Re: PICs on Linux

[Ryan Underwood <nemesis-lists@icequake.net>]

Hi Bart,

On Mon, Nov 08, 2004 at 02:21:16PM +1300, Bart Oldeman wrote:
> 
> Speed shouldn't be such a big deal: according to
> http://www.ussg.iu.edu/hypermail/linux/kernel/0303.0/0978.html
> you can only do about 300000 inb's and outb's per second
> anyway.

Speed betweeen a virtualized and non-virtualized parallel port I/O can
be a big deal in my experience.  I have at least one device that gives
up when parallel ports are passed through in non-fast mode.  It operates
with tristate data lines and one strobe pin.  I'm not sure what the
problem is, only that the port must be fast for it to work.  On a
(much?) faster machine, probably the port virtualization would not
affect it.  But requiring the use of ppdev doesn't make sense in this
case.

By the way, I have benchmarked ppdev vs raw portio performance on this
device with a linux program.  The difference is quite significant (ppdev
only performs around 60% of the rawio speed IIRC).  On a better device
which uses interrupt or DMA driven I/O, the difference would not be that
significant, but on this device, transfer rate depends on how fast the
registers of parallel port can be "twiddled", so it is greatly affected
by any delay therein.

-- 
Ryan Underwood, <nemesis@icequake.net>