Re: mxs auart timeout waiting for transmission with hw flow control

[Hector Palacios <hector.palacios@digi.com>]

Hello,

On 09/30/2013 06:41 PM, Hector Palacios wrote:
> Hi Uwe,
>
> On 09/30/2013 06:20 PM, Uwe Kleine-König wrote:
>> Hi Hector,
>>
>> On Mon, Sep 30, 2013 at 06:07:19PM +0200, Hector Palacios wrote:
>>> I saw your patch at
>>> http://www.spinics.net/lists/stable/msg12952.html and I believe I'm
>>> having a similar issue when not using the port as console.
>>>
>>> With hardware flow control enabled transmission fails at low
>>> baudrates (9600,38400). Apparently the transmitter closes the port
>> Which kernel are you using? I think there were some other fixes
>> concering flow control some time ago.
>
> v3.10. The driver is pretty close to current status.
>
>>> before the data has been really shifted out (or even transferred by
>>> the DMA). While the console functions check the status of
>>> AUART_STAT_BUSY, the standard functions don't, and the AUART is busy
>>> when shutdown is called at low baudrates.
>>>
>>> I also see the function uart_wait_until_sent() of serial_core.c
>>> getting out with timeout expired. At 9600 baud, this function waits
>>> 2 or 4 jiffies. Increasing (a lot) the timeout of this function
>>> solves the problem. I monitored the time it takes for TX to be empty
>>> and it resulted in sometimes around 200 jiffies (@9600).

The problem is that the shutdown callback does not wait for the DMA buffer to flush.
I will submit a patch that fixes it.

>> Again with flow control and another imx28 on the opposite side? Does it
>> work without flow control? Is CTS busy for some time? In that case a
>> simple timeout isn't appropriate. Using console and flow control on the
>> same port might be hairy, not sure though.
>
> Yes, two imx28 devices with hw flow control enabled. Without flow control it works.

The reason why it works without hardware flow control is that in this driver the DMA 
is only activated when hardware flow control is enabled.

By the way, where did the spinlocks go in this driver?
Ocasionally I saw unhandled NULL pointer exception in uart_write_wakeup(). Maybe my 
patch will make it even hardware to hit, but still...

Best regards,
--
Hector Palacios
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