>
>
> Please note that I am not *ever* inclined to include such a wasteful change
> into my driver. Furthermore, you need to also change the code in
> aic7xxx_alloc_scb() to go along with this change (or at least verify that
> allocation sizes are still reasonable).
May be not that wasteful if AIC7XXX_MAX_SG is a changable module paramteter?
The scatter_gather/segments concept is due to a limitation of x86 platform that
only
allow DMA up to a 4K boundary.
So a virtual memory buffer is splitted into segments of real physcal addresses
of 4K each
with the first and last segments less than or equal to 4K.
>From /var/log/messages, current st parameters are:
st: bufsize 32768, wrt 30720, max buffers 4, s/g segs 16.
So the current 32k bufsize must have split into 16 segments of 4K each.
The segments list is then transfered to the adaptor card for DMA.
I know currently st/sg drivers DMA to kernel space first and then copy
to user space later.
Can current linux support direct DMA to user space? If Yes, then it is not that
resource
demanding but can Linux locked/pinned over 1MB of physical memory for a user?
>
>
> I'm in complete agreement with Gerard Roudier that any piece of hardware that
> needs this kind of change to work efficiently is in need of a good firmware
> author to get things fixed in the hardware, which is where the obvious problem
> is.
It is a tape drive versus a hard disk which may support
linked/taged/overlap/non-sequential processing.
I also agree that our firmware can be improved.
However, there are real customers that require large tape block size (1MB) which
is allowable by SCSI-II standard
and supported on other UNIX platforms and tape drive vendors.
It will be bad if a tape created by NT or Solaris cannot be read by Linux. If
you only read 32K on
a 1MB tape block, you can only get 32K data and then the next 32K scsi read will
start on the next 1MB block.
So you cannot get back that (1MB-32K) data!
Thanks again.
--
=============================
Tony Chung
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