USB: ftdi_sio: added CS5 quirk for broken smartcard readers

Genuine FTDI chips support only CS7/8. A previous fix in commit
8704211f65a2 ("USB: ftdi_sio: fixed handling of unsupported CSIZE
setting") enforced this limitation and reported it back to userspace.

However, certain types of smartcard readers depend on specific
driver behaviour that requests 0 data bits (not 5) to change into a
different operating mode if CS5 has been set.

This patch reenables this behaviour for all FTDI devices.

Tagged to be added to stable, because it affects a lot of users of
embedded systems which rely on these readers to work properly.

Cc: stable <stable@vger.kernel.org>
Reported-by: Heinrich Siebmanns <H.Siebmanns@t-online.de>
Tested-by: Heinrich Siebmanns <H.Siebmanns@t-online.de>
Signed-off-by: Colin Leitner <colin.leitner@gmail.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 15 insertions, 2 deletions

diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
index a4bebacc5cae..5057201bbba8 100644
--- a/drivers/usb/serial/ftdi_sio.c
+++ b/drivers/usb/serial/ftdi_sio.c
@@ -2121,10 +2121,20 @@ static void ftdi_set_termios(struct tty_struct *tty,
         }
 
         /*
-         * All FTDI UART chips are limited to CS7/8. We won't pretend to
+         * All FTDI UART chips are limited to CS7/8. We shouldn't pretend to
          * support CS5/6 and revert the CSIZE setting instead.
+         *
+         * CS5 however is used to control some smartcard readers which abuse
+         * this limitation to switch modes. Original FTDI chips fall back to
+         * eight data bits.
+         *
+         * TODO: Implement a quirk to only allow this with mentioned
+         *       readers. One I know of (Argolis Smartreader V1)
+         *       returns "USB smartcard server" as iInterface string.
+         *       The vendor didn't bother with a custom VID/PID of
+         *       course.
          */
-        if ((C_CSIZE(tty) != CS8) && (C_CSIZE(tty) != CS7)) {
+        if (C_CSIZE(tty) == CS6) {
                 dev_warn(ddev, "requested CSIZE setting not supported\n");
 
                 termios->c_cflag &= ~CSIZE;
@@ -2171,6 +2181,9 @@ no_skip:
                 urb_value |= FTDI_SIO_SET_DATA_PARITY_NONE;
         }
         switch (cflag & CSIZE) {
+        case CS5:
+                dev_dbg(ddev, "Setting CS5 quirk\n");
+                break;
         case CS7:
                 urb_value |= 7;
                 dev_dbg(ddev, "Setting CS7\n");