linux-stable.git/drivers/tty, branch v5.4.185 Linux kernel stable tree tty: n_gsm: fix NULL pointer access due to DLCI release 2022-03-02T10:41:16+00:00 daniel.starke@siemens.com daniel.starke@siemens.com 2022-02-18T07:31:20+00:00 2782b05d02086d42ef63870c18749b17ca6cf157 commit 96b169f05cdcc844b400695184d77e42071d14f2 upstream. The here fixed commit made the tty hangup asynchronous to avoid a circular locking warning. I could not reproduce this warning. Furthermore, due to the asynchronous hangup the function call now gets queued up while the underlying tty is being freed. Depending on the timing this results in a NULL pointer access in the global work queue scheduler. To be precise in process_one_work(). Therefore, the previous commit made the issue worse which it tried to fix. This patch fixes this by falling back to the old behavior which uses a blocking tty hangup call before freeing up the associated tty. Fixes: 7030082a7415 ("tty: n_gsm: avoid recursive locking with async port hangup") Cc: stable@vger.kernel.org Signed-off-by: Daniel Starke <daniel.starke@siemens.com> Link: https://lore.kernel.org/r/20220218073123.2121-4-daniel.starke@siemens.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 96b169f05cdcc844b400695184d77e42071d14f2 upstream.

The here fixed commit made the tty hangup asynchronous to avoid a circular
locking warning. I could not reproduce this warning. Furthermore, due to
the asynchronous hangup the function call now gets queued up while the
underlying tty is being freed. Depending on the timing this results in a
NULL pointer access in the global work queue scheduler. To be precise in
process_one_work(). Therefore, the previous commit made the issue worse
which it tried to fix.

This patch fixes this by falling back to the old behavior which uses a
blocking tty hangup call before freeing up the associated tty.

Fixes: 7030082a7415 ("tty: n_gsm: avoid recursive locking with async port hangup")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220218073123.2121-4-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
tty: n_gsm: fix proper link termination after failed open 2022-03-02T10:41:16+00:00 daniel.starke@siemens.com daniel.starke@siemens.com 2022-02-18T07:31:19+00:00 c03a4958141752283e35dbdc4998e0403bb1332a commit e3b7468f082d106459e86e8dc6fb9bdd65553433 upstream. Trying to open a DLCI by sending a SABM frame may fail with a timeout. The link is closed on the initiator side without informing the responder about this event. The responder assumes the link is open after sending a UA frame to answer the SABM frame. The link gets stuck in a half open state. This patch fixes this by initiating the proper link termination procedure after link setup timeout instead of silently closing it down. Fixes: e1eaea46bb40 ("tty: n_gsm line discipline") Cc: stable@vger.kernel.org Signed-off-by: Daniel Starke <daniel.starke@siemens.com> Link: https://lore.kernel.org/r/20220218073123.2121-3-daniel.starke@siemens.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit e3b7468f082d106459e86e8dc6fb9bdd65553433 upstream.

Trying to open a DLCI by sending a SABM frame may fail with a timeout.
The link is closed on the initiator side without informing the responder
about this event. The responder assumes the link is open after sending a
UA frame to answer the SABM frame. The link gets stuck in a half open
state.

This patch fixes this by initiating the proper link termination procedure
after link setup timeout instead of silently closing it down.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220218073123.2121-3-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
tty: n_gsm: fix encoding of control signal octet bit DV 2022-03-02T10:41:16+00:00 daniel.starke@siemens.com daniel.starke@siemens.com 2022-02-18T07:31:17+00:00 912144e8a3b05d18b4d9e6dc5bac3335dfec04e0 commit 737b0ef3be6b319d6c1fd64193d1603311969326 upstream. n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010. See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516 The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to the newer 27.010 here. Chapter 5.4.6.3.7 describes the encoding of the control signal octet used by the MSC (modem status command). The same encoding is also used in convergence layer type 2 as described in chapter 5.5.2. Table 7 and 24 both require the DV (data valid) bit to be set 1 for outgoing control signal octets sent by the DTE (data terminal equipment), i.e. for the initiator side. Currently, the DV bit is only set if CD (carrier detect) is on, regardless of the side. This patch fixes this behavior by setting the DV bit on the initiator side unconditionally. Fixes: e1eaea46bb40 ("tty: n_gsm line discipline") Cc: stable@vger.kernel.org Signed-off-by: Daniel Starke <daniel.starke@siemens.com> Link: https://lore.kernel.org/r/20220218073123.2121-1-daniel.starke@siemens.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 737b0ef3be6b319d6c1fd64193d1603311969326 upstream.

