Subject: Re: com rumblings...
To: Garrett D'Amore <email@example.com>
From: Allen Briggs <firstname.lastname@example.org>
Date: 06/14/2006 09:29:33
On Wed, Jun 14, 2006 at 12:04:44AM -0700, Garrett D'Amore wrote:
> I'm in the process of hacking up a com.c that has an #ifdef
> COM_USE_REGMAP that does register remapping.
Is this primarily for console use, or something that you're planning
to handle more constant "high-speed" traffic?
I've often thought that we could use a simpler com(4) for console
use that could be separate from the com(4) that's been tweaked for
more performance on slower primarily-x86 boxes.
No clue on COM_HAYESP. Sorry.
On Wed, Jun 14, 2006 at 02:52:56AM -0700, Garrett D'Amore wrote:
> I'm actually starting to wonder whether about the costs vs. benefits of
> using a register map all the time.
I think the only way to justify this really acceptably is with
numbers. I can see a bus_space_read_1(t,h,reg) being basically
in some implmentations (pseudo-asm ;-), given an ignored bus_space_tag,
a bus_space_handle already loaded in register rH and 'reg' a constant.
You're talking about 'reg' being loaded from an array, so the pseudo-asm
becomes something like:
which requires one more temp register (offset), one more function-level
register (array base register, one more memory fetch (as you note,
likely cached after the first reference), and one more instruction.
The instruction cost is paid for each bus_space call, and the others
are kind of amortized over the function.
So it doesn't seem like it's a no-brainer that it's effectively zero-
cost. Nor does it seem given that it's more expensive.
> Its a real pain in neck to have the #ifdef logic that conditionally
> enables an indirect register map.
Can it be done by wrapping bus_space access in another layer of
preprocessor macro? Similar to how some register accesses are
handled in other drivers (e.g., CSR_READ() in if_wm.c).
> On systems that have a reasonable cache and have performance
> considerations for serial IO, the array is likely to be cached.
I'm not sure that we're talking about systems with reasonable cache.
What's "reasonable"? ;-)
> Does anyone strongly feel that an extra indirect reference on com(4)
> register accesses is unacceptable on some architectures?
I'm sure someone does, unless you have numbers to back up your
argument. ;-) I don't feel really strongly about it, but I haven't
used serial ports for data transfer for some years now...
Use NetBSD! http://www.NetBSD.org/