Messages in this thread | | | Date | Mon, 21 Jan 2002 10:00:42 +0100 | From | Jens Axboe <> | Subject | Re: Linux 2.5.3-pre1-aia1 |
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(have read up on mult now)
On Mon, Jan 21 2002, Andre Hedrick wrote: > > On Sun, Jan 20 2002, Andre Hedrick wrote: > > > > No it's not. By your standards, that would mean that if the device is > > > > setup for 16 sector multi mode, then I could never ever issue requests > > > > less than that (without doing some crap 'toss away extra data' stuff). > > > > How else would you handle, eg, 2 sector requests with multi mode set? > > > > > > Change the opcode in the command block to single sector, if > > > rq->current_nr_sectors != drive->multcount. > > > > That crossed my mind too, however that's not what we've been doing in > > the past and multi mode has worked fine. > > And we have not done a lot of things in the past. > Mind the fact, before you changed max-sectors from 128 to 255 != 256, he > problems maybe a direct result. Mind the fact, it is my fault for not > telling you about the issue. > > Since 128 and 256 are clearly 2,4,8,16 divisable and clean, as a result we > kind of masked the problem, but 255 is not at all the same issue.
But, eg, 24 sectors is not and we would still be starting a multi read/write for that...
> Mind you Mark Lord did get this correct in the copy buffer issue, but the > bug introduced by 255 is the only problem I can trace to be suspect.
255 is effectively 248 (256 - 8), however that is still not correct when modulo a 16 multi sector setting.
> > > the hardware to what you are sending down. The question you need to > > > answer is issuing a request for multi-sector transfers less than what the > > > device is expecting, sane and correct. If you tell me it is correct, > > > please show me where I read something wrong in the specification. > > > > You are saying that even when I do: > > > > /* this is our request */ > > rq->nr_sectors = 48; > > rq->current_nr_sectors = 8; > > > > /* drive->mult_count has been programmed to 16 */ > > You exectute WIN_MULTREAD and it behaves based on what the device has been > programmed to do respond. > > If you want 8 sectors only, by golly you had better tell it expect 8 > sectors and then you can interrupt upon completion. > > If it expects 16 sectors and you stop at 8, and issue a new command, > expect the device to go south.
This really sucks, it means we cannot safely use multi mode for a variety of request sizes. I agree with your earlier comment. Here's what I think we should be doing: when requesting multi mode, limit to 8 sectors like in your patch. This is by far the most commen multiple, that's why. When starting a request, use WIN_MULT* only for cases where (rq->nr_sectors % drive->mult_count) == 0. If that doesn't hold, simply use WIN_READ or WIN_WRITE.
Applied the 2.5.3-pre2 sched SMP fix, booting -pre2 and then hacking up a patch.
-- Jens Axboe
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