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.4.6.3.7 describes the encoding of the
control signal octet used by the MSC (modem status command). The same
encoding is also used in convergence layer type 2 as described in chapter
5.5.2. Table 7 and 24 both require the DV (data valid) bit to be set 1 for
outgoing control signal octets sent by the DTE (data terminal equipment),
i.e. for the initiator side.
Currently, the DV bit is only set if CD (carrier detect) is on, regardless
of the side.

This patch fixes this behavior by setting the DV bit on the initiator side
unconditionally.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220218073123.2121-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
serial: parisc: GSC: fix build when IOSAPIC is not set 2022-02-23T10:59:54+00:00 Randy Dunlap rdunlap@infradead.org 2022-02-14T18:00:19+00:00 bd282ee53e34c5bc7b2dc45bb5f1b7070923598e commit 6e8793674bb0d1135ca0e5c9f7e16fecbf815926 upstream. There is a build error when using a kernel .config file from 'kernel test robot' for a different build problem: hppa64-linux-ld: drivers/tty/serial/8250/8250_gsc.o: in function `.LC3': (.data.rel.ro+0x18): undefined reference to `iosapic_serial_irq' when: CONFIG_GSC=y CONFIG_SERIO_GSCPS2=y CONFIG_SERIAL_8250_GSC=y CONFIG_PCI is not set and hence PCI_LBA is not set. IOSAPIC depends on PCI_LBA, so IOSAPIC is not set/enabled. Make the use of iosapic_serial_irq() conditional to fix the build error. Signed-off-by: Randy Dunlap <rdunlap@infradead.org> Reported-by: kernel test robot <lkp@intel.com> Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com> Cc: Helge Deller <deller@gmx.de> Cc: linux-parisc@vger.kernel.org Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: linux-serial@vger.kernel.org Cc: Jiri Slaby <jirislaby@kernel.org> Cc: Johan Hovold <johan@kernel.org> Suggested-by: Helge Deller <deller@gmx.de> Signed-off-by: Helge Deller <deller@gmx.de> Cc: stable@vger.kernel.org Signed-off-by: Helge Deller <deller@gmx.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 6e8793674bb0d1135ca0e5c9f7e16fecbf815926 upstream.

There is a build error when using a kernel .config file from
'kernel test robot' for a different build problem:

hppa64-linux-ld: drivers/tty/serial/8250/8250_gsc.o: in function `.LC3':
(.data.rel.ro+0x18): undefined reference to `iosapic_serial_irq'

when:
  CONFIG_GSC=y
  CONFIG_SERIO_GSCPS2=y
  CONFIG_SERIAL_8250_GSC=y
  CONFIG_PCI is not set
    and hence PCI_LBA is not set.
  IOSAPIC depends on PCI_LBA, so IOSAPIC is not set/enabled.

