[PATCH v4 2/4] serial: mxs-auart: use mctrl_gpio helpers for handling modem signals

Janusz Użycki j.uzycki at elproma.com.pl
Fri Nov 7 08:29:33 PST 2014

W dniu 2014-11-07 o 15:48, Marek Vasut pisze:
> On Friday, November 07, 2014 at 02:23:23 PM, Janusz Użycki wrote:
> [...]
>>>> Hardware RTS/CTS lines can be occupied by RX/TX of other AUART port
>>>> in order to obtain as much uarts as possible using i.mx283.
>>>> Therefore gpios can be used for "hardware" flow control.
>>> Your logic is outright flawed here, the first sentence doesn't implicate
>>> the second sentence. In fact, those two are completely unrelated.
>> I didn't write MUST but CAN. There is a choice. Is flexibility of the
>> driver disadvantage?
> If the flexibility brings in known problems, then yes, it is a problem. Not
> because of the flexibility, but because it brings in bugs.

New features new bugs :) Does it mean to stop development?

>>>>>>      If we change them to gpio.  Could the DMA still
>>>>>> works fine?
>>>>>> did you test the DMA with this patch?
>>>>>> Add Marek for this patch too.
>>>>> I didn't look too deep into the patch, so here's just my experience:
>>>>> 1) The AUART block signals and GPIO block signals are not sychronised
>>>>> using the
>>>>>       same clock. Therefore, the latency between toggling of the AUART
>>>>>       lines and the GPIO-driven pins will not be deterministic and will
>>>>>       vary. There might be a way to approximate that, but that's
>>>>>       definitelly not a reliable solution.
>>>>>       This is very bad for example if you drive RS485 DIR line with the
>>>>>       RTS pin as a GPIO ; the RTS pin will toggle at non-deterministic
>>>>>       time compared to the end of UART transmission. This will trigger
>>>>>       bit-loss on the RS485 line and you just don't want that.
>>>>> 2) Speaking of RS485, there's [1] and [2]. which I believe apply to any
>>>>> combo
>>>>>       of UART+GPIO toggling.
>>>>> So I hate to bring the bad news , but UART+GPIO combo toggling is
>>>>> really a bad bad idea.
>>>> Unfortunately if hardware is limited there is no choice and UART+GPIO is
>>>> necessary.
>>> You will run into timing problems (see above).
>> A lot of 8250-compatible devices has no hardware flow control and in
>> most cases
>> they works and it is enough even for 115200 speed if CTS is handled by irq.
>> So it depends on your needs.
> I presume that in such a case , the 8250 still handles the CTS line, not some
> external GPIO block, yes ?

Yes. However mxs includes both GPIO and AUART. Clocks differs but it is 
still the same silicon.
External GPIO block is extreme example highly not recommended here.

>>> What you're proposing here is a workaround for broken hardware, which was
>>> proven to be a bad idea and NAK'd already multiple times in the past
>>> (please see the links I posted in my last email).
>> It is not broken  hardware - rather limited to lower speeds but still
>> very useful solution.
> What exact "lower speed" are you talking about here and why ?

For example not more than 115200 but it depends on CPU load of course, 
FIFO size
and device on the opposite site. RTS/CTS via GPIO require to know the limit
in an application.

I googled even so exotic thing like:
"8250: add support for DTR/DSR hardware flow control"

>>>> Your experience confirms the discussion [3] with Russell King. DMA
>>>> should be disabled and
>>>> the patch disables DMA support in mxs_auart_init_gpios() if RTS or CTS
>>>> line is set as gpio.
>>> DMA has nothing to do with those problems here. DMA can be safely ignored
>>> for the purpose of the discussion altogether.
>> When gpios are used for RTS/CTS DMA is not used. However DMA is related
>> due to the driver
>> and "fsl,uart-has-rtscts". If you look into code of the driver you
>> should agree.
> This makes me believe that the DMA introduces too many timing fluctuations,
> so it's really not possible for you to keep toggling the GPIOs such that the
> bus would work. Is that the case ?

Yes, for RTS/CTS based on gpio DMA is not used. They are just toggled.
So you probably misunderstood me.

> [...]
>>>> Now the question: "fsl,uart-has-rtscts" name seems to be misleading now,
>>>> do you agree? It rather should include "dma" word in the name. Any
>>>> suggestion?
>>>> [3] http://thread.gmane.org/gmane.linux.serial/16069/focus=16077
>>> The best suggestion I can give you is to fix your hardware early, before
>>> you run into nasty deep s.....tuff. These workarounds do not work and
>>> they will bit you later on, when it's hard to fix the hardware anymore.
>> The speed is limited but why don't you accept SW-HW mixed solutions?
> Did you read up on the RS485 timing problems and why that solution was never
> accepted for any driver ? I believe the threads explained that quite clearly.

Example of RS485 implementation where RTS is toggled by software:

>> Exactly the same method is accepted for 8250.
> Can you point out this code please ?

If 8250 doesn't support auto flow control RTS/CTS are also toggled by 
uart_trottle(), uart_untrothle(), uart_handle_cts_change():
Detected by irq (ms enabled when CRTSCTS) CTS change stops tx because
UPF_HARD_FLOW flag is not set by the mxs-auart driver.
Of course timing problem exists but in many cases it is not critical -
the toggle method was implemented many years ago and it seems to work.

>> It is good to have choice than not. We can comment that speed is limited
>> for gpio-based hardware flow control. So please, rethink again.
> [...]

The only problem for me is misleading "fsl,uart-has-rtscts" name because 
the flag
only enables DMA if CRTSCTS is set and hardware flow control of AUART 
block is used.

best regards

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