Re: [PATCH 2/10] seperate max sectors and max hw sectors

[Jens Axboe <axboe@suse.de>]

On Tue, Nov 08 2005, Mike Christie wrote:
> Jens Axboe wrote:
> >On Tue, Nov 08 2005, Mike Christie wrote:
> >
> >>Seperate max_hw_sectors and max_sectors.
> >>
> >>LLDs call blk_queue_max_hw_sectors() to set max_hw_sectors.
> >>blk_queue_max_sectors will also set max_sectors to a safe
> >>default value.
> >>
> >>blk_init_queue still calls blk_queue_max_sectors so if there
> >>are any LLDs that do not call blk_queue_max_hw_sectors() and
> >>were expecting both the max_sectors and max_hw_sectors to be
> >>255 they do not have to do anything.
> >>
> >>I was not able to test every driver I touched, but I think the
> >>only place I may have messed up is MD so some testing is needed.
> >
> >
> >->max_sectors will become less of a driver property and more of a
> >block/vm propery, so I think the best way to do this is just to have
> >blk_queue_max_sectors() set ->max_hw_sectors directly and lower
> >->max_sectors appropriately if it is lower. That also comes with the
> >bonus of not having to modify drivers.
> >
> 
> Ugggh. I did this in reverse to make the naming nicer. So I added a 
> blk_queue_max_hw_sectors() which sets ->max_sectors to some Block layer 
> default and ->max_hw_sectors to the hw limit (for SCSI this is the scsi 
> host template ->max_sectors). Is this ok? It is more clear for driver 
> writers that they are setting max_hw_sectors when calling 
> blk_queue_max_hw_sectors(). I also converted all the 
> blk_queue_max_sectors() to blk_queue_max_hw_sectors().

Driver writers need not know. They call blk_queue_max_sectors() to set
the maximum value of a request they can handle, they could not care less
what internal field in the queue is actually used for that. From their
perspective, blk_queue_max_sectors() defines their hard limit. We may
(and will) adjust the soft limit behind their back - which is ok, as
long as we honor the value they defined by calling
blk_queue_max_sectors().

-- 
Jens Axboe