Make the use of iosapic_serial_irq() conditional to fix the build error.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-parisc@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-serial@vger.kernel.org
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Johan Hovold <johan@kernel.org>
Suggested-by: Helge Deller <deller@gmx.de>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
n_tty: wake up poll(POLLRDNORM) on receiving data 2022-02-16T11:52:52+00:00 TATSUKAWA KOSUKE (立川 江介) tatsu-ab1@nec.com 2022-01-26T23:35:02+00:00 d4dc28db1b985d78ebcf0aa59b11377b6e1090ec commit c816b2e65b0e86b95011418cad334f0524fc33b8 upstream. The poll man page says POLLRDNORM is equivalent to POLLIN when used as an event. $ man poll <snip> POLLRDNORM Equivalent to POLLIN. However, in n_tty driver, POLLRDNORM does not return until timeout even if there is terminal input, whereas POLLIN returns. The following test program works until kernel-3.17, but the test stops in poll() after commit 57087d515441 ("tty: Fix spurious poll() wakeups"). [Steps to run test program] $ cc -o test-pollrdnorm test-pollrdnorm.c $ ./test-pollrdnorm foo <-- Type in something from the terminal followed by [RET]. The string should be echoed back. ------------------------< test-pollrdnorm.c >------------------------ #include <stdio.h> #include <errno.h> #include <poll.h> #include <unistd.h> void main(void) { int n; unsigned char buf[8]; struct pollfd fds[1] = {{ 0, POLLRDNORM, 0 }}; n = poll(fds, 1, -1); if (n < 0) perror("poll"); n = read(0, buf, 8); if (n < 0) perror("read"); if (n > 0) write(1, buf, n); } ------------------------------------------------------------------------ The attached patch fixes this problem. Many calls to wake_up_interruptible_poll() in the kernel source code already specify "POLLIN | POLLRDNORM". Fixes: 57087d515441 ("tty: Fix spurious poll() wakeups") Cc: stable@vger.kernel.org Signed-off-by: Kosuke Tatsukawa <tatsu-ab1@nec.com> Link: https://lore.kernel.org/r/TYCPR01MB81901C0F932203D30E452B3EA5209@TYCPR01MB8190.jpnprd01.prod.outlook.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit c816b2e65b0e86b95011418cad334f0524fc33b8 upstream.

The poll man page says POLLRDNORM is equivalent to POLLIN when used as
an event.
$ man poll
<snip>
              POLLRDNORM
                     Equivalent to POLLIN.

However, in n_tty driver, POLLRDNORM does not return until timeout even
if there is terminal input, whereas POLLIN returns.

The following test program works until kernel-3.17, but the test stops
in poll() after commit 57087d515441 ("tty: Fix spurious poll() wakeups").

[Steps to run test program]
  $ cc -o test-pollrdnorm test-pollrdnorm.c
  $ ./test-pollrdnorm
  foo          <-- Type in something from the terminal followed by [RET].
                   The string should be echoed back.

  ------------------------< test-pollrdnorm.c >------------------------
  #include <stdio.h>
  #include <errno.h>
  #include <poll.h>
  #include <unistd.h>

  void main(void)
  {
	int		n;
	unsigned char	buf[8];
	struct pollfd	fds[1] = {{ 0, POLLRDNORM, 0 }};

	n = poll(fds, 1, -1);
	if (n < 0)
		perror("poll");
	n = read(0, buf, 8);
	if (n < 0)
		perror("read");
	if (n > 0)
		write(1, buf, n);
  }
  ------------------------------------------------------------------------

The attached patch fixes this problem.  Many calls to
wake_up_interruptible_poll() in the kernel source code already specify
"POLLIN | POLLRDNORM".

Fixes: 57087d515441 ("tty: Fix spurious poll() wakeups")
Cc: stable@vger.kernel.org
Signed-off-by: Kosuke Tatsukawa <tatsu-ab1@nec.com>
Link: https://lore.kernel.org/r/TYCPR01MB81901C0F932203D30E452B3EA5209@TYCPR01MB8190.jpnprd01.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
vt_ioctl: add array_index_nospec to VT_ACTIVATE 2022-02-16T11:52:52+00:00 Jakob Koschel jakobkoschel@gmail.com 2022-01-27T14:44:05+00:00 0e31f914d761767c7a15bde4a6f1e55ea74ba09e commit 28cb138f559f8c1a1395f5564f86b8bbee83631b upstream. in vt_setactivate an almost identical code path has been patched with array_index_nospec. In the VT_ACTIVATE path the user input is from a system call argument instead of a usercopy. For consistency both code paths should have the same mitigations applied. Kasper Acknowledgements: Jakob Koschel, Brian Johannesmeyer, Kaveh Razavi, Herbert Bos, Cristiano Giuffrida from the VUSec group at VU Amsterdam. Co-developed-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com> Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com> Link: https://lore.kernel.org/r/20220127144406.3589293-2-jakobkoschel@gmail.com Cc: stable <stable@vger.kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 28cb138f559f8c1a1395f5564f86b8bbee83631b upstream.

in vt_setactivate an almost identical code path has been patched
with array_index_nospec. In the VT_ACTIVATE path the user input
is from a system call argument instead of a usercopy.
For consistency both code paths should have the same mitigations
applied.

