cdc-acm: fix abnormal DATA RX issue for Mediatek Preloader.

Mediatek Preloader is a proprietary embedded boot loader for loading Little Kernel and Linux into device DRAM.

This boot loader also handle firmware update. Mediatek Preloader will be enumerated as a virtual COM port when the device is connected to Windows or Linux OS via CDC-ACM class driver. When the USB enumeration has been done, Mediatek Preloader will send out handshake command "READY" to PC actively instead of waiting command from the download tool.

Since Linux 4.12, the commit "tty: reset termios state on device registration" (93857edd9829e144acb6c7e72d593f6e01aead66) causes Mediatek Preloader receiving some abnoraml command like "READYXX" as it sent. This will be recognized as an incorrect response. The behavior change also causes the download handshake fail. This change only affects subsequent connects if the reconnected device happens to get the same minor number.

By disabling the ECHO termios flag could avoid this problem. However, it cannot be done by user space configuration when download tool open /dev/ttyACM0. This is because the device running Mediatek Preloader will send handshake command "READY" immediately once the CDC-ACM driver is ready.

This patch wants to fix above problem by introducing "DISABLE_ECHO" property in driver_info. When Mediatek Preloader is connected, the CDC-ACM driver could disable ECHO flag in termios to avoid the problem.

Signed-off-by Macpaul Lin <[email protected]>

Cc [email protected]

---
Changes for v2:
- Move quirks testing of DISABLE_ECHO flag into acm_tty_install().
- Change quirks testing into bitwise comparison.
Changes for v3:
- Replace quirks testing from init_termios to tty->termios.
- Remove parenthesis for ECHO flag.
Changes for v4:
- Drop quirks varible to simplify the patch.
- Move termios operation right after the driver_data has been installed.
- Write general style comment for suppressing initial echoing.
Changes for v5:
- Fix: termios operation right abover the driver_data has been installed.
- Update commit comment about this patch affects the reconnected device which get the same minor numbers.

 drivers/usb/class/cdc-acm.c | 12 +++++++++++-
 drivers/usb/class/cdc-acm.h |  1 +
 2 files changed, 12 insertions(+), 1 deletion(-)

drivers/usb/class/cdc-acm.c +7 -0

index 1b68fed..336cf13 100644
@@ -581,6 +581,13 @@ static int acm_tty_install(struct tty_driver *driver, struct tty_struct *tty)
 	if (retval)
 		goto error_init_termios;
 
+	/*
+	 * Suppress initial echoing for some devices which might send data
+	 * immediately after acm driver has been installed.
+	 */
+	if (acm->quirks & DISABLE_ECHO)
+		tty->termios.c_lflag &= ~ECHO;
+
 	tty->driver_data = acm;
 
 	return 0;