[PATCH i2c-tools v3 0/2] Add support for message modifier flags

[PATCH i2c-tools v3 0/2] Add support for message modifier flags

[Benoît Monin]

I2C messages can be modified with a set of flags covered by the protocol
mangling and the skip repeated start functionalities. This series add
support for such flags to i2cdetect and i2ctransfer.

The first patch shows the support of protocol mangling and repeated
start skipping in the output of 'i2cdetect -F'.

The second patch adds the parsing of optional flags to i2ctransfer
message description. Those command-line flags then set the i2c message
flags alongside the read/write flag.

I wrote these changes to test the insertion of I2C_M_STOP flag in
multi-message transactions with i2ctransfer, but the other flags may be
useful for various test scenarios. 

Signed-off-by: Benoît Monin <benoit.monin@bootlin.com>
---
Changes in v3:
- Use ifdefs to check that the flags and functionalities are known
  at compile-time.
- Check that the adapter supports the requested flags before using them
  in i2ctransfer.
- Add a warning in i2ctransfer man page about the risk of using these
  flags.
- Link to v2: https://lore.kernel.org/r/20251223-msg-flags-v2-0-8d934a4366e2@bootlin.com

Changes in v2:
- Document the flags in i2ctransfer.8 man page.
- Link to v1: https://lore.kernel.org/r/20251128-msg-flags-v1-0-6353f26fa6bc@bootlin.com

---
Benoît Monin (2):
      i2cdetect: Display mangling and nostart support
      i2ctransfer: Add optional message modifier flags

 tools/i2cdetect.c   |   8 ++++
 tools/i2ctransfer.8 |  28 ++++++++++++-
 tools/i2ctransfer.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++------
 3 files changed, 141 insertions(+), 14 deletions(-)
---
base-commit: c1b0305972e89aae048068a3b228f35ef0206243
change-id: 20251127-msg-flags-3d2b2da9ae28

Best regards,
-- 
Benoît Monin, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

[PATCH i2c-tools v3 1/2] i2cdetect: Display mangling and nostart support

[Benoît Monin]

Show the support of protocol mangling and repeated start elision in
the list of fonctionnalities displayed with '-F'. We only use these
functionnalities if they are known at compile-time so i2cdetect can be
built against older kernels.

Signed-off-by: Benoît Monin <benoit.monin@bootlin.com>
---
 tools/i2cdetect.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/i2cdetect.c b/tools/i2cdetect.c
index bb2f146..3c22f17 100644
--- a/tools/i2cdetect.c
+++ b/tools/i2cdetect.c
@@ -164,6 +164,14 @@ static const struct func all_func[] = {
 #if defined(I2C_FUNC_SLAVE)
 	{ .value = I2C_FUNC_SLAVE,
 	  .name = "Target mode" },
+#endif
+#if defined(I2C_FUNC_PROTOCOL_MANGLING)
+	{ .value = I2C_FUNC_PROTOCOL_MANGLING,
+	  .name = "Protocol mangling" },
+#endif
+#if defined(I2C_FUNC_NOSTART)
+	{ .value = I2C_FUNC_NOSTART,
+	  .name = "Repeated start skipping" },
 #endif
 	{ .value = 0, .name = "" }
 };

-- 
2.52.0

[PATCH i2c-tools v3 2/2] i2ctransfer: Add optional message modifier flags

[Benoît Monin]

Allow setting protocol mangling and repeated start elision flags of an i2c
message with a set of optional command-line flags. These optional flags
are parsed at the beginning of the DESC field up to a read or write flag.

For example, to read one byte from address 0x50 followed by a stop, then
write two bytes at 0x54 on bus 0, one would call i2ctransfer as follow:

    i2ctransfer 0 pr1@0x50 w2@0x54 0x10 0x20

Since the new flags are optional, this patch preserves the compatibility
of the i2ctransfer syntax.

Handling of the message flags is done in add_flag_if_supported(). This
function checks if the flag is defined at compile time and if the adapter
supports the required functionality to handle the flag.