Kasper Acknowledgements: Jakob Koschel, Brian Johannesmeyer, Kaveh
Razavi, Herbert Bos, Cristiano Giuffrida from the VUSec group at VU
Amsterdam.

Co-developed-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Link: https://lore.kernel.org/r/20220127144406.3589293-2-jakobkoschel@gmail.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
vt_ioctl: fix array_index_nospec in vt_setactivate 2022-02-16T11:52:51+00:00 Jakob Koschel jakobkoschel@gmail.com 2022-01-27T14:44:04+00:00 ae3d57411562260ee3f4fd5e875f410002341104 commit 61cc70d9e8ef5b042d4ed87994d20100ec8896d9 upstream. array_index_nospec ensures that an out-of-bounds value is set to zero on the transient path. Decreasing the value by one afterwards causes a transient integer underflow. vsa.console should be decreased first and then sanitized with array_index_nospec. Kasper Acknowledgements: Jakob Koschel, Brian Johannesmeyer, Kaveh Razavi, Herbert Bos, Cristiano Giuffrida from the VUSec group at VU Amsterdam. Co-developed-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com> Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com> Link: https://lore.kernel.org/r/20220127144406.3589293-1-jakobkoschel@gmail.com Cc: stable <stable@vger.kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 61cc70d9e8ef5b042d4ed87994d20100ec8896d9 upstream.

array_index_nospec ensures that an out-of-bounds value is set to zero
on the transient path. Decreasing the value by one afterwards causes
a transient integer underflow. vsa.console should be decreased first
and then sanitized with array_index_nospec.

Kasper Acknowledgements: Jakob Koschel, Brian Johannesmeyer, Kaveh
Razavi, Herbert Bos, Cristiano Giuffrida from the VUSec group at VU
Amsterdam.

Co-developed-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Link: https://lore.kernel.org/r/20220127144406.3589293-1-jakobkoschel@gmail.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
tty: Add support for Brainboxes UC cards. 2022-02-01T16:24:35+00:00 Cameron Williams cang1@live.co.uk 2022-01-24T09:42:23+00:00 a06cba5ad1256a27b8e18b9ddd7957a5df506311 commit 152d1afa834c84530828ee031cf07a00e0fc0b8c upstream. This commit adds support for the some of the Brainboxes PCI range of cards, including the UC-101, UC-235/246, UC-257, UC-268, UC-275/279, UC-302, UC-310, UC-313, UC-320/324, UC-346, UC-357, UC-368 and UC-420/431. Signed-off-by: Cameron Williams <cang1@live.co.uk> Cc: stable <stable@vger.kernel.org> Link: https://lore.kernel.org/r/AM5PR0202MB2564688493F7DD9B9C610827C45E9@AM5PR0202MB2564.eurprd02.prod.outlook.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 152d1afa834c84530828ee031cf07a00e0fc0b8c upstream.

This commit adds support for the some of the Brainboxes PCI range of
cards, including the UC-101, UC-235/246, UC-257, UC-268, UC-275/279,
UC-302, UC-310, UC-313, UC-320/324, UC-346, UC-357, UC-368
and UC-420/431.

