Date:	Wed, 2 Dec 2015 18:20:02 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Matwey V. Kornilov" <matwey@....msu.ru>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Greg KH <gregkh@...uxfoundation.org>
Cc:	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

On 11/18/2015 02:49 PM, Matwey V. Kornilov wrote:
> 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:
[...]
>>>>>> 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_.

Is there consensus then about what the semantics of unsupported RS485 delay
values are? I (or someone else) can trivially add the documentation and
fixes to the existing in-tree drivers.


>>> 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

For the moment then, can we add a UART_CAP_SW485 (not exposed to userspace)
that enables this algorithm only for h/w that supports a both-empty interrupt
mode. The probe or driver (ala 8250_omap) would opt-in and configure the h/w much
like the omap-serial driver does now (with the SCR register).

Does that seem like an acceptable compromise?

Regards,
Peter Hurley

PS - I still need to review this series for how the timer logic works esp. wrt
teardown.

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