On Thu, 6 Jul 2000, Kenneth D. Merry wrote:
> On Thu, Jul 06, 2000 at 15:51:34 -0400, Zhihui Zhang wrote:
> >
> > Can anyone tell me what factors determine the max DMA size (DMA counter on
> > each controller or PCI bus related)? What is the typical max DMA size for
> > a SCSI disk connected to a PCI bus? It seems to be much larger than
> > MAXPHYS (128K). If so, does it mean we are not using full potential of
> > DMA? So what's the problem if we enlarge MAXPHYS?
> >
> > Any help is appreciated.
>
> MAXPHYS determines the size of struct buf, which at the moment determines
> the maximum size of a given DMA transaction to a SCSI controller.
>
> Typical modern SCSI controllers can handle much more than MAXPHYS data
> (currently 128K) at a time. An exception is the Adaptec 154x controllers,
> which can only handle about 64K of data. (Thus the reason I/O through the
> CAM passthrough interface is limited to 64K instead of the full 128K. We
> will have that limitation until we implement a way of determining the
> maximum DMA size allowable for a given controller.)
>
> However, as Matt said, you have to be careful about increasing MAXPHYS too
> much, since you could end up allocating too much memory.
>
> I think a better approach to increasing the amount of data that can be sent
> at one time to a SCSI controller would be to implement some sort of buffer
> chaining scheme. Most SCSI controllers can do scatter/gather DMA, and CAM
> has facilities for it, so that would probably be the easiest way to go.
>
Hmm. My knowledge may be a bit dated in this matter, but as I recall the
8237 DMA controller standard on PCs only supports DMA requests up to 128k (and
then only on the upper 4 DMA channels). The low 4 DMA channels were
byte-granular and could only transfer 64k. Am I correct in assuming from this
discussion that the state of affairs is somewhat different nowadays?
Kelly
--
Kelly Yancey - [EMAIL PROTECTED] - Belmont, CA
System Administrator, eGroups.com http://www.egroups.com/
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