Signed-off-by: Benoît Monin <benoit.monin@bootlin.com>
---
 tools/i2ctransfer.8 |  28 ++++++++++++-
 tools/i2ctransfer.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 133 insertions(+), 14 deletions(-)

diff --git a/tools/i2ctransfer.8 b/tools/i2ctransfer.8
index 4bdf436..ca41241 100644
--- a/tools/i2ctransfer.8
+++ b/tools/i2ctransfer.8
@@ -96,8 +96,11 @@ The number of blocks is limited by the Linux Kernel and defined by I2C_RDWR_IOCT
 .I desc
 blocks are composed like this:
 
-.I {r|w}<length_of_message>[@address]
+.I [inpst]{r|w}<length_of_message>[@address]
 
+.TP
+.B [inpst]
+If supported, specifies optional MESSAGE MODIFIER FLAGS. See the section below for details.
 .TP
 .B {r|w}
 specifies if the message is read or write
@@ -141,6 +144,29 @@ decrease value by 1 until end of message (i.e. 0xff- means 0xff, 0xfe, 0xfd, ...
 p
 use value as seed for an 8 bit pseudo random sequence (i.e. 0p means 0x00, 0x50, 0xb0, ...)
 
+.SH "MESSAGE MODIFIER FLAGS"
+.PP
+These optional flags can be used to change the handling of a message when it is sent on the bus.
+They allow you to deviate from the I2C standard to work around device issues.
+They should be used with care, as they can confuse your I2C bus.
+Note that not all flags (or any) may be supported by your particular hardware and your kernel version.
+
+.TP
+.B i
+ignore NACK from client, treat them as ACK.
+.TP
+.B n
+in a read message, master ACK/NACK bit is skipped.
+.TP
+.B p
+emit a STOP after the message.
+.TP
+.B s
+skip repeated start.
+.TP
+.B t
+toggle read/write bit.
+
 .SH EXAMPLES
 .PP
 On bus 0, from an EEPROM at address 0x50, read 8 byte from offset 0x64
diff --git a/tools/i2ctransfer.c b/tools/i2ctransfer.c
index 4db98e3..446a174 100644
--- a/tools/i2ctransfer.c
+++ b/tools/i2ctransfer.c
@@ -52,9 +52,16 @@ static void help(void)
 		"           -V version info\n"
 		"           -y yes to all confirmations\n"
 		"  I2CBUS is an integer or an I2C bus name\n"
-		"  DESC describes the transfer in the form: {r|w}LENGTH[@address]\n"
-		"    1) read/write-flag 2) LENGTH (range 0-65535, or '?')\n"
-		"    3) I2C address (use last one if omitted)\n"
+		"  DESC describes the transfer in the form: [inpst]{r|w}LENGTH[@address]\n"
+		"    1) optional message modifier flags, if supported\n"
+		"       i: ignore NACK from client\n"
+		"       n: no master ACK/NACK bit in a read message\n"
+		"       p: emit a STOP after the message\n"
+		"       s: skip repeated start\n"
+		"       t: toggle read/write bit\n"
+		"    2) mandatory read/write flag\n"
+		"    3) LENGTH (range 0-65535, or '?')\n"
+		"    4) I2C address (use last one if omitted)\n"
 		"  DATA are LENGTH bytes for a write message. They can be shortened by a suffix:\n"
 		"    = (keep value constant until LENGTH)\n"
 		"    + (increase value by 1 until LENGTH)\n"
@@ -66,17 +73,20 @@ static void help(void)
 		"  # i2ctransfer 0 w17@0x50 0x42 0xff-\n");
 }
 
-static int check_funcs(int file)
+static int get_funcs(int file, unsigned long *funcs)
 {
-	unsigned long funcs;
-
 	/* check adapter functionality */
-	if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
+	if (ioctl(file, I2C_FUNCS, funcs) < 0) {
 		fprintf(stderr, "Error: Could not get the adapter "
 			"functionality matrix: %s\n", strerror(errno));
 		return -1;
 	}
 
+	return 0;
+}
+
+static int check_funcs(unsigned long funcs)
+{
 	if (!(funcs & I2C_FUNC_I2C)) {
 		fprintf(stderr, MISSING_FUNC_FMT, "I2C transfers");
 		return -1;
@@ -140,6 +150,85 @@ static int confirm(const char *filename, struct i2c_msg *msgs, __u32 nmsgs)
 	return 1;
 }
 
