(in)appropriate uses for MAXBSIZE

[Scott Long]

On Apr 9, 2010, at 7:53 AM, Rick Macklem wrote:
> 
>> - What are appropriate and inappropriate uses for MAXBSIZE given the questions
>> above?  In other words, what would immediately break were we to simplemindedly
>> bump MAXBSIZE value.
>> 
>> I have no answers but have some observations.
>> I strongly believe that all uses of MAXBSIZE in sys/dev/ are inappropriate.  For
>> me it's inconceivable that a hardware driver would need to know maximum size of
>> a filesystem block.
> 
> I think you are on the right track here. It seems to me that MAXBSIZE
> (or some new constant instead of it) should define the maximum block
> size handled by the buffer cache. I have no idea what the implications
> of increasing it are, but it sounds like some drivers will need to be
> fixed, as you've noted.

Storage drivers are insulated from the details of MAXBSIZE by GEOM honoring
the driver's advertised max-i/o-size attribute.  What I see when I grep through the
sources are mostly uses in busdma attributes, which themselves probably came
via cut-n-paste from prior drivers.  I can't come up with any explanation for that
which makes good design sense, so I'll agree that storage drivers shouldn't
reference MAXBSIZE.

Scott