patch "tty: serial: OMAP: ensure FIFO levels are set correctly in non-DMA" added to tty tree

[NeilBrown]

On Fri, 3 Feb 2012 13:42:13 +0200 Grazvydas Ignotas <notasas at gmail.com> wrote:

> On Fri, Feb 3, 2012 at 11:54 AM, NeilBrown <neilb at suse.de> wrote:
> > On Thu, 2 Feb 2012 22:45:53 -0700 (MST) Paul Walmsley <paul at pwsan.com> wrote:
> >> On Fri, 3 Feb 2012, NeilBrown wrote:
> >>
> >> > then CPUIDLE enters lower states and I think it uses less power but I
> >> > sometimes lose the first char I type (that is known) but also I sometimes get
> >> > corruption on output.
> >>
> >> I don't see any output corruption on 35xx BeagleBoard, either with or
> >> without these patches.  So this is presumably a 37xx-centric problem, and
> >> unrelated to these patches, I would guess.
> >
> > Maybe it is 37xx specific.  I think this is a DM3730.
> 
> Not sure if it's the same problem but with 3530 on 3.2 with
> sleep_timeout set, I usually get first char dropped (as expected) but
> sometimes I get corrupted char instead too. Corrupt char seems to
> almost always happen if I set cpufreq to powersave, on performace it's
> almost always ok, so maybe it's some timing problem,

I see that too - I'm glad someone else does :-)

If I look at the corruption as compared to the expected character, it is
always the case some it has been shifted down a few bits, with '1's shifted
in the top.
As bytes are sent lsb first followed by stop bits which are '1', this is
completely consistent with clocking the bits in a little bit late.

I see this with baud rates of 115200 and higher, but never with lower.

My theory is that there is a delay between the falling RX line waking the
system up, and the CPU enabling the UART - whether enabling the clocks or
doing a full config, I am not sure - though I think the former.

Maybe if we could enable the UART clocks immediately after returning from the
WFI instruction we could avoid the corruption....

NeilBrown

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