Re: [PATCH] ieee1394: sbp2: lower block queue alignment requirement

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

On 15 Jan, Kristian Høgsberg wrote:
> On 1/14/07, Stefan Richter <stefanr@s5r6.in-berlin.de> wrote:
>> OK, thanks. I will keep the explicit setting of the mask then.
>> I actually think that everything should work just fine with 4 bytes
>> alignment, but the SBP-2 spec mentiones 8 bytes alignment for the S/G
>> tables.  So maybe device firmwares will expect it that way even though
>> IEEE 1394 doesn't impose such a restriction even on transfers with a
>> payload of a multiple of 8 bytes.
> 
> Do you have a reference to this 8 byte alignment requirement?  I
> couldn't find it browsing through the standard.

SBP-2 clause 5.2. It's also in SBP-3 even though there is no obvious
benefit for this rule.

> Also, since the SBP-2
> driver has to  convert the scatterlist to an SBP-2 page table, it can
> just allocate the page table on a 8-byte boundary if that is required.

Yes, after the discussion this occurred to me too but my energy reserves
were already too low to wrap this up.

>  The entries in the scatter table have no alignment restrictions from
> the SBP-2 side.
> 
> By the way, is there a way to ask the SCSI stack to limit the size of
> the entries in the scatterlist to 64k?  This would greatly simplify
> the conversion to SBP-2 page tables, since this is the maximum size
> these can hold.

Douglas has a very informative text on this:
http://sg.torque.net/sg/sg_io.html#mozTocId154063

| [...] since lk 2.6.16 another limit comes into play: the
| MAX_SEGMENT_SIZE define which is set to 64 KB. MAX_SEGMENT_SIZE is a
| default and can be overridden by the LLD calling
| blk_queue_max_segment_size().

Looks like we can yank out some cruft off sbp2.
-- 
Stefan Richter
-=====-=-=== ---= -====
http://arcgraph.de/sr/

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