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

Peter Hurley
2016-01-15T16:40:53Z

On 01/15/2016 08:14 AM, Peter Hurley wrote:
> 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.

More comments below.

>> Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
>> ---
>> 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;

Actually, just refactor the mod_timer() below into __start_tx_rs485();
'delay' and assignment-in-conditional goes away.

>> +
>> + 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)
>>
>