Date:	Fri, 15 Jan 2016 23:20:43 +0300
From:	"Matwey V. Kornilov" <matwey@....msu.ru>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>, Jiri Slaby <jslaby@...e.com>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH v6 2/3] tty: Add software emulated RS485 support for 8250

2016-01-15 22:45 GMT+03:00 Peter Hurley <peter@...leysoftware.com>:
> On 01/15/2016 10:42 AM, Matwey V. Kornilov wrote:
>> 2016-01-15 19:14 GMT+03:00 Peter Hurley <peter@...leysoftware.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@....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
>  *
>  *      ....
>  */
>

I would like to provide init and destroy and left the rest for user.
The reason is simple.
When serial8250_em485_init is called a driver has to enable interrupt
on empty shift register and probably disable it after
serial8250_em485_destroy.
And I believe that the driver knows better how to do it
transactionally and correctly.
The 8250_omap doesn't do that just because it enables the interrupt
normally on start and always keep it enabled.

>
>
>>>> +     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)
>>>>
>>>
>>
>>
>>
>



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

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