Revision d9a047aeffcef5755952d18f2901d8777d84019d authored by Doug Ledford on 09 July 2015, 14:21:08 UTC, committed by Doug Ledford on 14 July 2015, 17:20:15 UTC
There is little chance our memory allocation will fail, so we can combine initializing the work structs with allocating them instead of looping through all of them once to allocate and again to initialize. Then when we need to actually find out if our device is up or in the process of going down, have all of our work structs batched up, take the spin_lock once and only once, and do all of the batch under the one spin_lock invocation instead of incurring all of the locked memory cycles we would otherwise incur to take/release the spin_lock over and over again. Signed-off-by: Doug Ledford <dledford@redhat.com>
1 parent 9bbf282
serial-rs485.txt
RS485 SERIAL COMMUNICATIONS
1. INTRODUCTION
EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the
electrical characteristics of drivers and receivers for use in balanced
digital multipoint systems.
This standard is widely used for communications in industrial automation
because it can be used effectively over long distances and in electrically
noisy environments.
2. HARDWARE-RELATED CONSIDERATIONS
Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in
half-duplex mode capable of automatically controlling line direction by
toggling RTS or DTR signals. That can be used to control external
half-duplex hardware like an RS485 transceiver or any RS232-connected
half-duplex devices like some modems.
For these microcontrollers, the Linux driver should be made capable of
working in both modes, and proper ioctls (see later) should be made
available at user-level to allow switching from one mode to the other, and
vice versa.
3. DATA STRUCTURES ALREADY AVAILABLE IN THE KERNEL
The Linux kernel provides the serial_rs485 structure (see [1]) to handle
RS485 communications. This data structure is used to set and configure RS485
parameters in the platform data and in ioctls.
The device tree can also provide RS485 boot time parameters (see [2]
for bindings). The driver is in charge of filling this data structure from
the values given by the device tree.
Any driver for devices capable of working both as RS232 and RS485 should
implement the rs485_config callback in the uart_port structure. The
serial_core calls rs485_config to do the device specific part in response
to TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callback
receives a pointer to struct serial_rs485.
4. USAGE FROM USER-LEVEL
From user-level, RS485 configuration can be get/set using the previous
ioctls. For instance, to set RS485 you can use the following code:
#include <linux/serial.h>
/* RS485 ioctls: */
#define TIOCGRS485 0x542E
#define TIOCSRS485 0x542F
/* Open your specific device (e.g., /dev/mydevice): */
int fd = open ("/dev/mydevice", O_RDWR);
if (fd < 0) {
/* Error handling. See errno. */
}
struct serial_rs485 rs485conf;
/* Enable RS485 mode: */
rs485conf.flags |= SER_RS485_ENABLED;
/* Set logical level for RTS pin equal to 1 when sending: */
rs485conf.flags |= SER_RS485_RTS_ON_SEND;
/* or, set logical level for RTS pin equal to 0 when sending: */
rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
/* Set logical level for RTS pin equal to 1 after sending: */
rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
/* or, set logical level for RTS pin equal to 0 after sending: */
rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
/* Set rts delay before send, if needed: */
rs485conf.delay_rts_before_send = ...;
/* Set rts delay after send, if needed: */
rs485conf.delay_rts_after_send = ...;
/* Set this flag if you want to receive data even whilst sending data */
rs485conf.flags |= SER_RS485_RX_DURING_TX;
if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
/* Error handling. See errno. */
}
/* Use read() and write() syscalls here... */
/* Close the device when finished: */
if (close (fd) < 0) {
/* Error handling. See errno. */
}
5. REFERENCES
[1] include/uapi/linux/serial.h
[2] Documentation/devicetree/bindings/serial/rs485.txt
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