[SeaBIOS] [PATCH] Extend tools/readserial.py to support serial port timing calibration.

[Kevin O'Connor]

On Sat, Jul 09, 2011 at 03:44:50PM -0500, Scott Duplichan wrote:
> Kevin O'Connor wrote:
> 
> ]Some serial ports have slightly different timing.  These timing
> ]variations result in less accurate boot time reporting.  So, add a
> ]calibration mechanism to the tool so that one can determine how much
> ]time a specific machine's serial port uses.
> 
> Hello Kevin,
> 
> A while back I noticed both SeaBIOS and coreboot poll for TEMP (line status
> register bit for transmitter empty) before transmitting a character. An
> alternative is to poll THRE (transmitter holding register empty) instead.
> 
> Polling TEMP forces the code to wait until the final stop bit time period is
> complete before it can load the next character. It is understandable that on
> some implementations this will cause one idle bit time to pass before
> transmission of the next character starts.
> 
> When THRE is polled, the code has a period of nearly 10 bit times where the
> next character can be loaded such that it will transmit without any unneeded
> idle time between characters.

Thanks Scott, in some of my recent tests I noticed that in the SeaBIOS
code as well.  I just sent a patch in a separate email.
Unfortunately, I didn't see an improvement in the timing accuracy even
after I made that change.  Maybe I've missed something else.

> Here is the result of a transmit test using both methods on ASRock E350M1.
> The test works by timing a 5 second transmit:
> 
>     TX baud rate test, TEMP method (SeaBIOS and coreboot)
>     selected    Measured    Percent Error
>          110         100        9
>          300         273        9
>          600         546        9
>         1200        1092        9
>         2400        2185        8
>         4800        4370        8
>         9600        8742        8
>        19200       17484        8
>        28800       26225        8
>        38400       34967        8
>        57600       52451        8
>       115200      104899        8

I'm seeing something very similar when running timing tests under
Linux.  I get ~93.9us per char instead of the 86.8us per character at
115200 baud - a difference of 8%.  This was under Linux though.

Oddly, under Linux, if I use bigger transmissions (eg, 10K) instead of
just 4000 byte transmissions I get much better timings - as little as
88.1us.  Maybe Linux is using the fifos when the transmission is large
enough.

>     TX baud rate test, THRE method
>     selected    Measured    Percent Error
>          110         110        0
>          300         300        0
>          600         600        0
>         1200        1201        0
>         2400        2403        0
>         4800        4807        0
>         9600        9615        0
>        19200       19232        0
>        28800       28848        0
>        38400       38464        0
>        57600       57697        0
>       115200      115394        0
> 
> I also noticed coreboot enables the uart fifos. This is not beneficial
> because the simple polling logic used by coreboot and SeaBIOS cannot utilize
> the transmit fifo.

Could the fifos being enabled be the cause of the slow down even after
the SeaBIOS serial polling fix?

-Kevin