[v7] cdc-acm: fix abnormal DATA RX issue for Mediatek Preloader.

[Johan Hovold <johan@kernel.org>]

On Wed, Dec 19, 2018 at 12:11:03PM +0800, Macpaul Lin wrote:
> 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 <macpaul.lin@mediatek.com>
> Cc: stable@vger.kernel.org
> ---
> 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.
> Changes for v6:
>  - Update VID/PID:0x0e8d/0x0003 as Mediatek Inc BROM.
>  - Update VID/PID:0x0e8d/0x2000 as Mediatek Inc Preloader.
> Changes for v7:
>  - Keep VID/PID:0x0e8d/0x0003 unchanged because of 2 different UNION
>    descriptor implementated in Mediatek Inc BROM (MT6589/MT6765).

Reviewed-by: Johan Hovold <johan@kernel.org>

Johan