Re: [dm-devel] [PATCH 1/3] block: Introduce blk_set_stacking_limits function

[Mike Snitzer <snitzer redhat com>]

On Thu, May 26 2011 at 10:42pm -0400,
Martin K. Petersen <martin petersen oracle com> wrote:

> Stacking driver queue limits are typically bounded exclusively by the
> capabilities of the low level devices, not by the stacking driver
> itself.
> 
> Stacking drivers are typically permissive. A feature is supported unless
> an incompatible bottom device causes it to be disabled. Low-level
> drivers on the other hand are restrictive and want features disabled by
> default. Low-level drivers explicitly enable features as part of their
> device discovery process.
> 
> This patch introduces blk_set_stacking_limits() which has more liberal
> metrics than the default queue limits function. This allows us to
> inherit topology parameters from bottom devices without manually
> tweaking the default limits in each driver prior to calling the stacking
> function.
> 
> Since there is now a clear distinction between stacking and low-level
> devices, blk_set_default_limits() has been modified to carry the more
> conservative values that we used to manually set in
> blk_queue_make_request().
> 
> Signed-off-by: Martin K. Petersen <martin petersen oracle com>

Acked-by: Mike Snitzer <snitzer redhat com>