Signed-off-by: Cameron Williams <cang1@live.co.uk>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/AM5PR0202MB2564688493F7DD9B9C610827C45E9@AM5PR0202MB2564.eurprd02.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
tty: n_gsm: fix SW flow control encoding/handling 2022-02-01T16:24:35+00:00 daniel.starke@siemens.com daniel.starke@siemens.com 2022-01-20T10:18:57+00:00 f5e6c946732a8201ad6d2ba7b4d259b5a103ce2e commit 8838b2af23caf1ff0610caef2795d6668a013b2d upstream. n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010. See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516 The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to the newer 27.010 here. Chapter 5.2.7.3 states that DC1 (XON) and DC3 (XOFF) are the control characters defined in ISO/IEC 646. These shall be quoted if seen in the data stream to avoid interpretation as flow control characters. ISO/IEC 646 refers to the set of ISO standards described as the ISO 7-bit coded character set for information interchange. Its final version is also known as ITU T.50. See https://www.itu.int/rec/T-REC-T.50-199209-I/en To abide the standard it is needed to quote DC1 and DC3 correctly if these are seen as data bytes and not as control characters. The current implementation already tries to enforce this but fails to catch all defined cases. 3GPP 27.010 chapter 5.2.7.3 clearly states that the most significant bit shall be ignored for DC1 and DC3 handling. The current implementation handles only the case with the most significant bit set 0. Cases in which DC1 and DC3 have the most significant bit set 1 are left unhandled. This patch fixes this by masking the data bytes with ISO_IEC_646_MASK (only the 7 least significant bits set 1) before comparing them with XON (a.k.a. DC1) and XOFF (a.k.a. DC3) when testing which byte values need quotation via byte stuffing. Fixes: e1eaea46bb40 ("tty: n_gsm line discipline") Cc: stable@vger.kernel.org Signed-off-by: Daniel Starke <daniel.starke@siemens.com> Link: https://lore.kernel.org/r/20220120101857.2509-1-daniel.starke@siemens.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 8838b2af23caf1ff0610caef2795d6668a013b2d upstream.

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.2.7.3 states that DC1 (XON) and DC3 (XOFF)
are the control characters defined in ISO/IEC 646. These shall be quoted if
seen in the data stream to avoid interpretation as flow control characters.

ISO/IEC 646 refers to the set of ISO standards described as the ISO
7-bit coded character set for information interchange. Its final version
is also known as ITU T.50.
See https://www.itu.int/rec/T-REC-T.50-199209-I/en

To abide the standard it is needed to quote DC1 and DC3 correctly if these
are seen as data bytes and not as control characters. The current
implementation already tries to enforce this but fails to catch all
defined cases. 3GPP 27.010 chapter 5.2.7.3 clearly states that the most
significant bit shall be ignored for DC1 and DC3 handling. The current
implementation handles only the case with the most significant bit set 0.
Cases in which DC1 and DC3 have the most significant bit set 1 are left
unhandled.

This patch fixes this by masking the data bytes with ISO_IEC_646_MASK (only
the 7 least significant bits set 1) before comparing them with XON
(a.k.a. DC1) and XOFF (a.k.a. DC3) when testing which byte values need
quotation via byte stuffing.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220120101857.2509-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
serial: stm32: fix software flow control transfer 2022-02-01T16:24:35+00:00 Valentin Caron valentin.caron@foss.st.com 2022-01-11T16:44:41+00:00 05b3301188887f9aa9f71fb4a9be7f0e68eeb84c commit 037b91ec7729524107982e36ec4b40f9b174f7a2 upstream. x_char is ignored by stm32_usart_start_tx() when xmit buffer is empty. Fix start_tx condition to allow x_char to be sent. Fixes: 48a6092fb41f ("serial: stm32-usart: Add STM32 USART Driver") Cc: stable <stable@vger.kernel.org> Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com> Signed-off-by: Valentin Caron <valentin.caron@foss.st.com> Link: https://lore.kernel.org/r/20220111164441.6178-3-valentin.caron@foss.st.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 037b91ec7729524107982e36ec4b40f9b174f7a2 upstream.

x_char is ignored by stm32_usart_start_tx() when xmit buffer is empty.

Fix start_tx condition to allow x_char to be sent.

Fixes: 48a6092fb41f ("serial: stm32-usart: Add STM32 USART Driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20220111164441.6178-3-valentin.caron@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>