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

[Oliver Neukum <oneukum@suse.com>]

On Mi, 2018-12-19 at 10:00 +0100, Johan Hovold wrote:
> 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>
Acked-by: Oliver Neukum <oneukum@suse.com>