[PATCH 15/15] tty: serial: 8250: omap: add dma support

[Sebastian Reichel]

On Tue, Sep 02, 2014 at 01:15:37PM -0700, Tony Lindgren wrote:
> * Sebastian Andrzej Siewior <bigeasy at linutronix.de> [140902 11:40]:
> > On 09/01/2014 07:47 PM, Sebastian Andrzej Siewior wrote:
> > > Comparing it with serial-omap I see the same thing: I takes approx the
> > > same amount of data until the first one is displayed. After a lot of
> > > "long" writes which wake the chip up from idle I manage to freeze both,
> > > the serial-omap driver and mine driver.
> > 
> > So after some testing:
> > - it happens with omap-serial as well. Especially after disabling the
> >   LED trigger for both LEDs.
> > 
> > - it seemed that disabling the MDR1 check whether or not we lost
> >   context made the problem appear less often but it was a trick. Even
> >   with restoring the context each time I see the same problem.
> > 
> > - it seems to be easier to trigger with the LED trigger switched off.
> >   However sometimes it works for 10 minutes, sometimes it triggers
> >   after one.
> > 
> > - I see to face two kind of "deaths":
> >   - the LED still goes on and off and the uart just does not respond
> >     even if I tell the button print something on the screen (the button
> >     also changes the frequency of the LED so I know that the button is
> >     doing something).
> >     Also from dumping the content of /proc/interrupts it seems that a
> >     wake up is made, the uart should have restored the registers.
> 
> OK yeah this is the case I was seeing too. So do you just set the
> LED triggers to none in sysfs to make it easier to reproduce?
> 
> >   - one where the system is dead and the LED does not blink anymore.
> >     Also my button is dead.
> 
> This I don't think I've seen. This could also be the errata issue on
> your earlier rev beagleboard-xm with off-idle.
>  
> > - disabling DMA makes the problem not go away.
> 
> OK
>  
> > - mdelay(25) in omap8250_lost_context() is long enough to drop the 403
> >   bytes I send in my testcase. That means I see only "good" characters.
> >   With this the box remained alive for 2h. However the uart died anyway.
> 
> I wonder if doing some TX on the uart restores it? So maybe try
> 
> $ while [ 1 ]; do date; sleep 10 done &
> 
> To have it occasionally print something in the background.

If there is a way to detect the hangup you may try to recover by
resetting the serial device using omap_hwmod_reset().

-- Sebastian
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