On Sat, Jun 27 2009, Neil Brown wrote:

quoted

There's no such thing as first or second class block devices. The fact
that drivers using ->make_request_fn directly do not utilize the full
scope of the queue isn't a very interesting fact, imho.

 Your phrase "drivers using ->make_request_fn directly" seems to
 suggest you are looking at things very differently to me.

 From my perspective, all drivers use ->make_request_fn equally.
 Some set it to "__make_request", some to "md_make_request", others to
 "dm_request" or "loop_make_request" etc.

Neil, will you please stop these silly games. Stop trying to invent
differences based on interpretations of what you read into my replies.

 Each of these different drivers need some private storage.
 __make_request uses struct request_queue
 md_make_request uses struct mddev_s
 dm_request uses struct mapped_device
 loop_make_request uses struct loop_device
  etc

 These structures are all attached to gendisk one way or another.

 Of these examples, the first three have an extra level.  They are
 intermediaries or "midlayers" for multiple drivers and perform some
 processing before passing the request down.
 __make_request provides this for ide and scsi (etc) via ->request_fn and
    ->queuedata in struct request_queue (and other fields).
 md_make_request provides this for raid1 and raid5 (etc) via
    ->pers->make_request and  ->private is struct mddev_s (and other
    fields). 
 dm_request provides this for crypt and multipath (etc) via
    ->map->targets[]->type->map and ->map->targets[]->private (and
    other fields).

Nothing - I repeat nothing - stops md/dm from removing that layer. It's
a layer they imposed themselves based on the design they chose to
implement internally. It has NOTHING to do with how the block layer is
designed. If md raid1 assigned raid1_dev (or whatever raid1 uses a its
device identifier structure) to ->queuedata, and had an mddev_s in its
raid1 structure, that would be a perfectly viable design as well.

Loop does that. md/dm have their own internal layering, if anything is a
"midlayer" (to keep to the apparent theme of design patterns), it's the
code md and dm bits.

 Looked at from this perspective, the fact that some drivers 'do not
 utilise the full scope of the queue' certainly isn't the interesting
 point.  The interesting point is that they have to use parts of the
 queue at all.

 And from this perspective, __make_request is a class above everything
 else.  __make_request gets a dedicate field in gendisk (->queue) and
 every driver has to provide a queue.  Other (lower class) drivers get
 to share gendisk->private_date and/or gendisk->queue->queuedata.

That's just utter nonsense.

-- 
Jens Axboe