[PATCH i2c-tools] i2ctransfer: add support for I2C_M_RECV_LEN

[Wolfram Sang <wsa+renesas@sang-engineering.com>]

Strictly spoken, this is for emulating SMBus transfers, not I2C. But
hey, we still want to test devices supporting these transfers.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
---

I created this to be able to play around with I2C_M_RECV_LEN and with
SMBUS2 and SMBUS3 max block sizes. Tested with a Renesas Lager board and
my slave-testunit hacked with SMBus block transfer support (to be
upstreamed later).

Fun fact: printing the correct length in the output took longer than
implementing the actual functionality.

 tools/i2ctransfer.8 |  1 +
 tools/i2ctransfer.c | 42 +++++++++++++++++++++++++++++++-----------
 2 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/tools/i2ctransfer.8 b/tools/i2ctransfer.8
index d16e34e..152d20d 100644
--- a/tools/i2ctransfer.8
+++ b/tools/i2ctransfer.8
@@ -91,6 +91,7 @@ specifies if the message is read or write
 .B <length_of_message>
 specifies the number of bytes read or written in this message.
 It is parsed as an unsigned 16 bit integer, but note that the Linux Kernel applies an additional upper limit (8192 as of v4.10).
+For read messages to targets which support SMBus Block transactions, it can also be '?', then the target will determine the length.
 .TP
 .B [@address]
 specifies the 7-bit address of the chip to be accessed for this message, and is an integer.
diff --git a/tools/i2ctransfer.c b/tools/i2ctransfer.c
index 7b95d48..2b8a3fb 100644
--- a/tools/i2ctransfer.c
+++ b/tools/i2ctransfer.c
@@ -45,7 +45,8 @@ static void help(void)
 		"Usage: i2ctransfer [-f] [-y] [-v] [-V] [-a] I2CBUS DESC [DATA] [DESC [DATA]]...\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) 3) I2C address (use last one if omitted)\n"
+		"    1) read/write-flag 2) LENGTH (range 0-65535, or '?')\n"
+		"    3) 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"
@@ -84,17 +85,24 @@ static void print_msgs(struct i2c_msg *msgs, __u32 nmsgs, unsigned flags)
 
 	for (i = 0; i < nmsgs; i++) {
 		int read = msgs[i].flags & I2C_M_RD;
+		int recv_len = msgs[i].flags & I2C_M_RECV_LEN;
 		int print_buf = (read && (flags & PRINT_READ_BUF)) ||
 				(!read && (flags & PRINT_WRITE_BUF));
-
-		if (flags & PRINT_HEADER)
-			fprintf(output, "msg %u: addr 0x%02x, %s, len %u",
-				i, msgs[i].addr, read ? "read" : "write", msgs[i].len);
+		__u16 len = recv_len ? msgs[i].buf[0] + 1 : msgs[i].len;
+
+		if (flags & PRINT_HEADER) {
+			fprintf(output, "msg %u: addr 0x%02x, %s, len ",
+				i, msgs[i].addr, read ? "read" : "write");
+			if (!recv_len || flags & PRINT_READ_BUF)
+				fprintf(output, "%u", len);
+			else
+				fprintf(output, "TBD");
+		}
 
 		if (msgs[i].len && print_buf) {
 			if (flags & PRINT_HEADER)
 				fprintf(output, ", buf ");
-			for (j = 0; j < msgs[i].len - 1; j++)
+			for (j = 0; j < len - 1; j++)
 				fprintf(output, "0x%02x ", msgs[i].buf[j]);
 			/* Print final byte with newline */
 			fprintf(output, "0x%02x\n", msgs[i].buf[j]);
@@ -192,13 +200,23 @@ int main(int argc, char *argv[])
 				goto err_out_with_arg;
 			}
 
-			len = strtoul(arg_ptr, &end, 0);
-			if (len > 0xffff || arg_ptr == end) {
-				fprintf(stderr, "Error: Length invalid\n");
-				goto err_out_with_arg;
+			if (*arg_ptr == '?') {
+				if (!(flags & I2C_M_RD)) {
+					fprintf(stderr, "Error: variable length not allowed with write\n");
+					goto err_out_with_arg;
+				}
+				len = 256; /* SMBUS3_MAX_BLOCK_LEN + 1 */
+				flags |= I2C_M_RECV_LEN;
+				arg_ptr++;
+			} else {
+				len = strtoul(arg_ptr, &end, 0);
+				if (len > 0xffff || arg_ptr == end) {
+					fprintf(stderr, "Error: Length invalid\n");
+					goto err_out_with_arg;
+				}
+				arg_ptr = end;
 			}
 
-			arg_ptr = end;
 			if (*arg_ptr) {
 				if (*arg_ptr++ != '@') {
 					fprintf(stderr, "Error: Unknown separator after length\n");
@@ -237,6 +255,8 @@ int main(int argc, char *argv[])
 				}
 				memset(buf, 0, len);
 				msgs[nmsgs].buf = buf;
+				if (flags & I2C_M_RECV_LEN)
+					buf[0] = 1; /* number of extra bytes */
 			}
 
 			if (flags & I2C_M_RD || len == 0) {
-- 
2.27.0