Date:	Wed, 18 Nov 2015 22:49:57 +0300
From:	"Matwey V. Kornilov" <matwey@....msu.ru>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.com,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH v3 2/5] tty: Introduce SER_RS485_SOFTWARE read-only flag
 for struct serial_rs485

2015-11-18 22:39 GMT+03:00 Matwey V. Kornilov <matwey@....msu.ru>:
> 2015-11-18 21:33 GMT+03:00 Peter Hurley <peter@...leysoftware.com>:
>> On 11/17/2015 03:20 AM, Matwey V. Kornilov wrote:
>>> 2015-11-16 22:18 GMT+03:00 Peter Hurley <peter@...leysoftware.com>:
>>>> On 11/14/2015 10:25 AM, One Thousand Gnomes wrote:
>>>>>> I specifically asked for it.
>>>>>>
>>>>>> I can think of 2 reasons that userspace wants to know:
>>>>>> 1. Because the characteristics of the software emulation are unacceptable so
>>>>>>    the application wants to terminate w/error rather than continue.
>>>>>
>>>>> But that could equally be true of hardware.
>>>>
>>>> I had this exact same thought, but concluded that it argues for a way
>>>> to select the software implementation even when h/w supports RS485.
>>>>
>>>>> In fact your software
>>>>> emulation is going to behave vastly better than many of the hardware ones.
>>>>>
>>>>>> 2. Because userspace will use different values for h/w vs. s/w. For example,
>>>>>>    right now, the emulation will raise/lower RTS prematurely when tx ends if
>>>>>>    the rts-after-send timer is 0.
>>>>>
>>>>> That's a bug then. It should be fixed as part of the merge or future
>>>>> patches - if they are not providing that emulation then they ought to do
>>>>> so and at least adjust the timing based on the baud rate so you don't
>>>>> have to spin polling the 16x50 uart to check the last bit fell out of the
>>>>> register.
>>>>
>>>> I suppose the timer(s) could be fudged and then TEMT polled (or polled every
>>>> char baud cycles). But I don't see how this will behave better than a h/w
>>>> implementation; the granularity of the jiffy clock alone will guarantee
>>>> sub-optimal turnaround, even at 9600.
>>>>
>>>>> I'd have no problem with an API that was about asking what features are
>>>>> available : both hardware and software - but the software flag seems to
>>>>> make no sense at all. Software doesn't imply anything about quality or
>>>>> feature set. If there is something the emulation cannot support then
>>>>> there should be a flag indicating that feature is not supported, not a
>>>>> flag saying software (which means nothing - as it may be supported in
>>>>> future, or may differ by uart etc).
>>>>
>>>> Fair enough.
>>>>
>>>>> It's also not "easy to drop". If it ever goes in we are stuck with a
>>>>> pointless impossible to correctly set flag for all eternity.
>>>>>
>>>>> Please explain the correct setting for this flag when a device driver
>>>>> uses hardware or software or a mix according to what the silicon is
>>>>> capable of and what values are requested ? How will an application use the
>>>>> flag meaningfully. Please explain what will happen if someone discovers a
>>>>> silicon bug and in a future 4.x release turns an implementation from
>>>>> hardware to software - will they have to lie about the flag to avoid
>>>>> breaking their application code - that strikes me as a bad thing.
>>>>
>>>> The existing driver behavior is already significantly variant and needs
>>>> to be converged, which shouldn't be too difficult. Here's a quick summary:
>>>>
>>>> mcfuart         ignores delay values, delays unsupported
>>>> imx             clamps delay values to 0, delays unsupported
>>>> atmel           only delay_rts_after_send used; delay_rts_before_send does nothing
>>>> 8250_fintek     clamps delay values to 1, unclear if h/w delay is msecs
>>>> omap-serial*    software emulation (but tx empty polling not reqd)
>>>> lpc18xx-uart    clamps delay_rts_before_send to 0, unsupported
>>>>                 clamps delay_rts_after_send to max h/w value
>>>> max310x         returns -ERANGE if either delay value > h/w support (15 msecs)
