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

Peter Hurley
2016-01-15T19:45:19Z

On 01/15/2016 10:42 AM, Matwey V. Kornilov wrote:
> 2016-01-15 19:14 GMT+03:00 Peter Hurley <peter@hurleysoftware.com>:
>> 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@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).
>
> This is required to call del_timer_sync(&p->em485->start_tx_timer);
> from __stop_tx_rs485

I know; that doesn't mean it's ok.

>>> + 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?
>
> Both start_tx and rs485_config (which calls destroy) are wrapped with
> port->lock in serial_core.c

Ahh, missed that.

Maybe it would be better simply to implement the config_rs485()
generically, and just call it from the omap_8250 config_rs485().

And put a note about the locking in a function comment header

/**
* serial8250_config_em485() - rs485 config helper
*
* ....
*/

>>> + 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.
>
> I use double __ for consistency with __start_tx. Now I have:
>
> if (up->em485)
> __start_tx_rs485(port);
> else
> __start_tx(port);

But __start_tx() is labelled that way to differentiate it from being identified
as the start_tx() handler (which is serial8250_start_tx()). IOW, contributors
unfamiliar with the 8250 driver itself won't become confused when grepping
for start_tx (or at least the idea is to minimize that confusion).

start_tx_rs485() doesn't need differentiation, so doesn't require the
double __ leader.

FWIW, this is consistent and typical elsewhere in the kernel.

>>> +{
>>> + 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 (!(....) !=
>> !(....)) {
>>
>
> I would like to keep the double negation, in my opinion it is more
> clear to the reader and I believe that the compiler is able to
> optimize it.
>
>>> + 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?
>>
>
> Why not?

The expected yardstick for inline is some demonstrable speed improvement;
otherwise, size is favored.

And __stop_tx() is already inlined in 3 places, which really doesn't
need inlining either -- a call/ret is nothing compared to device i/o.

Regards,
Peter Hurley

>>> +{
>>> + 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)
>>>
>>
>
>
>