flags. I2C_M_STOP is the controller's native
+ * behaviour and must not be rejected.
+ */
+ for (i = 0; i < num; i++)
+ if (msgs[i].flags & mangling)
+ return -EOPNOTSUPP;
/* Start I2C transfer */
result = i2c_imx_start(i2c_imx, atomic);
@@ -1604,6 +1622,14 @@ static int i2c_imx_xfer_common(struct i2c_adapter *adapter,
use_dma = i2c_imx->dma && msgs[i].len >= DMA_THRESHOLD &&
msgs[i].flags & I2C_M_DMA_SAFE;
+
+ /*
+ * I2C_M_IGNORE_NAK is honoured on the PIO paths only; a
+ * NAK'd byte does not generate a further DMA request
+ * (mirrors i2c-img-scb).
+ */
+ if (msgs[i].flags & I2C_M_IGNORE_NAK)
+ use_dma = false;
if (msgs[i].flags & I2C_M_RD) {
int block_data = msgs->flags & I2C_M_RECV_LEN;
@@ -1697,7 +1723,7 @@ static int i2c_imx_init_recovery_info(struct imx_i2c_struct *i2c_imx,
static u32 i2c_imx_func(struct i2c_adapter *adapter)
{
return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL
- | I2C_FUNC_SMBUS_READ_BLOCK_DATA;
+ | I2C_FUNC_SMBUS_READ_BLOCK_DATA | I2C_FUNC_PROTOCOL_MANGLING;
}
static const struct i2c_algorithm i2c_imx_algo = {
base-commit: 04e9bf1648f846976b543e91c1838a712433772a
--
2.34.1
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