>>>> sc16is7xx*      returns -EINVAL if delay_rts_after_send is set
>>>> crisv10*        clamps delay_rts_before_send to 1000 msecs
>>>>                 ignores delays_rts_after_send (after dma is delayed by 2 * chars)
>>>> * implements delay(s) in software
>>>>
>>>> The omap-serial emulation should not have been merged in its current form.
>>>>
>>>> IMO the proper driver behavior should be clamp to h/w limit so an application
>>>> can determine the maximum delay supported. If a delay is unsupported, it should
>>>> be clamped to 0. The application should check the RS485 settings returned by
>>>> TIOCSRS485 to determine how the driver set them.
>>>> [ Documentation/serial/serial-rs485.txt should suggest/model this action ]
>>>
>>> But the similar could be true for minimal supported delay. If user
>>> requires delay which is less than lower bound, the delay is raised to
>>> the lower bound. If user requires delay which is greater than upper
>>> bound, the delay is set to the upper bound. Then software
>>> implementation could use (tx fifo size / baudrate) as lower bound for
>>> delay_after_send.
>>
>> From the application point-of-view (really the only relevant semantics),
>> delay_dts_after_send refers to the number of milliseconds to delay the
>> toggle of RTS after the last bit has been _transmitted_.
>>
>> I agree with Alan that any adjustment to the delay to adhere to that
>> meaning needs to be transparent to user-space.
>>
>>
>>>> Are TIOCGRS485 and TIOCSRS485 documented in tty_ioctl man page? (I haven't
>>>> updated my man pages in a while)
>>>>
>>>> As far as software vs. hardware and a query api, what I care about is
>>>> conveying to userspace whether the implementation will be adequate for purpose,
>>>> with the main issue being the true delay from actual EOT to RTS toggle
>>>> when delay_after_rts_send == 0.
>>>
>>> Or I just can internally add (tx fifo size / baudrate) to the user
>>> supplied value to take care of the bytes in tx fifo.
>>
>> Yes. Or poll every jiffy.
>>
>> But either will be far too coarse for many users; a delay_rts_after_send of
>> 0 could still produce multi- _msec_ delays when the application expects
>> turnaround of ~1 char time. At a leisurely 19200 baud, that's ~520us which will
>> not be possible with this emulation.
>
> If we want real-time, then we have to spin on LSR waiting for TXSRE be 1.
>
>>
>> A couple of possibilities for improving the emulation are:
>> 1) Optionally using an HR timer for sub-jiffy turnaround.
>> 2) Only supporting 8250-based hardware that can be set to interrupt when
>>    both tx fifo and transmitter shift register are empty.
>
> This is to support the RS485 API with already exists in omap_serrial,
> but not in 8250_omap. And OMAP does not support tx line interrupt in
> UART mode. So the latter is not an option.

Oh, I am sorry, it does support. There is "Supplementary Control
Register" described in 19.5.1.39

>
>>
>> But regardless, the driver should still advertise whether direction control
>> is realtime or not (ie., software or not).
>>
>> Regards,
>> Peter Hurley
>>
>>
>>>> Since that delay is unbounded with software methods, I thought it made sense to
>>>> indicate that with a read-only bit. Naming it something else is fine too;
>>>> SER_RS485_NOT_REALTIME_EOT?
>>>>
>>>> A more comprehensive approach might be to add a capabilities word
>>>> to struct serial_rs485 which would allow the driver to report what
>>>> it supports; eg. realtime turnaround or not, etc. (Not sure if extending
>>>> struct serial_rs485 is really possible; the serial core hasn't been
>>>> clearing padding on the driver's behalf).
>>>>
>>>>> At the very least the above should be clearly explained in the
>>>>> documentation and patch covering notes - and if nobody can explain those
>>>>> then IMHO the flag is broken.
>>>>
>>>> Yep.
>>>>
>>>> Regards,
>>>> Peter Hurley
>>
>
>
>
> --
> 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
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