W dniu 2014-11-07 o 09:03, Marek Vasut pisze:
On Friday, November 07, 2014 at 02:34:31 AM, Huang Shijie wrote:
why change them to gpio?
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.
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.
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.
So I didn't test the patch with the DMA - I've just disabled it.
The question is different. The driver supports the cases for RTS/CTS:
1) both RTS/CTS assigned to hardware AUART (pinmux configured by DT,
DT sets fsl,uart-has-rtscts)
a) settermios() sets CRTSCTS: DMA enabled, auto RTS/CTS is enabled
b) settermios() doesn't set CRTSCTS: DMA disabled, RTS/CTS
controlled by get/set_mctrl()
2) both RTS/CTS assigned to hardware AUART (pinmux configured by DT,
DT doesn't set fsl,uart-has-rtscts)
a) settermios() sets CRTSCTS: DMA disabled, auto RTS/CTS is enabled
b) settermios() doesn't set CRTSCTS: DMA disabled, RTS/CTS
controlled by get/set_mctrl()
and after the patch it is more complex (because of mixed cases):
3) both RTS/CTS assigned to gpios (DT sets or not fsl,uart-has-rtscts):
the patch cancels fsl,uart-has-rtscts flag to disable DMA support,
settermios() sets or not CRTSCTS: DMA disabled, RTS/CTS controlled
by get/set_mctrl(),
both lines by gpios. In addition:
a) settermios() doesn't set CRTSCTS: RTS of hardware AUART is also
controlled but
the case assumes it is not selected by pinmux in DT
b) settermios() sets CRTSCTS: auto RTS of hardware AUART is also
enabled but
the case assumes it is not selected by pinmux in DT
4) RTS assigned to hardware AUART (pinmux configured by DT,
fsl,uart-has-rtscts set or not),
CTS assigned to gpio: the patch cancels fsl,uart-has-rtscts flag to
disable DMA support,
a) settermios() doesn't set CRTSCTS: DMA disabled, CTS (as gpio)
read by get_mctrl(),
RTS of hardware AUART controlled by set_mctrl()
b) settermios() sets CRTSCTS: DMA disabled, CTS (as gpio) read by
get_mctrl(),
auto RTS is enabled
So case 4) is exactly 3) case with just different pinmux
configuration and the patch
threats 3) and 4) as the same case.
5) CTS assigned to hardware AUART (pinmux configured by DT,
fsl,uart-has-rtscts set or not),
RTS assigned to gpio: the patch cancels fsl,uart-has-rtscts flag to
disable DMA support,
a) settermios() doesn't set CRTSCTS: DMA disabled, CTS of hardware
AUART read by
get_mctrl(), RTS (as gpio) controlled by set_mctrl(), in
addition RTS of hardware AUART
is also controlled but the case assumes it is not selected by
pinmux in DT
b) settermios() sets CRTSCTS: DMA disabled, auto RTS/CTS is enabled but
RTS is controlled by set_mctrl() as gpio because the case
assumes it is not selected
by pinmux in DT
It is not nice description but I hadn't idea how to write it more clear.
The mixed cases 4) and 5) are likely not so useful but possible and not
so expensive
to support.
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
best regards
Janusz
HTH
[1] http://comments.gmane.org/gmane.linux.serial/6770
[2] http://article.gmane.org/gmane.linux.serial/3619/
Best regards,
Marek Vasut
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