+/*
+ * Parse one message flag and check if the adapter supports it.
+ * Return:
+ * 1 if we have the direction of the message (read or write)
+ * 0 if the argument is a valid and supported optional flag
+ * -1 for unsupported flag
+ */
+static int add_flag_if_supported(__u16 *flags, unsigned long funcs, char arg)
+{
+	int mangling = 0, nostart = 0;
+
+	/* Get the message flag from the argument */
+	switch (arg) {
+		/* mandatory direction flag: ends the parsing of flags */
+		case 'r':
+			*flags |= I2C_M_RD;
+			return 1;
+		case 'w':
+			return 1;
+
+		/* optional flags */
+#ifdef I2C_M_IGNORE_NAK
+		case 'i':
+			*flags |= I2C_M_IGNORE_NAK;
+			mangling = 1;
+			break;
+#endif
+#ifdef I2C_M_NO_RD_ACK
+		case 'n':
+			*flags |= I2C_M_NO_RD_ACK;
+			mangling = 1;
+			break;
+#endif
+#ifdef I2C_M_STOP
+		case 'p':
+			*flags |= I2C_M_STOP;
+			mangling = 1;
+			break;
+#endif
+#ifdef I2C_M_NOSTART
+		case 's':
+			*flags |= I2C_M_NOSTART;
+			nostart = 1;
+			break;
+#endif
+#ifdef I2C_M_REV_DIR_ADDR
+		case 't':
+			*flags |= I2C_M_REV_DIR_ADDR;
+			mangling = 1;
+			break;
+#endif
+
+		default:
+			fprintf(stderr, "Error: Unsupported flag '%c'\n", arg);
+			return -1;
+	}
+
+	/* Check that the adapter supports the requested flags */
+#ifdef I2C_FUNC_PROTOCOL_MANGLING
+	if (funcs & I2C_FUNC_PROTOCOL_MANGLING)
+		mangling = 0;
+#endif
+	if (mangling) {
+		fprintf(stderr, MISSING_FUNC_FMT, "protocol mangling");
+		return -1;
+	}
+
+#ifdef I2C_FUNC_NOSTART
+	if (funcs & I2C_FUNC_NOSTART)
+		nostart = 0;
+#endif
+	if (nostart) {
+		fprintf(stderr, MISSING_FUNC_FMT, "repeated start skipping");
+		return -1;
+	}
+
+	return 0;
+}
+
 int main(int argc, char *argv[])
 {
 	char filename[20];
@@ -148,6 +237,7 @@ int main(int argc, char *argv[])
 	struct i2c_msg msgs[I2C_RDRW_IOCTL_MAX_MSGS];
 	enum parse_state state = PARSE_GET_DESC;
 	unsigned int buf_idx = 0;
+	unsigned long funcs;
 
 	memset(msgs, 0, sizeof(msgs));
 
@@ -182,7 +272,7 @@ int main(int argc, char *argv[])
 		exit(1);
 
 	file = open_i2c_dev(i2cbus, filename, sizeof(filename), 0);
-	if (file < 0 || check_funcs(file))
+	if (file < 0 || get_funcs(file, &funcs) || check_funcs(funcs))
 		exit(1);
 
 	while (optind < argc) {
@@ -191,6 +281,7 @@ int main(int argc, char *argv[])
 		__u16 flags;
 		__u8 data, *buf;
 		char *end;
+		int ret;
 
 		if (nmsgs == I2C_RDRW_IOCTL_MAX_MSGS) {
 			fprintf(stderr, "Error: Too many messages (max: %d)\n",
@@ -202,11 +293,13 @@ int main(int argc, char *argv[])
 		case PARSE_GET_DESC:
 			flags = 0;
 
-			switch (*arg_ptr++) {
-			case 'r': flags |= I2C_M_RD; break;
-			case 'w': break;
-			default:
-				fprintf(stderr, "Error: Invalid direction\n");
+			for (ret = 0; *arg_ptr && !ret; arg_ptr++) {
+				ret = add_flag_if_supported(&flags, funcs, *arg_ptr);
+				if (ret < 0)
+					goto err_out_with_arg;
+			}
+			if (ret != 1) {
+				fprintf(stderr, "Error: Missing direction flag 'r' or 'w'\n");
 				goto err_out_with_arg;
 			}
 

-- 
2.52.0

Re: [PATCH i2c-tools v3 0/2] Add support for message modifier flags

[Jean Delvare]

