Re: [PATCH v4] USB: serial: xr: Add TIOCGRS485 and TIOCSRS485 ioctls

From: Jarkko Sonninen
Date: Thu Jul 06 2023 - 15:38:08 EST

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On 6/20/23 15:38, Johan Hovold wrote:

Hi Jarkko,

I've been waiting for you to send a v5 that addresses the issue that the
kernel test robot found, but I guess I can review the other changes in
v4 first.

Thank you. Sorry for slow responses. My mind has been elsewhere.

On Sun, Apr 23, 2023 at 09:59:28PM +0300, Jarkko Sonninen wrote:

Exar devices like XR21B1411 can control an RS485 transceiver by
automatically asserting the RTS#/RS485 pin before sending data
and deasserting it when the last stop bit has been transmitted.
The polarity of the RST#/RS485 signal is configurable and the
hardware also supports half-duplex turn-around delay and
address matching mode.

Add support for enabling and disabling RS-485 mode and
configuring the RST#/RS485 signal polarity using the TIOCGRS485
and TIOCSRS485 ioctls. Support for half-duplex turn-around delay
and address matching mode are left unimplemented for now.

User enables RS-485 mode by setting SER_RS485_ENABLED flag in
struct serial_rs485 flags. User should also set either
SER_RS485_RTS_ON_SEND or SER_RS485_RTS_AFTER_SEND to select the
behaviour of the RTS#/RS485 pin. Setting SER_RS485_RTS_ON_SEND
will drive RTS#/RS485 high during transmission. As this is the
typical application described by Exar, it is selected when
user sets neither or both flags.

Thanks for updating the commit message here.

Since RTS# is active low, shouldn't SER_RS485_RTS_ON_SEND drive RTS# low
rather than high during transmission as I also pointed out earlier?

I guess you are right. I'll change that.

I use an exar usb adapter to control a solar charging controller. I haven't found any other type of exar adapters in ebay.

This adapter uses high level (RTS off) on TX. So I really would like it to work with the default configuration.

I hope it is ok to use SER_RS485_RTS_AFTER_SEND as the default

Signed-off-by: Jarkko Sonninen <kasper@xxxxxx>
---
Changes in v3:
- In this version only rs485.flags are stored to state.
- There is no locking as only one bit of the flags is used.
- ioctl returns -ENOIOCTLCMD as the actual error handling is in tty code.
Changes in v4:
- Store struct rs485 to data
- Add mutex to protect data->rs485.
- Implement SER_RS485_RTS_ON_SEND or SER_RS485_RTS_AFTER_SEND flags
- SER_RS485_RTS_ON_SEND is the default like in serial_core.c

drivers/usb/serial/xr_serial.c | 93 +++++++++++++++++++++++++++++++++-
1 file changed, 92 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/serial/xr_serial.c b/drivers/usb/serial/xr_serial.c
index fdb0aae546c3..a2157619b63c 100644
--- a/drivers/usb/serial/xr_serial.c
+++ b/drivers/usb/serial/xr_serial.c
@@ -19,6 +19,7 @@
#include <linux/usb.h>
#include <linux/usb/cdc.h>
#include <linux/usb/serial.h>
+#include <linux/mutex.h>

Nit: keep the includes sorted alphabetically.

struct xr_txrx_clk_mask {
u16 tx;
@@ -93,6 +94,7 @@ struct xr_txrx_clk_mask {
#define XR_GPIO_MODE_SEL_DTR_DSR 0x2
#define XR_GPIO_MODE_SEL_RS485 0x3
#define XR_GPIO_MODE_SEL_RS485_ADDR 0x4
+#define XR_GPIO_MODE_RS485_TX_H 0x8
#define XR_GPIO_MODE_TX_TOGGLE 0x100
#define XR_GPIO_MODE_RX_TOGGLE 0x200
@@ -237,6 +239,8 @@ static const struct xr_type xr_types[] = {
struct xr_data {
const struct xr_type *type;
u8 channel; /* zero-based index or interface number */
+ struct serial_rs485 rs485;
+ struct mutex lock;
};
static int xr_set_reg(struct usb_serial_port *port, u8 channel, u16 reg, u16 val)
@@ -630,6 +634,7 @@ static void xr_set_flow_mode(struct tty_struct *tty,
const struct xr_type *type = data->type;
u16 flow, gpio_mode;
int ret;
+ bool rs485_enabled;

Nit: move above ret to maintain some approximation of reverse xmas style
declaration.

