i2c: testunit: return current command on read messages

Because the testunit can start tests in the future via the DELAY
register, it may happen that a command is still pending. Support
detecting that by returning the number of a command in progress (if
there is one).
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Documentation/i2c/slave-testunit-backend.rst

@@ -20,11 +20,13 @@ Instantiating the device is regular. Example for bus 0, address 0x30::
 
   # echo "slave-testunit 0x1030" > /sys/bus/i2c/devices/i2c-0/new_device
 
-After that, you will have a write-only device listening. Reads will just return
-an 8-bit version number of the testunit. When writing, the device consists of 4
-8-bit registers and, except for some "partial" commands, all registers must be
-written to start a testcase, i.e. you usually write 4 bytes to the device. The
-registers are:
+After that, you will have the device listening. Reading will return a single
+byte. Its value is 0 if the testunit is idle, otherwise the command number of
+the currently running command.
+
+When writing, the device consists of 4 8-bit registers and, except for some
+"partial" commands, all registers must be written to start a testcase, i.e. you
+usually write 4 bytes to the device. The registers are:
 
 .. csv-table::
   :header: "Offset", "Name", "Description"
@@ -170,4 +172,4 @@ are not equivalent to a REPEATED START. As an example, this returns just the
 default response::
 
   # i2cset -y 0 0x30 4 0 0 i; i2cget -y 0 0x30
-  0x01
+  0x00

drivers/i2c/i2c-slave-testunit.c

@@ -15,7 +15,6 @@
 #include <linux/slab.h>
 #include <linux/workqueue.h> /* FIXME: is system_long_wq the best choice? */
 
-#define TU_CUR_VERSION 0x01
 #define TU_VERSION_MAX_LENGTH 128
 
 enum testunit_cmds {
@@ -159,7 +158,8 @@ static int i2c_slave_testunit_slave_cb(struct i2c_client *client,
 		else if (is_proc_call)
 			*val = tu->regs[TU_REG_DATAH];
 		else
-			*val = TU_CUR_VERSION;
+			*val = test_bit(TU_FLAG_IN_PROCESS, &tu->flags) ?
+					tu->regs[TU_REG_CMD] : 0;
 		break;
 	}