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

[Johan Hovold <johan@kernel.org>]

On Thu, Dec 13, 2018 at 11:27:56AM +0800, macpaul.lin@mediatek.com wrote:
> From: 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 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@mediatek.com>
> ---
>  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