Re: [PATCH v6 2/3] tty: Add software emulated RS485 support for 8250

From: Peter Hurley
Date: Fri Jan 15 2016 - 11:14:39 EST

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On 12/21/2015 10:26 AM, Matwey V. Kornilov wrote:
> Implementation of software emulation of RS485 direction handling is based
> on omap_serial driver.
> Before and after transmission RTS is set to the appropriate value.
>
> Note that before calling serial8250_em485_init the caller has to
> ensure that UART will interrupt when shift register empty. Otherwise,
> emultaion cannot be used.
>
> Both serial8250_em485_init and serial8250_em485_destroy are
> idempotent functions.

Apologies for the long delay; comments below.

> Signed-off-by: Matwey V. Kornilov <matwey@xxxxxxxxxx>
> ---
> drivers/tty/serial/8250/8250.h | 6 ++
> drivers/tty/serial/8250/8250_port.c | 161 +++++++++++++++++++++++++++++++++++-
> include/linux/serial_8250.h | 7 ++
> 3 files changed, 170 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index d54dcd8..0189cb3 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -117,6 +117,12 @@ static inline void serial_dl_write(struct uart_8250_port *up, int value)
> struct uart_8250_port *serial8250_get_port(int line);
> void serial8250_rpm_get(struct uart_8250_port *p);
> void serial8250_rpm_put(struct uart_8250_port *p);
> +int serial8250_em485_init(struct uart_8250_port *p);
> +void serial8250_em485_destroy(struct uart_8250_port *p);
> +static inline bool serial8250_em485_enabled(struct uart_8250_port *p)
> +{
> + return p->em485 && (p->port.rs485.flags & SER_RS485_ENABLED);

Under what circumstances is p->em485 != NULL but
(p->port.rs485.flags & SER_RS485_ENABLED) is true?

ISTM, p->em485 is necessary and sufficient to determine if em485 is enabled.

In which case, this function can be eliminated and callers can be reduced to

if (p->em485)
....

> +}
>
> #if defined(__alpha__) && !defined(CONFIG_PCI)
> /*
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 8ad0b2d..d67a848 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -37,6 +37,7 @@
> #include <linux/slab.h>
> #include <linux/uaccess.h>
> #include <linux/pm_runtime.h>
> +#include <linux/timer.h>
>
> #include <asm/io.h>
> #include <asm/irq.h>
> @@ -504,6 +505,31 @@ static void serial8250_clear_fifos(struct uart_8250_port *p)
> }
> }
>
> +static inline void serial8250_em485_rts_on_send(struct uart_8250_port *p)

Only one call site, so please drop inline.

> +{
> + unsigned char mcr = serial_in(p, UART_MCR);
> +
> + if (p->port.rs485.flags & SER_RS485_RTS_ON_SEND)
> + mcr |= UART_MCR_RTS;
> + else
> + mcr &= ~UART_MCR_RTS;
> + serial_out(p, UART_MCR, mcr);
> +}
> +
> +static inline void serial8250_em485_rts_after_send(struct uart_8250_port *p)

Doesn't really need to be inline.

> +{
> + unsigned char mcr = serial_in(p, UART_MCR);
> +
> + if (p->port.rs485.flags & SER_RS485_RTS_AFTER_SEND)
> + mcr |= UART_MCR_RTS;
> + else
> + mcr &= ~UART_MCR_RTS;
> + serial_out(p, UART_MCR, mcr);
> +}
> +
> +static void serial8250_em485_handle_start_tx(unsigned long arg);
> +static void serial8250_em485_handle_stop_tx(unsigned long arg);
> +
> void serial8250_clear_and_reinit_fifos(struct uart_8250_port *p)
> {
> serial8250_clear_fifos(p);
> @@ -528,6 +554,42 @@ void serial8250_rpm_put(struct uart_8250_port *p)
> }
> EXPORT_SYMBOL_GPL(serial8250_rpm_put);
>
> +int serial8250_em485_init(struct uart_8250_port *p)
> +{
> + if (p->em485 != NULL)
> + return 0;
> +
> + p->em485 = kmalloc(sizeof(struct uart_8250_em485), GFP_KERNEL);
> + if (p->em485 == NULL)
> + return -ENOMEM;
> +
> + init_timer(&p->em485->stop_tx_timer);
> + p->em485->stop_tx_timer.function = serial8250_em485_handle_stop_tx;
> + p->em485->stop_tx_timer.data = (unsigned long)p;
> + p->em485->stop_tx_timer.flags |= TIMER_IRQSAFE;

Not sure this is going to fly; this would be the only user of TIMER_IRQSAFE
(which was specifically introduced to workaround workqueue issues and not
meant for general use).

