Hi Peter,
On Thu, Mar 24, 2016 at 1:13 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
> On 03/23/2016 01:00 PM, Geert Uytterhoeven wrote:
>> On Wed, Mar 23, 2016 at 6:11 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>>> gpio-based RTS/CTS is mutually exclusive with autoRTS/autoCTS. But I'm not
>>> seeing any logic in these patches to prevent that.
>>
>> Sure. Currently nothing sets SCIx_HAVE_RTSCTS, so this can't happen yet.
>
> Anything that enables the gpios which will be in DT and beyond our control
> needs to exclude autoRTS/autoCTS within the patch that provides the
> DT functionality.
>
>> I went for GPIO-based RTS/CTS first, as we now have a framework for that,
>> and I can use it as a known-working base.
>
> Right. And I'm saying that a condition of gpio-based RTS/CTS support needs to
> contain the logic that prevents autoRTS/autoCTS, and not rely on orthogonal knobs.
AutoRTS/autoCTS cannot be enabled yet from DT.
Hence on DT platforms, the driver uses either gpio-based RTS/CTS, or no RTS/CTS
at all.
>>> autoCTS without a way to read CTS input level means that although transmission
>>> stops, the driver has no way to update port->hw_stopped so the write buffer
>>> will keep filling up until it's full @ 4k.
>>
>> SCIF allows to read CTS input level, regardless of automatic RTS/CTS is
>> enabled or not.
>
> I see that but crucial support for CRTSCTS is missing, AFAICT; namely,
> a modem status interrupt.
There's indeed no interrupt support for that, unless you use a GPIO.
> There's no way to determine when the remote re-enables CTS without polling,
> and the serial core provides no mechanism for the driver to poll CTS.
OK, so it's the responsibility of the driver to poll CTS, if there's no CTS
interrupt support. Even when autoCTS is enabled.
> So again, the driver has no way to trigger stopping/restarting tx when CTS
> changes without autoCTS. (A modem status interrupt for dCTS should call
> uart_handle_cts_change() to perform this operation).
But if the hardware has autoCTS, we should use it, right?
Hence "... without autoCTS" is never true if the hardware has autoCTS?
BTW, this means that drivers using mctrl_gpio_init_noauto(), but not
implementing autoCTS(), and not polling CTS are broken?
drivers/tty/serial/clps711x.c looks fishy: it doesn't poll CTS, but always sets
TIOCM_CTS, probably to work around this?
>>> autoRTS without a way to override RTS output complicates throttling.
>>
>> SCIF allows to override RTS output.
>
> Not the way I read the TRM: 16.3.11 Serial Port Register, bit 7 states
> "When the RTS pin is actually used as a port outputting
> the RTSDT bit value, the MCE bit in SCFCR should be
> cleared to 0."
>
> IOW, autoRTS/autoCTS needs to be disabled to get the RTS override value
> outputting at the pin.
That's also my understanding: to manually control the RTS pin, you have to
disable automatic flow control (or use pinmux to change the pin to a GPIO).
[*]
>>>> 2. If the user didn't set CRTSCTS, the RTS output pin should be controlled by
>>>> the serial core, through .set_mctrl(),
>>>
>>> Not quite.
>>>
>>> _Regardless of CRTSCTS setting_, the RTS output pin should be controlled
>>> through .set_mctrl() method. The serial core takes CRTSCTS into account on
>>> behalf of the driver when necessary.
>>
>> What does this mean exactly?
>> AFAIU, there are three states:
>> - Force RTS asserted,
(let's call this state 1)
>> - Force RTS deasserted,
(state 2)
>> - Use hardware-controlled automatic RTS,
(state 3)
>> while .set_mctrl() just provides the boolean TIOCM_RTS flag?
>
> Since there is no userspace knob for autoRTS, drivers that enable autoRTS
> with CRTSCTS should force RTS to inactive for set_mctrl(!TIOCM_RTS) and cause
> RTS to return to normal operation (autoRTS, if enabled, or RTS active otherwise)
> for set_mctrl(TIOCM_RTS).
OK, makes sense. That's also what I was guessing. So when autoRTS is enabled,
we have either state 2 or state 3.
Now, why would you want to override RTS output, and is [*] above an issue?
Is it because you want autoCTS, but not autoRTS?
Thanks for your answers, and sorry for still having that many questions...
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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