[PATCH v2] cdc-acm: fix abnormal DATA RX issue for Mediatek Preloader.

[Macpaul Lin <macpaul.lin@mediatek.com>]

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.

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 <macpaul.lin@mediatek.com>
---
Change for v2:
 - Move quirks testing of DISABLE_ECHO flag into acm_tty_install().
 - Change quirks testing into bitwise comparison.

 drivers/usb/class/cdc-acm.c | 13 ++++++++++++-
 drivers/usb/class/cdc-acm.h |  1 +
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 1b68fed..f1a914d 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -571,12 +571,20 @@ static void acm_softint(struct work_struct *work)
 static int acm_tty_install(struct tty_driver *driver, struct tty_struct *tty)
 {
 	struct acm *acm;
+	unsigned long quirks;
 	int retval;
 
 	acm = acm_get_by_minor(tty->index);
 	if (!acm)
 		return -ENODEV;
 
+	/* get normal quirks */
+	quirks = acm->quirks;
+
+	/* handle active handshake triggered by device */
+	if (quirks & DISABLE_ECHO)
+		driver->init_termios.c_lflag &= ~(ECHO);
+
 	retval = tty_standard_install(driver, tty);
 	if (retval)
 		goto error_init_termios;
@@ -1655,7 +1663,10 @@ static int acm_pre_reset(struct usb_interface *intf)
 	.driver_info = NO_UNION_NORMAL, /* has no union descriptor */
 	},
 	{ USB_DEVICE(0x0e8d, 0x0003), /* FIREFLY, MediaTek Inc; andrey.arapov@gmail.com */
-	.driver_info = NO_UNION_NORMAL, /* has no union descriptor */
+	.driver_info = DISABLE_ECHO, /* DISABLE ECHO in termios flag */
+	},
+	{ USB_DEVICE(0x0e8d, 0x2000), /* FIREFLY, MediaTek Inc; Preloader */
+	.driver_info = DISABLE_ECHO, /* DISABLE ECHO in termios flag */
 	},
 	{ USB_DEVICE(0x0e8d, 0x3329), /* MediaTek Inc GPS */
 	.driver_info = NO_UNION_NORMAL, /* has no union descriptor */
diff --git a/drivers/usb/class/cdc-acm.h b/drivers/usb/class/cdc-acm.h
index ca06b20..515aad0 100644
--- a/drivers/usb/class/cdc-acm.h
+++ b/drivers/usb/class/cdc-acm.h
@@ -140,3 +140,4 @@ struct acm {
 #define QUIRK_CONTROL_LINE_STATE	BIT(6)
 #define CLEAR_HALT_CONDITIONS		BIT(7)
 #define SEND_ZERO_PACKET		BIT(8)
+#define DISABLE_ECHO			BIT(9)
-- 
1.9.1