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:
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/drivers/tty/serial/omap-serial.c?id=4a0ac0f55b18dc297a87a85417fcf068658bf103
https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/drivers/tty/serial/omap-serial.c
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
software,
uart_trottle(), uart_untrothle(), uart_handle_cts_change():
http://lxr.free-electrons.com/source/drivers/tty/serial/serial_core.c#L635
http://lxr.free-electrons.com/source/drivers/tty/serial/serial_core.c#L2796
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
Janusz
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