On Sat, Jun 27 2009, Neil Brown wrote: > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
