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Many serial drivers do the same thing: * send x_char if set * keep sending from the xmit circular buffer until either - the loop reaches the end of the xmit buffer - TX is stopped - HW fifo is full * check for pending characters and: - wake up tty writers to fill for more data into xmit buffer - stop TX if there is nothing in the xmit buffer The only differences are: * how to write the character to the HW fifo * the check of the end condition: - is the HW fifo full? - is limit of the written characters reached? So unify the above into two helpers: * uart_port_tx_limited() -- it performs the above taking the written characters limit into account, and * uart_port_tx() -- the same as above, except it only checks the HW readiness, not the characters limit. The HW specific operations (as stated as "differences" above) are passed as arguments to the macros. They are: * tx_ready -- returns true if HW can accept more data. * put_char -- write a character to the device. * tx_done -- when the write loop is done, perform arbitrary action before potential invocation of ops->stop_tx() happens. Note that the above are macros. This means the code is generated in place and the above 3 arguments are "inlined". I.e. no added penalty by generating call instructions for every single character. Nor any indirect calls. (As in some previous versions of this patchset.) Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com> Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org> Link: https://lore.kernel.org/r/20221004104927.14361-2-jirislaby@kernel.org Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
107 lines
3.3 KiB
ReStructuredText
107 lines
3.3 KiB
ReStructuredText
====================
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Low Level Serial API
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====================
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This document is meant as a brief overview of some aspects of the new serial
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driver. It is not complete, any questions you have should be directed to
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<rmk@arm.linux.org.uk>
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The reference implementation is contained within amba-pl011.c.
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Low Level Serial Hardware Driver
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--------------------------------
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The low level serial hardware driver is responsible for supplying port
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information (defined by uart_port) and a set of control methods (defined
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by uart_ops) to the core serial driver. The low level driver is also
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responsible for handling interrupts for the port, and providing any
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console support.
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Console Support
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---------------
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The serial core provides a few helper functions. This includes identifing
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the correct port structure (via uart_get_console()) and decoding command line
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arguments (uart_parse_options()).
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There is also a helper function (uart_console_write()) which performs a
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character by character write, translating newlines to CRLF sequences.
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Driver writers are recommended to use this function rather than implementing
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their own version.
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Locking
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-------
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It is the responsibility of the low level hardware driver to perform the
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necessary locking using port->lock. There are some exceptions (which
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are described in the struct uart_ops listing below.)
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There are two locks. A per-port spinlock, and an overall semaphore.
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From the core driver perspective, the port->lock locks the following
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data::
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port->mctrl
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port->icount
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port->state->xmit.head (circ_buf->head)
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port->state->xmit.tail (circ_buf->tail)
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The low level driver is free to use this lock to provide any additional
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locking.
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The port_sem semaphore is used to protect against ports being added/
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removed or reconfigured at inappropriate times. Since v2.6.27, this
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semaphore has been the 'mutex' member of the tty_port struct, and
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commonly referred to as the port mutex.
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uart_ops
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--------
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.. kernel-doc:: include/linux/serial_core.h
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:identifiers: uart_ops
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Other functions
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---------------
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.. kernel-doc:: drivers/tty/serial/serial_core.c
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:identifiers: uart_update_timeout uart_get_baud_rate uart_get_divisor
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uart_match_port uart_write_wakeup uart_register_driver
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uart_unregister_driver uart_suspend_port uart_resume_port
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uart_add_one_port uart_remove_one_port uart_console_write
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uart_parse_earlycon uart_parse_options uart_set_options
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uart_get_lsr_info uart_handle_dcd_change uart_handle_cts_change
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uart_try_toggle_sysrq uart_get_console
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.. kernel-doc:: include/linux/serial_core.h
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:identifiers: uart_port_tx_limited uart_port_tx
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Other notes
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-----------
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It is intended some day to drop the 'unused' entries from uart_port, and
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allow low level drivers to register their own individual uart_port's with
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the core. This will allow drivers to use uart_port as a pointer to a
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structure containing both the uart_port entry with their own extensions,
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thus::
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struct my_port {
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struct uart_port port;
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int my_stuff;
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};
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Modem control lines via GPIO
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----------------------------
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Some helpers are provided in order to set/get modem control lines via GPIO.
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.. kernel-doc:: drivers/tty/serial/serial_mctrl_gpio.c
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:identifiers: mctrl_gpio_init mctrl_gpio_free mctrl_gpio_to_gpiod
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mctrl_gpio_set mctrl_gpio_get mctrl_gpio_enable_ms
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mctrl_gpio_disable_ms
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