Re: [PATCH v3 2/5] tty: Introduce SER_RS485_SOFTWARE read-only flag for struct serial_rs485

From: Matwey V. Kornilov
Date: Wed Nov 18 2015 - 14:40:08 EST

2015-11-18 21:33 GMT+03:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
> On 11/17/2015 03:20 AM, Matwey V. Kornilov wrote:
>> 2015-11-16 22:18 GMT+03:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
>>> 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.

> 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;
>>> 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
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