[openib-general] [RFC] IB_AT_MOST

[Fab Tillier]

Hi Michael,

> From: Michael S. Tsirkin [mailto:mst at mellanox.co.il]
> Sent: Friday, December 16, 2005 5:58 AM
> 
> Hi!
> I recently noted that some middleware seems to use the "as much
> as possible" approach, for example, using maximum possible value
> for max_rd_atomic or other fields, in create/modify qp.
> 
> An obvious thing could be to perform query_device and use max.
> values from there. However, it turns out that hardware max supported
> values might not be easy to express in terms of a single constant.
> Consider for example the max number of s/g entries supported per
> WQE: mellanox HCAs support different number of these for RC and UD
> QPs. So whatever single number query device reports, using it will
> never achieve what the user wants for all QP types.
> 
> Rather than extending the device query for all thinkable hardware
> weirdness, I'd like to propose, instead, the following API extension
> (below): passing a negative value in e.g. qp attribute would have the
> meaning: let hardware use at most the specified value.
> This, as opposed to the usual "at least the specified value" meaning
> for positive values.
> 
> How does the following work, for an API? Please comment.

I don't understand the IB_AT_MOST macro.  If someone uses IB_AT_MOST( 1 ) and
the hardware supports 4, they will get 4, which is definitely not "at most 1".

I would rename it to IB_MAX, and define it a -1 or something like that.

- Fab