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

PATCH Superseded Johan Hovold 1 participant 1 message Dec 18, 2018

Subsystems (2)
Stable branch USB

v4 of 7 Superseded Latest is v7 →

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Johan Hovold

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PATCH v4 1/1 cdc-acm: fix abnormal DATA RX issue for Mediatek Preloader.

Quoted 13 lines · Macpaul Lin

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.

Please mention that this change only affects subsequent connects if the reconnected device happens to get the same minor number.

Quoted 11 lines

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]>

And you can add a CC stable tag here.

Quoted 10 lines

---
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.

No, it should go after the *tty* has been installed (and *before* setting driver_data)

Quoted 11 lines

- Write general style comment for suppressing initial echoing.

 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 +0 -0
index 1b68fed..ab8609e 100644

drivers/usb/class/cdc-acm.c static int acm_tty_install(struct tty_driver *driver, struct tty_struct *tty) · line 591

Quoted 9 lines
drivers/usb/class/cdc-acm.c +6 -0
@@ -583,6 +583,13 @@ static int acm_tty_install(struct tty_driver *driver, struct tty_struct *tty)
 
 	tty->driver_data = acm;
 
+	/*
+	 * 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;

You forgot to move this above setting tty->driver data.

drivers/usb/class/cdc-acm.c static int acm_tty_install(struct tty_driver *driver, struct tty_struct *tty) · line 593

Quoted 2 lines
drivers/usb/class/cdc-acm.c +1 -0
+
 	return 0;

Thanks,
Johan