ret = xr_get_reg_uart(port, type->gpio_mode, &gpio_mode);
if (ret)
@@ -645,7 +650,17 @@ static void xr_set_flow_mode(struct tty_struct *tty,
/* Set GPIO mode for controlling the pins manually by default. */
gpio_mode &= ~XR_GPIO_MODE_SEL_MASK;
- if (C_CRTSCTS(tty) && C_BAUD(tty) != B0) {
+ rs485_enabled = !!(data->rs485.flags & SER_RS485_ENABLED);
+ if (rs485_enabled) {
+ dev_dbg(&port->dev, "Enabling RS-485\n");
+ gpio_mode |= XR_GPIO_MODE_SEL_RS485;
+ if (data->rs485.flags & SER_RS485_RTS_ON_SEND)
+ gpio_mode |= XR_GPIO_MODE_RS485_TX_H;
+ else
+ gpio_mode &= ~XR_GPIO_MODE_RS485_TX_H;

As mentioned above, should this not be inverted?

+ }
+
+ if (C_CRTSCTS(tty) && C_BAUD(tty) != B0 && !rs485_enabled) {
dev_dbg(&port->dev, "Enabling hardware flow ctrl\n");
gpio_mode |= XR_GPIO_MODE_SEL_RTS_CTS;
flow = XR_UART_FLOW_MODE_HW;
@@ -809,6 +824,80 @@ static void xr_cdc_set_line_coding(struct tty_struct *tty,
kfree(lc);
}
+static void xr_sanitize_serial_rs485(struct serial_rs485 *rs485)
+{
+ if (!(rs485->flags & SER_RS485_ENABLED)) {
+ memset(rs485, 0, sizeof(*rs485));
+ return;
+ }

This looks odd to me, but I see that this is what serial core is
currently doing...

+
+ /* Select RTS on send if the user hasn't selected the mode properly */
+ if (!!(rs485->flags & SER_RS485_RTS_ON_SEND) ==
+ !!(rs485->flags & SER_RS485_RTS_AFTER_SEND)) {
+ rs485->flags |= SER_RS485_RTS_ON_SEND;
+ rs485->flags &= ~SER_RS485_RTS_AFTER_SEND;
+ }
+
+ /* Only the flags are implemented at the moment */
+ rs485->flags &= SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND |
+ SER_RS485_RTS_AFTER_SEND;
+ rs485->delay_rts_before_send = 0;
+ rs485->delay_rts_after_send = 0;
+ memset(rs485->padding, 0, sizeof(rs485->padding));
+}
+
+static int xr_get_rs485_config(struct tty_struct *tty,
+ struct serial_rs485 *argp)

You still need __user annotation here as the build robot reported.

+{
+ struct usb_serial_port *port = tty->driver_data;
+ struct xr_data *data = usb_get_serial_port_data(port);
+
+ mutex_lock(&data->lock);

This does not prevent the termios structure from changing underneath
you. You need to take a write lock on the termios rw sem here instead.

I'll change these to "down_read(&tty->termios_rwsem);" and "up_read(&tty->termios_rwsem);"

I would feel safer by using a local variable "struct serial_rs485" to minimize the holding of the semaphore.

+ if (copy_to_user(argp, &data->rs485, sizeof(data->rs485))) {
+ mutex_unlock(&data->lock);
+ return -EFAULT;
+ }
+ mutex_unlock(&data->lock);
+
+ return 0;
+}
+
+static int xr_set_rs485_config(struct tty_struct *tty,
+ struct serial_rs485 *argp)

missing __user

+{
+ struct usb_serial_port *port = tty->driver_data;
+ struct xr_data *data = usb_get_serial_port_data(port);
+ struct serial_rs485 rs485;
+
+ if (copy_from_user(&rs485, argp, sizeof(rs485)))
+ return -EFAULT;
+ xr_sanitize_serial_rs485(&rs485);
+
+ mutex_lock(&data->lock);

Same here as set_termios() (and xr_set_flow_mode()) can otherwise be
called in parallel.

I'll change these to "down_write(&tty->termios_rwsem);" and "up_write(&tty->termios_rwsem);"

+ data->rs485 = rs485;
+ xr_set_flow_mode(tty, port, NULL);
+ mutex_unlock(&data->lock);
+
+ if (copy_to_user(argp, &rs485, sizeof(rs485)))
+ return -EFAULT;
+
+ return 0;
+}

Looks good otherwise.

Johan

- Jarkko

Next message: Wolfram Sang: "Re: [PATCH v3] i2c: busses: i2c-nomadik: Remove a useless call in the remove function"
Previous message: Wolfram Sang: "Re: [PATCH] i2c: bcm-iproc: Fix bcm_iproc_i2c_isr deadlock issue"
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