+	switch (event) {
+	case I2C_SLAVE_STOP:
+		command_size = panel->idx;
+		fallthrough;
+	case I2C_SLAVE_WRITE_REQUESTED:
+		panel->idx = 0;
+		break;
+	case I2C_SLAVE_WRITE_RECEIVED:
+		if (panel->idx < sizeof(panel->command))
+			panel->command[panel->idx++] = *val;
+		else
+			dev_dbg(&panel->input->dev, "command truncated\n");

Just double checking: Do you really want to process truncated commands?
Since you detect the state here, you could also choose to reject such
commands?