> + init_timer(&p->em485->start_tx_timer);
> + p->em485->start_tx_timer.function = serial8250_em485_handle_start_tx;
> + p->em485->start_tx_timer.data = (unsigned long)p;
> + p->em485->start_tx_timer.flags |= TIMER_IRQSAFE;
> +
> + serial8250_em485_rts_after_send(p);
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(serial8250_em485_init);

Newline.

> +void serial8250_em485_destroy(struct uart_8250_port *p)
> +{
> + if (p->em485 == NULL)
> + return;
> +
> + del_timer_sync(&p->em485->start_tx_timer);
> + del_timer_sync(&p->em485->stop_tx_timer);

What keeps start_tx() from restarting a new timer right here?

> + kfree(p->em485);
> + p->em485 = NULL;
> +}
> +EXPORT_SYMBOL_GPL(serial8250_em485_destroy);
> +
> /*
> * These two wrappers ensure that enable_runtime_pm_tx() can be called more than
> * once and disable_runtime_pm_tx() will still disable RPM because the fifo is
> @@ -1293,7 +1355,61 @@ static void serial8250_stop_rx(struct uart_port *port)
> serial8250_rpm_put(up);
> }
>
> -static inline void __stop_tx(struct uart_8250_port *p)
> +static __u32 __start_tx_rs485(struct uart_8250_port *p)
^^^^^
No need to preserve the userspace type here.

The double underline leader in an identifier is typically used to distinguish
an unlocked version from a locked version. I don't think it's necessary here
or any of the other newly-introduced functions.

> +{
> + if (!serial8250_em485_enabled(p))
> + return 0;

Already checked that em485 was enabled in lone caller.

> + if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX))
> + serial8250_stop_rx(&p->port);
> +
> + del_timer_sync(&p->em485->stop_tx_timer);
> +
> + if (!!(p->port.rs485.flags & SER_RS485_RTS_ON_SEND) != !!(serial_in(p, UART_MCR) & UART_MCR_RTS)) {

Line too long. And just one negation is sufficient, ie.

if (!(....) !=
!(....)) {

> + serial8250_em485_rts_on_send(p);
> + return p->port.rs485.delay_rts_before_send;
> + }
> +
> + return 0;
> +}
> +
> +static inline void __do_stop_tx_rs485(struct uart_8250_port *p)

Does this really need to be inline?

> +{
> + if (!serial8250_em485_enabled(p))
> + return;
> +
> + serial8250_em485_rts_after_send(p);
> + /*
> + * Empty the RX FIFO, we are not interested in anything
> + * received during the half-duplex transmission.
> + */

Malformed block comment.

/*
*
*
*/

> + if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX))
> + serial8250_clear_fifos(p);
> +}
> +
> +static void serial8250_em485_handle_stop_tx(unsigned long arg)
> +{
> + struct uart_8250_port *p = (struct uart_8250_port *)arg;
> +
> + __do_stop_tx_rs485(p);
> +}
> +
> +static inline void __stop_tx_rs485(struct uart_8250_port *p)

Single caller so drop inline.

> +{
> + if (!serial8250_em485_enabled(p))
> + return;
> +
> + del_timer_sync(&p->em485->start_tx_timer);
> +
> + /* __do_stop_tx_rs485 is going to set RTS according to config AND flush RX FIFO if required */

Block comment.

> + if (p->port.rs485.delay_rts_after_send > 0) {
> + mod_timer(&p->em485->stop_tx_timer, jiffies + p->port.rs485.delay_rts_after_send * HZ / 1000);

Line too long; please re-format.
This is one problem with overly long identifiers.

