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USB: serial: ch341: simulate break condition if not supported

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A subset of all CH341 devices don't support a real break condition. This
fact is already used in the "ch341_detect_quirks" function. With this
change a quirk is implemented to simulate a break condition by
temporarily lowering the baud rate and sending a NUL byte.

The primary drawbacks of this approach are that the duration of the
break can't be controlled by userland and that data incoming during
a simulated break is corrupted.

The "TTY_DRIVER_HARDWARE_BREAK" serial driver flag was investigated as
an alternative. It's a driver-wide flag and would've required
significant changes to the serial and USB-serial driver frameworks to
expose it for individual USB-serial adapters.

Tested by sending a break condition and watching the TX pin using an
oscilloscope.

Signed-off-by: Michael Hanselmann <public@hansmi.ch>
Link: https://lore.kernel.org/r/f34a9b6e-ec2a-0873-e97b-2d5b2170e2ff@msgid.hansmi.ch
[ johan: condense info message ]
Signed-off-by: Johan Hovold <johan@kernel.org>
This commit is contained in:
Michael Hanselmann 2020-07-04 20:25:03 +02:00 committed by Johan Hovold
parent cabe0785ff
commit 0580baa46e
1 changed files with 93 additions and 9 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

102
drivers/usb/serial/ch341.c
View file

@ -78,6 +78,7 @@
#define CH341_LCR_CS5 0x00
#define CH341_QUIRK_LIMITED_PRESCALER BIT(0)
#define CH341_QUIRK_SIMULATE_BREAK BIT(1)
static const struct usb_device_id id_table[] = {
{ USB_DEVICE(0x4348, 0x5523) },
@ -94,6 +95,7 @@ struct ch341_private {
u8 msr;
u8 lcr;
unsigned long quirks;
unsigned long break_end;
};
static void ch341_set_termios(struct tty_struct *tty,
@ -170,10 +172,9 @@ static const speed_t ch341_min_rates[] = {
* 2 <= div <= 256 if fact = 0, or
* 9 <= div <= 256 if fact = 1
*/
static int ch341_get_divisor(struct ch341_private *priv)
static int ch341_get_divisor(struct ch341_private *priv, speed_t speed)
{
unsigned int fact, div, clk_div;
speed_t speed = priv->baud_rate;
bool force_fact0 = false;
int ps;
@ -236,15 +237,16 @@ static int ch341_get_divisor(struct ch341_private *priv)
}
static int ch341_set_baudrate_lcr(struct usb_device *dev,
struct ch341_private *priv, u8 lcr)
struct ch341_private *priv,
speed_t baud_rate, u8 lcr)
{
int val;
int r;
if (!priv->baud_rate)
if (!baud_rate)
return -EINVAL;
val = ch341_get_divisor(priv);
val = ch341_get_divisor(priv, baud_rate);
if (val < 0)
return -EINVAL;
@ -324,7 +326,7 @@ static int ch341_configure(struct usb_device *dev, struct ch341_private *priv)
if (r < 0)
goto out;
r = ch341_set_baudrate_lcr(dev, priv, priv->lcr);
r = ch341_set_baudrate_lcr(dev, priv, priv->baud_rate, priv->lcr);
if (r < 0)
goto out;
@ -357,8 +359,8 @@ static int ch341_detect_quirks(struct usb_serial_port *port)
USB_TYPE_VENDOR | USB_RECIP_DEVICE | USB_DIR_IN,
CH341_REG_BREAK, 0, buffer, size, DEFAULT_TIMEOUT);
if (r == -EPIPE) {
dev_dbg(&port->dev, "break control not supported\n");
quirks = CH341_QUIRK_LIMITED_PRESCALER;
dev_info(&port->dev, "break control not supported, using simulated break\n");
quirks = CH341_QUIRK_LIMITED_PRESCALER | CH341_QUIRK_SIMULATE_BREAK;
r = 0;
goto out;
}
@ -539,7 +541,8 @@ static void ch341_set_termios(struct tty_struct *tty,
if (baud_rate) {
priv->baud_rate = baud_rate;
r = ch341_set_baudrate_lcr(port->serial->dev, priv, lcr);
r = ch341_set_baudrate_lcr(port->serial->dev, priv,
priv->baud_rate, lcr);
if (r < 0 && old_termios) {
priv->baud_rate = tty_termios_baud_rate(old_termios);
tty_termios_copy_hw(&tty->termios, old_termios);
@ -558,15 +561,96 @@ static void ch341_set_termios(struct tty_struct *tty,
ch341_set_handshake(port->serial->dev, priv->mcr);
}
/*
* A subset of all CH34x devices don't support a real break condition and
* reading CH341_REG_BREAK fails (see also ch341_detect_quirks). This function
* simulates a break condition by lowering the baud rate to the minimum
* supported by the hardware upon enabling the break condition and sending
* a NUL byte.
*
* Incoming data is corrupted while the break condition is being simulated.
*
* Normally the duration of the break condition can be controlled individually
* by userspace using TIOCSBRK and TIOCCBRK or by passing an argument to
* TCSBRKP. Due to how the simulation is implemented the duration can't be
* controlled. The duration is always about (1s / 46bd * 9bit) = 196ms.
*/
static void ch341_simulate_break(struct tty_struct *tty, int break_state)
{
struct usb_serial_port *port = tty->driver_data;
struct ch341_private *priv = usb_get_serial_port_data(port);
unsigned long now, delay;
int r;
if (break_state != 0) {
dev_dbg(&port->dev, "enter break state requested\n");
r = ch341_set_baudrate_lcr(port->serial->dev, priv,
CH341_MIN_BPS,
CH341_LCR_ENABLE_RX | CH341_LCR_ENABLE_TX | CH341_LCR_CS8);
if (r < 0) {
dev_err(&port->dev,
"failed to change baud rate to %u: %d\n",
CH341_MIN_BPS, r);
goto restore;
}
r = tty_put_char(tty, '\0');
if (r < 0) {
dev_err(&port->dev,
"failed to write NUL byte for simulated break condition: %d\n",
r);
goto restore;
}
/*
* Compute expected transmission duration and add a single bit
* of safety margin (the actual NUL byte transmission is 8 bits
* plus one stop bit).
*/
priv->break_end = jiffies + (10 * HZ / CH341_MIN_BPS);
return;
}
dev_dbg(&port->dev, "leave break state requested\n");
now = jiffies;
if (time_before(now, priv->break_end)) {
/* Wait until NUL byte is written */
delay = priv->break_end - now;
dev_dbg(&port->dev,
"wait %d ms while transmitting NUL byte at %u baud\n",
jiffies_to_msecs(delay), CH341_MIN_BPS);
schedule_timeout_interruptible(delay);
}
restore:
/* Restore original baud rate */
r = ch341_set_baudrate_lcr(port->serial->dev, priv, priv->baud_rate,
priv->lcr);
if (r < 0)
dev_err(&port->dev,
"restoring original baud rate of %u failed: %d\n",
priv->baud_rate, r);
}
static void ch341_break_ctl(struct tty_struct *tty, int break_state)
{
const uint16_t ch341_break_reg =
((uint16_t) CH341_REG_LCR << 8) | CH341_REG_BREAK;
struct usb_serial_port *port = tty->driver_data;
struct ch341_private *priv = usb_get_serial_port_data(port);
int r;
uint16_t reg_contents;
uint8_t *break_reg;
if (priv->quirks & CH341_QUIRK_SIMULATE_BREAK) {
ch341_simulate_break(tty, break_state);
return;
}
break_reg = kmalloc(2, GFP_KERNEL);
if (!break_reg)
return;