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

Matwey V. Kornilov
2016-01-15T21:16:53Z

2016-01-15 23:01 GMT+03:00 Matwey V. Kornilov <matwey@sai.msu.ru>:
> 2016-01-15 22:45 GMT+03:00 Peter Hurley <peter@hurleysoftware.com>:
>> 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.
>>
>
> What do you suggest? Run __stop_tx as a tasklet? I am not sure whether
> it introduces races or not.

Would it be fine to use workqueues instead of timers? I mean
schedule_delayed_work and cancel_delayed_work_sync.
They use same timers with TIMER_IRQSAFE under the hood.
Or it is better to allocate separate work queue in order to achieve
better latency than shared system wq can provide?

>
>>
>>>>> + 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.
>>
>
> Ok then, probably I am biased with my C++ experience and I am used to
> think that compiler considers `inline` only as a hint.
>
>>
>> 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)
>>>>>
>>>>
>>>
>>>
>>>
>>
>
>
>
> --
> With best regards,
> Matwey V. Kornilov.
> Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
> 119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382

--
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382