AHC_ALLOW_MEMIO 5.2.1

[Scott Long]

Andy Farkas wrote:

> On Wed, 28 Apr 2004, J.D. Bronson wrote:
> 
> 
>>What happened to this kernel option on 5.2.1?
>>
>>AHC_ALLOW_MEMIO
>>
>>
>>It does not seem to be in any file at all anymore?
>>
>>  -JDB
> 
> 
> Its in /sys/conf/NOTES :)
> 
> The generated file opt_aic7xxx.h will contain it.
> 
> I sometimes enable this option in my kernels. Whether it does anything or
> not (performance-wise or other), I don't know.
> 

On x86, MEMIO is generally faster and causes the CPU to spin less than
IOPORT cycles do.  It can result in a minor but measurable speed
difference, though the effects of HyperThreading, when in use, could
offset the benefits.  In any case, the reason that this option exists
is to work around motherboards that incorrectly do write-combining on
MEMIO registers, something that most ahc/ahd chips do not support nor
handle well.  We added a somewhat sophisticated runtime test for this
to the driver last year, so there really isn't a reason to not enable
the option.  If you do and the test detects problems, it will
automatically throttle back to IOPORT.  We should probably just remove
the option all-together, and just use the sysctl/tunable as a backup in
case problems develope.

Scott