Re: floppy driver 2.6.3 question

[Jens Axboe <axboe@suse.de>]

On Thu, Mar 18 2004, Peter T. Breuer wrote:
> "Also sprach Jens Axboe:"
> > On Thu, Mar 18 2004, Peter T. Breuer wrote:
> > > Hmm .. I though SPECIAL was "just" to ensure ordering of requests and
> > > I went to some lengths to ensure that if I receive a request then we
> > > start diverting incoming requests to an alternate queue until we have
> > > treated all the requests already on the device queue! Then we ack the
> > > special and pass the requests back from the alternate queue. You are
> > > telling me that I needn't have bothered since I'm the only one who
> > > could generate a special? Owww.
> > 
> > No, ->special is for the driver receiving the request,
> 
> I mean that I thought that if ->flags & REQ_SPECIAL were set, then
> we were obliged to flush the driver request queue before treating 
> this request, and I also thought that the driver was liable to receive
> such things from somewhere else.

I dunno where you get those crazy ideas, REQ_SPECIAL has absolutely
nothing to do with that.

> What I now suppose from your words above is that 
> 
>   a) ->flags & REQ_SPECIAL is something I can set all on my own.
>   b) if I do (or even if I don't?), the ->special field is mine to use?
>      (I am surprised since I dimly recall spotting it used for
>      something).

You may use REQ_SPECIAL bit as you see fit, and ->special as well. You
don't have to use them together, must do though. However, as I said
earlier, if you push these requests on to someone else request queue,
you must not fiddle with REQ_SPECIAL and/or ->special. In that case you
cannot touch/use more than what the block layer already does.

> > I don't know what
> > kind of bastard driver you are trying to create :-). If you pass it on,
> > you cannot use it.
> > 
> > But your code just using the after-last bit is still severly broken, you
> > must not play tricks like that.
> 
> It gets even worse, since I was using the very highest bits of ->flag
> for a sequence counter.  Very successfully, I may add.  Is there any

Oh god that's horrible.

> other place I can simply add to the request a counter for the number of
> (write) requests having gone through the driver?  The idea is that the
> driver is the only one who knows the real time order in which requests
> are received, so it must be the one to stamp them, and if somebody else
> wants to receive these and preserve order, this sequence number must be
> observed. But I didn't see anywhere to put them.
> 
> Are you going to say, set REQ_SPECIAL on everything and add all the
> real request info and a bit more to ->special? I suspect you are!

That's where to put it, indeed. But if you pass it on...

> > Actually, sounds like you are attempting to create an io stack at the
> > wrong level. Why aren't you just hooking into ->make_request_fn()
> 
> I don't quite understand. Using a different make_request_fn for these
> "special" requests is useful and indeed I do use one. But an "io
> stack"?

Sounds like you are acting as a middle man, hence an io stack. And the
place to do the stacking is at make_request_fn time, not at request_fn
time. If you happen to get it working, it'll be fragile at best.

> > instead? You are in for all sorts of pains doing what you describe
> > above.
> 
> Well, everything was working fine apart from death after revalidation!
> But I am attempting a rewrite now in accordance with what you say,
> which is why I am happy for elaboration/confirmation of (a-b) above.
> 
> I presume I can't get "special" requests off of anywhere but the READ
> and WRITE free lists via blk_get_request?  It would be easier to debug
> if there were a separate list.

SPECIAL isn't a request type, it's not anything actually (it's to use
for the driver for whatever he wants it to mean). But typically drivers
us it to indicate that they have attached a driver_rq structure to
->special.

-- 
Jens Axboe