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

[Paul Walmsley]

On Sat, 4 Feb 2012, NeilBrown wrote:

> On Fri, 3 Feb 2012 12:42:22 -0700 (MST) Paul Walmsley <paul at pwsan.com> wrote:
>
> > Case 1 is expected and is almost certainly not a bug.  As Neil mentioned 
> > it should be bps-rate dependent.  It occurs when the first character 
> > transmitted to the OMAP wakes the chip up via I/O ring/chain wakeup.
> > I/O ring/chain wakeup is driven by a 32KiHz clock and is therefore 
> > relatively high-latency.  So this could easily cause the first character 
> > or first few characters to be lost or corrupted, depending on the exact 
> > sequence of events, the low power state that the chip was in, etc.
> 
> A 32KiHz cycles every 30mSec.
> At 115200 bps, there is one bit every 8.7 microseconds.
> 
> When I type "return" - which looks like 0101100001111... on the wire,
> I see '0xC3' which looks like 011000011111... on the wire.
> So we lost exactly 2 bits, or a delay around 17 microseconds.
> 
> I find it hard to reconcile that delay with the cause being a 32KiHZ clock.

If you're not disabling the HF clock oscillator via the AUTOEXTCLKMODE 
bits, the wakeup logic may be getting clocked by the sys_clk, which is 
quite a bit faster.  That might explain the corruption patterns you're 
seeing.


- Paul