On Thu, Dec 13, 2018 at 11:27:56AM +0800, macpaul.lin@xxxxxxxxxxxx wrote: > From: Macpaul Lin <macpaul.lin@xxxxxxxxxxxx> > > Mediatek Preloader is a proprietary embedded boot loader for loading > Little Kernel and Linux into device DRAM. > > This boot loader also handle firmware updating. 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 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 sended. > Which will be recognized as an incorrect response. This behavior change > also causes the 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@xxxxxxxxxxxx> > --- > drivers/usb/class/cdc-acm.c | 9 ++++++++- > drivers/usb/class/cdc-acm.h | 1 + > 2 files changed, 9 insertions(+), 1 deletion(-) > > diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c > index 1b68fed..2f744bb 100644 > --- a/drivers/usb/class/cdc-acm.c > +++ b/drivers/usb/class/cdc-acm.c > @@ -1156,6 +1156,10 @@ static int acm_probe(struct usb_interface *intf, > goto skip_normal_probe; > } > > + /* handle active handshake triggered by device */ > + if (quirks == DISABLE_ECHO) > + acm_tty_driver->init_termios.c_lflag &= ~(ECHO); You cannot change the driver init_termios like this as that will affect all cdc-acm devices that are probed later. If this is at all needed, this will have to be done at tty install time. Note that 93857edd9829 ("tty: reset termios state on device registration") only makes sure that you get the default terminal setting whenever you plug in a device (rather than reuse settings from a previously connected device which happened to be assigned the same minor number). Specifically, it should not change any behaviour for the first time a cdc-acm device is plugged in. >From just a quick look, it seems you need to prevent your download tool from sending "XX" before disabling ECHO. Why wouldn't that work? Thanks, Johan