Hi Benoît,

On Tue, 27 Jan 2026 10:25:56 +0100, Benoît Monin wrote:
> I2C messages can be modified with a set of flags covered by the protocol
> mangling and the skip repeated start functionalities. This series add
> support for such flags to i2cdetect and i2ctransfer.
> 
> The first patch shows the support of protocol mangling and repeated
> start skipping in the output of 'i2cdetect -F'.
> 
> The second patch adds the parsing of optional flags to i2ctransfer
> message description. Those command-line flags then set the i2c message
> flags alongside the read/write flag.
> 
> I wrote these changes to test the insertion of I2C_M_STOP flag in
> multi-message transactions with i2ctransfer, but the other flags may be
> useful for various test scenarios. 
> 
> Signed-off-by: Benoît Monin <benoit.monin@bootlin.com>
> ---
> Changes in v3:
> - Use ifdefs to check that the flags and functionalities are known
>   at compile-time.
> - Check that the adapter supports the requested flags before using them
>   in i2ctransfer.
> - Add a warning in i2ctransfer man page about the risk of using these
>   flags.
> - Link to v2: https://lore.kernel.org/r/20251223-msg-flags-v2-0-8d934a4366e2@bootlin.com
> 
> Changes in v2:
> - Document the flags in i2ctransfer.8 man page.
> - Link to v1: https://lore.kernel.org/r/20251128-msg-flags-v1-0-6353f26fa6bc@bootlin.com
> 
> ---
> Benoît Monin (2):
>       i2cdetect: Display mangling and nostart support
>       i2ctransfer: Add optional message modifier flags
> 
>  tools/i2cdetect.c   |   8 ++++
>  tools/i2ctransfer.8 |  28 ++++++++++++-
>  tools/i2ctransfer.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++------
>  3 files changed, 141 insertions(+), 14 deletions(-)

Both patches look very good to me. I was able to test the i2cdetect
patch and did not find any issue. I can't easily test the i2ctransfer
patch though.

Wolfram, can you please give some basic testing to the i2ctransfer
patch? Not necessarily with the mangling flags, but at least test that
the changes do not introduce any regression for your own use cases?

Thanks,
-- 
Jean Delvare
SUSE L3 Support

Re: [PATCH i2c-tools v3 0/2] Add support for message modifier flags

[Benoît Monin]

Hello Wolfram,

On Tuesday, 17 February 2026 at 13:13:48 CET, Jean Delvare wrote:
> Hi Benoît,
> 
> On Tue, 27 Jan 2026 10:25:56 +0100, Benoît Monin wrote:
> > I2C messages can be modified with a set of flags covered by the protocol
> > mangling and the skip repeated start functionalities. This series add
> > support for such flags to i2cdetect and i2ctransfer.
> > 
> > The first patch shows the support of protocol mangling and repeated
> > start skipping in the output of 'i2cdetect -F'.
> > 
> > The second patch adds the parsing of optional flags to i2ctransfer
> > message description. Those command-line flags then set the i2c message
> > flags alongside the read/write flag.
> > 
> > I wrote these changes to test the insertion of I2C_M_STOP flag in
> > multi-message transactions with i2ctransfer, but the other flags may be
> > useful for various test scenarios. 
> > 
> > Signed-off-by: Benoît Monin <benoit.monin@bootlin.com>
> > ---
> > Changes in v3:
> > - Use ifdefs to check that the flags and functionalities are known
> >   at compile-time.
> > - Check that the adapter supports the requested flags before using them
> >   in i2ctransfer.
> > - Add a warning in i2ctransfer man page about the risk of using these
> >   flags.
> > - Link to v2: https://lore.kernel.org/r/20251223-msg-flags-v2-0-8d934a4366e2@bootlin.com
> > 
> > Changes in v2:
> > - Document the flags in i2ctransfer.8 man page.
> > - Link to v1: https://lore.kernel.org/r/20251128-msg-flags-v1-0-6353f26fa6bc@bootlin.com
> > 
> > ---
> > Benoît Monin (2):
> >       i2cdetect: Display mangling and nostart support
> >       i2ctransfer: Add optional message modifier flags
> > 
> >  tools/i2cdetect.c   |   8 ++++
> >  tools/i2ctransfer.8 |  28 ++++++++++++-
> >  tools/i2ctransfer.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++------
> >  3 files changed, 141 insertions(+), 14 deletions(-)
> 
> Both patches look very good to me. I was able to test the i2cdetect
> patch and did not find any issue. I can't easily test the i2ctransfer
> patch though.
> 
> Wolfram, can you please give some basic testing to the i2ctransfer
> patch? Not necessarily with the mangling flags, but at least test that
> the changes do not introduce any regression for your own use cases?
> 
> Thanks,
> 
Did you have a chance to test the patches?

Best regards,
-- 
Benoît Monin, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com