From: eric@tantalus.nrl.navy.mil (Eric Youngdale) Subject: Re: SCSI Performance (Yet Date: Thu, 26 Aug 1993 16:05:50 GMT
In article <1.11636.2381.0N279333@satalink.com> john.will@satalink.com (John Will) writes:
>B >On *my* system this won't cause a problem; I only have 12MB. But on a 32MB
>B >ISA machine the driver had better check the physical address of the user's
>B >buffer and do a copy if it's over 16MB, or things will go BOOM! Not to
>B >mention the 64K boundary problem...
>
>That's only when DMA is being used, and since I have problems with the
>current kernel (99pl12) and my Adaptec 1542B when I go over 16mb on my
>system, I'd say it wouldn't get much worse. With PIO disk controllers,
>like IDE drives, the 16mb boundary doesn't exist.
I should point out that you are an isolated case. I have 20Mb and an
Adaptec, and I have no trouble. I know other people with memory in the 20-32
Mb range, and it works fine. There us code in the top level drivers to
allocate special bounce buffers that are guaranteed to be at addresses < 16Mb
for use in DMA I/O, and I have had no indications that this is not working
correctly.
If you have problems with the Adaptec when going > 16Mb, then I suggest
that you check your SIMMS, and make sure that you have them installed
correctly. If some of them have different speeds, or they are not seated
correctly this could explain part of the problem.
The 64Kb boundary problem is not relevant to scsi controllers - all of
the supported cards that use DMA use bus mastering DMA, which means that the
scsi adapters and not the motherboards drive the address lines, and hence you
do not need to worry about the 64Kb boundary problem.
-Eric
-- "When Gregor Samsa woke up one morning from unsettling dreams, he found himself changed in his bed into a lawyer."