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.
Ken
--
Kenneth Merry
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