fast ethernet driver MII phy serial clock rates

[Bruce Evans]

On Sun, 25 Apr 2004, David Burns wrote:

> Mike Silbersack wrote:
> >
> > On Sat, 24 Apr 2004, David Burns wrote:
> >>NB this assumes that a DELAY(1) is really a delay of 1?s! Which I don't
> >>think it is ... :-(
> >
> > Correct, DELAY takes far longer than it should.

Actually, it takes at least as long as it should (normally a few
microseconds longer than the specified delay, but hundreds or thousands
of microseconds longer if it is interrupted).  The bus ISA accesses in
it can also be delayed by (PCI) bus activity (I've measured 170 usec for
the 3 accesses in getit() which normally take 3-4 usec).

> > If you're really interested in fixing the problem and not inadvertantly
> > breaking older cards, what you should do is implement a nanodelay function
> > that actually delays for the time it's supposed to and then delay the
> > rated amount.  Removing all delays will probably break something
> > somewhere.
>
> We could probably build a driver specific nanodelay function based on
> dummy PCI operations. Some will say this sucks but then I'd argue it's
> better than the current DELAY implementation.

No, it would be considerably worse.  DELAY() has a poor resolution
because non-dummy ISA operations that it uses to read the time are
slow.  Dummy PCI operations aren't much faster, depending on which
address they are at.  They would be at least 3 times faster in practice
since the current implementation of DELAY() needs 3 ISA operations.
DELAY() could probably use only the low byte of an unlatched counter
if its efficiency were important.  I think it is unimportant, since
only broken code uses busy-wait.

Anyway, you won't get near nansoseconds reasolution or PCI clock
resolution (30 nsec) using PCI i/o instructions.  rdtsc on i386's and
delay loops on all machines can easily do better provided the CPU
doesn't get throttled.

> Of course just sending one bit of data on the MDIO will take us about
> 600 nanoseconds - resulting in a 1.6MHz clock.

Except some machines add lots of wait states.  I have a PCI card which
can usually be accessed in 467 nsec (write) and 150 nsec (read) on one
machine, but on a newer machine which is otherwise 6 times faster but
appears to have a slow PCI bugs (ASUS A7N8X-E), the access times
increase to 943 nsec (write) and 290 nsec (read).

Bruce