> + } else {
> + __do_stop_tx_rs485(p);
> + }
> +}
> +
> +static inline void __do_stop_tx(struct uart_8250_port *p)
> {
> if (p->ier & UART_IER_THRI) {
> p->ier &= ~UART_IER_THRI;
> @@ -1302,6 +1418,21 @@ static inline void __stop_tx(struct uart_8250_port *p)
> }
> }
>
> +static inline void __stop_tx(struct uart_8250_port *p)
> +{
> + if (serial8250_em485_enabled(p)) {
> + unsigned char lsr = serial_in(p, UART_LSR);
> + /* To provide required timeing and allow FIFO transfer,
> + * __stop_tx_rs485 must be called only when both FIFO and shift register
> + * are empty. It is for device driver to enable interrupt on TEMT.
> + */

Block indent.

This code path should cancel start timer also.

> + if (!((lsr & UART_LSR_TEMT) && (lsr & UART_LSR_THRE)))

if ((lsr & BOTH_EMPTY) != BOTH_EMPTY)

> + return;
> + }
> + __do_stop_tx(p);
> + __stop_tx_rs485(p);
> +}
> +
> static void serial8250_stop_tx(struct uart_port *port)
> {
> struct uart_8250_port *up = up_to_u8250p(port);
> @@ -1319,12 +1450,10 @@ static void serial8250_stop_tx(struct uart_port *port)
> serial8250_rpm_put(up);
> }
>
> -static void serial8250_start_tx(struct uart_port *port)
> +static inline void __start_tx(struct uart_port *port)
> {
> struct uart_8250_port *up = up_to_u8250p(port);
>
> - serial8250_rpm_get_tx(up);
> -
> if (up->dma && !up->dma->tx_dma(up))
> return;
>
> @@ -1350,6 +1479,30 @@ static void serial8250_start_tx(struct uart_port *port)
> }
> }
>
> +static void serial8250_em485_handle_start_tx(unsigned long arg)
> +{
> + struct uart_8250_port *p = (struct uart_8250_port *)arg;
> +
> + __start_tx(&p->port);
> +}
> +
> +static void serial8250_start_tx(struct uart_port *port)
> +{
> + struct uart_8250_port *up = up_to_u8250p(port);
> + __u32 delay;

int delay;

> +
> + serial8250_rpm_get_tx(up);
> +
> + if (up->em485 && timer_pending(&up->em485->start_tx_timer))
> + return;
> +
> + if (up->em485 && (delay = __start_tx_rs485(up))) {

No assignment in conditional please.

> + mod_timer(&up->em485->start_tx_timer, jiffies + delay * HZ / 1000);
> + } else {
> + __start_tx(port);
> + }

Generally, braces aren't used for single statement if..else.
That probably won't apply here after removing the assignment-in-conditional,
but I thought it worth mentioning just so you know.

Regards,
Peter Hurley

> +}
> +
> static void serial8250_throttle(struct uart_port *port)
> {
> port->throttle(port);
> diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
> index faa0e03..71516ec 100644
> --- a/include/linux/serial_8250.h
> +++ b/include/linux/serial_8250.h
> @@ -76,6 +76,11 @@ struct uart_8250_ops {
> void (*release_irq)(struct uart_8250_port *);
> };
>
> +struct uart_8250_em485 {
> + struct timer_list start_tx_timer; /* "rs485 start tx" timer */
> + struct timer_list stop_tx_timer; /* "rs485 stop tx" timer */
> +};
> +
> /*
> * This should be used by drivers which want to register
> * their own 8250 ports without registering their own
> @@ -122,6 +127,8 @@ struct uart_8250_port {
> /* 8250 specific callbacks */
> int (*dl_read)(struct uart_8250_port *);
> void (*dl_write)(struct uart_8250_port *, int);
> +
> + struct uart_8250_em485 *em485;
> };
>
> static inline struct uart_8250_port *up_to_u8250p(struct uart_port *up)
>

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Next in thread: Peter Hurley: "Re: [PATCH v6 2/3] tty: Add software emulated RS485 support for 8250"
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