PCI write oddity

Daniel O'Connor doconnor at gsoft.com.au
Wed Sep 30 11:19:49 UTC 2009

I have a custom device driver for a PCI RS485 card - it has a 16550 and 
a PLX PCI bridge on it and I recently found something quite odd.

We used to run the link at 9600 but with some more recent modules we've 
decided to up it to 115200, so I changed the baud rate easily enough.. 

However while the bit rate is correct each byte going out has a 1 
millisecond delay(!). What is really odd is that when I added some 
debugging to log how long each write to the FIFO register took I 
discovered it was 1 millisecond.

Reads from the same register take ~5 usec which seems more like it.

Does anyone have any idea why it would be so damn slow?

Daniel O'Connor software and network engineer
for Genesis Software - http://www.gsoft.com.au
"The nice thing about standards is that there
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