Date:	Fri, 15 Jan 2016 11:45:19 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Matwey V. Kornilov" <matwey@....msu.ru>
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

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
 *
 *	....
 */



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

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